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The Role of Plants in Controlling Rates and Products of Weathering: Importance of Biological Pumping

May 2001
Annual Review of Earth and Planetary Sciences 13(29):135-63

May 2001
13(29):135-63

DOI:10.1146/annurev.earth.29.1.135

Authors:

Université de Toulon

Key Words plant cycling, organometallic complexes, biological weathering, soil genesis, stability of soil minerals s Abstract The recycling of elements by plants and plant-induced biological ac-tivity cause the rates and products of weathering to be markedly different from what would result in abiotic processes. Plants directly control water dynamics, weathering, and the chemistry of weathering solutions, which is clearly exhibited in equatorial areas where old weathering mantles are greatly influenced by biological activity. Depending on the dynamics of plant-induced organometallic compounds, this weathering results in either clayey soils, which are in a dynamic equilibrium sustained by the forest's cycling of elements, or sandy soils. In most places (tropical as well temperate areas), the weathering mantle can be regarded as being in a dynamic equilibrium sustained by plants.

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Typical composition of plants a

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Plant cycling interaction with soil minerals. For explanations of the processes illustrated in parts A , B , and C , see text.

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are plotted within the hatched rectangle.

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Figures - uploaded by Yves Lucas

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Content uploaded by Yves Lucas

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Annu. Rev. Earth Planet. Sci. 2001. 29:135–63

THE ROLE OF PLANTS IN CONTROLLING RATES

AND PRODUCTS OF WEATHERING:Importance

of Biological Pumping

Y Lucas

Laboratoire des Echanges Particulaires aux Interfaces, Universit´

e de Toulon et du Var,

BP 132, 83957 La Garde Cedex, France; e-mail: lucas@isitv.univ-tln.fr

Key Words plant cycling, organometallic complexes, biological weathering,

soil genesis, stability of soil minerals

■Abstract The recycling of elements by plants and plant-induced biological ac-

tivity cause the rates and products of weathering to be markedly different from what

would result in abiotic processes. Plants directly control water dynamics, weathering,

andthe chemistryof weatheringsolutions, whichisclearly exhibitedin equatorialareas

where old weathering mantles are greatly inﬂuenced by biological activity. Depending

on the dynamics of plant-induced organometallic compounds, this weathering results

in either clayey soils, which are in a dynamic equilibrium sustained by the forest’s

cycling of elements, or sandy soils. In most places (tropical as well temperate areas),

the weathering mantle can be regarded as being in a dynamic equilibrium sustained by

plants.

INTRODUCTION

The characteristics of soils and weathering mantles reﬂect the integrated effects of

water, atmosphere, and organisms acting on rocks. Most rocks formed in environ-

mental conditions different from those that occur at the Earth’s surface. When ex-

posedatthesurfacetheyare generally subjected to a thermodynamic readjustment.

The result is the dissolution of primary minerals in the rocks and the formation

of newly generated secondary minerals that compose the overlying weathering

horizons. The weathering mantle deepens when the rate of downward advance of

the weathering front exceeds the rate of surface removal by agents of erosion. Be-

cause of many environmental problems, there is a great interest in understanding

the processes that control the interactions between minerals and aqueous solution

in the weathering mantle. Quantitative data are also necessary, for incorporation

into numerical models that couple transport of solutes with the chemical reactions,

thereby simulating the genesis and the dynamics of the weathering mantle.

0084-6597/01/0515-0135$14.00 135

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136 LUCAS

In the upper decimeters of the weathering mantle, the role of living organisms is

obvious; for example, organic matter gives a darker color. Soil fauna, plant roots,

and organic matter directly affect soil structure and porosity. Sulfates, nitrates, and

organicacids released by bacterial activity control the acidity of soil solutions. The

role of plants in determining the rates of weathering at the mineral, the catchment,

or the continent scale was addressed in several studies (Likens et al 1977, Velbel

1985, Berner 1992, Bluth & Kump 1994, Cochran & Berner 1992, Drever 1994,

Moulton & Berner 1998). The role of plants in determining the mineral products of

weathering,that is, mostofthe soilminerals, is, however,notgenerally appreciated,

althoughLovering(1959) stressed this problem more than40yearsago. Byplants,

I consider both the direct effect of higher plants and the effect of the associated

living organisms (mainly bacteria, fungi, and microfauna). The purpose of this

paper is to examine how plants control the residual product of weathering, that is,

the soil proﬁle.

The Products of Weathering from an Abiotic Point of View

On a global scale and particularly in the tropics, the spatial distribution of sec-

ondary minerals is frequently interpreted as a function of their stability in aqueous

solutions without consideration of biological processes. There is indeed a close

relationship between the availability of water, which depends on climate, and the

bulk result of weathering, that is, the average mineralogical composition of the

weathering mantle. This is obvious when considering aluminosilicate minerals

that are mainly precipitated in soils of the humid to the dry tropics. In warm and

humid climates the new products are mainly gibbsite [Al(OH)3] and kaolinite

[Si2O5Al2(OH)4], and almost all of the primary minerals are dissolved, including

quartz (SiO2). The soils that are formed under these conditions are called ferral-

sols, oxisols, or lateritic soils. With decreasing rainfall and increasing length of

the dry season, gibbsite ceases to be formed. With further drying, smectite [ap-

proximately Si4O10Al2(OH)2nH2O] rather than kaolinite becomes the dominant

clay, and quartz and many primary minerals are preserved. As a result, the Si/Al

ratio of the weathering cover progressively increases with increasing aridity of

the climate. This change with climate corresponds to the sequence of thermody-

namic stability of aluminosilicates, which is the order in which minerals reach

saturation in an Si-Al system (Figure 1), as pointed out by Pedro (1966) and

described by leaching models (Fritz & Tardy 1973, Fouillac & Michard 1977,

Tardy & Nahon 1985, Ambrosi 1990). Most of these models are based on the

assumption of local thermodynamic equilibrium between minerals and solution.

Some more recent models also consider the kinetics of the reactions (Mad´eetal

1994, Soler & Lasaga 1996). In simulations at 20◦Cor25

◦

C, dilute rainwater

enters the topsoil and, as it percolates downward, reactions between solutions and

minerals lead to a progressive increase in the concentrations of Fe, Si, Al, and

other elements in the soil solution. Saturation is reached successively for goethite

(FeOOH), gibbsite, kaolinite, Fe-smectites, Fe-Al smectites, calcite (CaCO3),

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THE ROLE OF PLANTS IN WEATHERING 137

Figure 1 Mineral stability in the Si-Al system at 25◦C. Considering a pH varying from

4.5 to 5.5 and all elements being free ions, the soil solutions from columns A through E of

Table 3 are plotted within the hatched rectangle.

and Mg-smectites. At lower temperatures the rates of reactions are lower, and the

equilibrium constants change, so that newly formed phases are different [e.g. illite,

approximately (Si(4−x)Alx)O10Al2(OH)2Kx] and they are formed slowly or not

at all.

The results of such modeling are in agreement with the observed latitudinal

zonality of the weathering mantle, especially with respect to bulk mineralogical

composition in tropical soils. If water percolates into the weathering mantle at

a high rate, as in humid climates, solute concentrations in the soil water will be

low and the main newly generated minerals will be gibbsite and kaolinite. If water

percolates into the weathering mantle at a low rate, as in drier climates, the solute

concentrations increase, and the newly generated minerals are primarily smectite

and calcite.

The Limits of Abiotic Leaching Models

When leaching models are used without considering the role of plants and biolo-

gical activity, these models cannot describe some important features of the weath-

ering mantle. For example, the vertical sequence of horizons predicted by the

abiological models is as follows: (a) highly leached, aluminous, gibbsitic horizons

near the surface; (b) more siliceous kaolinitic horizons below; and (c) smec-

titic horizons at still greater depths. Actually, this structure is rarely obser-

ved, even in highly leached equatorial areas. There, kaolinite is the prevailing

secondary mineral in the topsoil horizons of most of the well-drained ferral-

sols. When more gibbsitic horizons are observed, they are situated beneath more

kaolinitic horizons, either as gibbsitic regoliths (Leneuf 1959, Delvigne 1965,

Sieffermann & Millot 1969, Novikoff 1974, Furian 1994), as bauxitic horizons

(Dennen & Norton 1977, Lucas et al 1989, Boulang´e & Carvalho 1997), or as

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138 LUCAS

gibbsite accumulations in a kaolinitic matrix (Lucas et al 1986). Thus, observa-

tions of kaolinite in the uppermost soil horizons are inconsistent with the results

of thermodynamically based models in which the predominance of gibbsite is

predicted.

Another example of the inadequacy of the thermodynamically based mo-

dels is the frequent association of podzols with ferralsols in the humid tropics.

Podzols, also called spodosols, are characterized by thick, sandy horizons

composed of residual, resistant primary minerals, mainly quartz, and only scarce

secondary minerals. Podzols have been reported in South America (Klinge

1965, Sombroek 1966, Volkoff 1985), in Africa (Brammer 1973, Schwartz

1988), and in Asia (Brabant 1987, Siefferman 1990). In the Amazon Basin,

podzols cover >250,000 km2(Martinelli et al 1996). Despite this great ext-

ent in area, until recently, tropical podzols were commonly unmentioned on

soil maps of the world (Food and Agricultural Organization 1974). One reason

for this omission is that podzols, although mineralogically quite different

from them, are often closely associated with ferralsols, coexisting on the same

landscape and often developing from the same parent rock (Turenne 1977,

Lucas et al 1984, Chauvelet al 1987, Veillon 1990, Boulet et al 1997, Dubroeucq &

Volkoff 1998). This close association cannot be understood solely from

leaching models without considering the role of plants and associated biologic

activity.

In this paper, I consider more speciﬁcally the role of plants on (a) the mo-

bility at the proﬁle or catchment scale of less mobile elements, mainly Si, Al,

and Fe, which play a major role on soil composition and properties because they

are the only constituents of many secondary minerals and the main constituent

of most of the others; and (b) the genesis of the proﬁle, that is, the vertical

succession of horizons from the rock to the topsoil. After a brief review of the

main processes by which plants affect weathering and soil genesis, with a fo-

cus on the role of biological pumping (that is, the recycling of inorganic con-

stituents by plants) in the chemistry of soil solutions, a case study is discussed in

detail.

PROCESSES INVOLVING PLANTS IN WEATHERING

AND SOIL GENESIS

Plants absorb soil water through their roots and take in atmospheric CO2through

their leaves. The water is transferred by capillary and osmotic processes to the

leaves where it is evaporated and returns to the atmosphere. The uptake water

contains dissolved elements, which permit the plant to satisfy its nutrient needs.

In the plant cells, water, CO2, and nutrients are combined to produce the organic

matter of the plant tissues. As the plant or its parts die, the organic matter returns

to the topsoil, where it is consumed by other living organisms—fauna, fungi,

and bacteria. Most of the organic matter is thus oxidized. The CO2returns to the

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THE ROLE OF PLANTS IN WEATHERING 139

atmosphere, and most other elements are released into the soil solution. A part

of the organic matter is incompletely oxidized and reacts to form the soil’s humic

substances.

The associated organisms are also involved in the processes of weathering and

soil genesis. Fungi and bacteria help the plants obtain nutrients from the soil

solution and the soil atmosphere, and they are indispensable to the decomposition

of dead organic matter. Plants also favor the development of a thicker soil by

diminishing the rate of surface removal by agents of erosion. There are close,

complex relationships and feedback processes between plants, microorganisms

and fauna, the soil solutions, and the structure and mineralogical composition

of the soil. Details on these processes can be found in generalized (Jenny 1980,

Duchaufour & Souchier 1991) or specialized literature (Callot et al 1982, Paul &

Clark 1989, Robert & Berthelin 1986, Robert & Chenu 1992). In this review,

three main consequences of these interactions are described: the enhancement of

weathering by plants, the control of water available for weathering processes, and

more speciﬁcally the geochemical consequences of the cycling of elements in the

upper part of the weathering mantle.

Enhancement of Weathering by Vegetation

Vegetation, with its associated fungi and bacteria, promotes mineral dissolution by

modifying the chemical characteristics of the soil solution. This occurs mainly in

the upper horizons, where the litterfall is decomposed, and in the volume immedi-

ately surrounding the ﬁne roots (a few millimeters), called the rhizosphere, where

the activity of microorganisms is intensely stimulated by organic compounds ex-

creted by the roots. Plants alter the weathering conditions mainly by changing

the pH, the complexing capacity, concentrations of dissolved elements in the soil

solution, and, to a lesser extent, the Eh of the soil solutions.

The ﬁne roots directly modify the pH in the rhizosphere by excreting pro-

tons (H+) or hydroxyl ions (OH−). Jaillard et al (1996) mapped pH around

ﬁne roots grown in pH-sensitive gel, and they showed that pH can be raised or,

more commonly, lowered by more than 2.5 pH units in the rhizosphere, depending

on the physiological state and the nutrient supply of the plant (B Jaillard, personal

communication). Acidiﬁcation results from the combined effects of several pro-

cesses at the root-soil interface. The root actively excretes H+, which exchanges

for other nutrients (Mg2+,Ca

,NH

, and so on), and thus maintains a

charge balance (Briskin 1994). It takes up basic cations and excretes acid cations

[Al3+(Andersson 1988)] and organic acids (Jones & Brassington 1998). Other

processes related to the oxidation of the dead organic matter tend to lower the

pH. The release of CO2by respiration of the root and associated organisms

increases the carbonic acid content of the soil solution and, hence, its acidity.

The nitrogen and sulfur released from plant tissues are biologically oxidized to

form nitrate NO3−and sulfate SO42−, and organic acids are also excreted by

microorganisms.

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140 LUCAS

The biota also enhances the formation of organometallic complexes, because

mostof the released organic acidshavethe ability to formcomplexes(Bufﬂe 1988).

This enhances the mobility of metals whose solubility is low in abiologic systems,

such as Al3+,Fe

, or other transition metals. It thereby enhances the dissolution

of metal-bearing minerals (Robert & Berthelin 1986, Tan 1991, Drever & Vance

1994). For example, Robert et al (1979) showed that the dissolution of a mica

(phlogopite) in a reactor is eightfold faster when a 10−4molar oxalic acid solution

is used than when pure water is used and threefold faster than when 10−4molar

sulfuric acid is used. The redox potential may be locally changed as the result

of depletion of dissolved O2through the oxidation of organic matter, which can

result in increased dissolution of redox-sensitive elements such as iron.

The effects of these processes on weathering have been quantiﬁed in the labo-

ratory as well as in some ﬁeld experiments. Among numerous studies, Hinsinger

et al (1993) grew plants with a mineral (phlogopite) as the sole source for both

K and Mg, and they found that the plants were able to increase the release of K

and Mg by a factor of two to four after four days. In a similar experiment using

phosphate rock and alumina sand as the mineral substrate, dissolution was signi-

ﬁcantly enhanced by the action of the roots (Hinsinger & Gilkes 1997). On a ﬁeld

scale, Moulton & Berner (1998) measured the release of Ca and Mg into streams

and growing trees in two areas of basaltic rocks—one with vegetation and one

barren—in Iceland. The rate of release was two- to ﬁvefold higher in the area with

vegetation than in the barren area.

The effect of plants on the weathering rates, deﬁned as equivalent to the ﬂux

of rock-derived base cations (Ca2+,Mg

,Na

, and K+) leaving a catchment in

aqueous solutions, was addressed in detail by Drever (1994). The effect of plants

on weathering rates of silicate rocks is probably a factor between 1.5 and 10 and

likely no more than a factor of 2. Drever emphasized that a decrease of the pH

signiﬁcantly enhances weathering of silicate only when it turns lower than 4.5,

and that the concentration of organic ligands is insufﬁcient to cause a signiﬁcant

increase in dissolution rate.

Anotherimportantfactor when considering the weathering rate at thecatchment

scaleis the thickness andnatureof the soil cover. Asemphasizedby Stallard (1985)

andfurther discussed byBluth& Kump (1994), weatheringratewill decrease asthe

soil gets deeper, isolating the bedrock from precipitation. Most of the aggressive

processes caused by plants are actually more strongly expressed in the upper

soil horizons and in the rhizosphere. Because the ﬁne roots have a rapid turn-

over, all of the soil material is affected by rhizospheric processes at a rate that is

considered rapid with respect to the times of development of a soil proﬁle. As

argued further, there is in the upper horizons great variation in biological activity,

and therefore great variations of pH and concentration of organic ligands. The

mobility of organic ligands is, however, very low in horizons containing clay,

oxide, or hydroxide minerals because organic compounds are easily adsorbed on

mineral surfaces. At greater depths, roots are scarce or missing, the biological

activity is reduced, the pH is buffered, and the concentration of the organic ligands

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THE ROLE OF PLANTS IN WEATHERING 141

is quite low because those produced in the topsoil horizons were mostly retained

there.

Control of Water Dynamics

The water available for weathering processes depends on the balance between the

inﬁltration of rainwater into topsoil and the output of water by evapotranspiration.

The inﬁltration into the topsoil depends on the soil permeability, which by itself is

sustained by plant-controlled processes (Colin et al 1992). The evapotranspiration

is the sum of evaporation from topsoil and transpiration through the plant leaves.

In areas where vegetation cover is continuous, transpiration is the main process

of water loss. The corresponding uptake of water by roots has two main effects;

it reduces the deep drainage, that is, the ﬂux of water that percolates downward,

and it creates a buffer that absorbs percolating free water, thus regulating deep

drainage, and contributes to reduction of the surface runoff.

Thereduction ofdeepdrainage byrootuptake isevident inmost dry ortemperate

areas, but is also quite signiﬁcant in the humid tropics. In the Amazon Basin,

rainfall ranges from 1500 to 3500 mm year−1, and the average evapotranspiration

for tropical forests is ∼1460 mm year−1(Bruijnzeel 1990). Based on the values

listed by Fritsch (1992), it appears that most of the rainforest areas have deep

drainage of ∼500–800 mm. The reduction of the deep drainage is typically linked

to an increase of the residence time of the weathering solutions, so that these are

likely to reach higher concentrations for some dissolved elements such as Si, Al,

orFe. If concentrations getcloseto saturation withSi-,Al-,orFe-bearing minerals,

the dissolution of these minerals is reduced or stopped (Lasaga 1984).

Root uptake modiﬁes the location of water in the soil voids, and thus the energy

state of water and itsmovement. In soil material, the pore diameters are not equally

distributed. The pore size distribution depends on the size and arrangement of soil

particles (Figure 2). Pores larger than ∼30 µm (equivalent void radius) allow a

fast downward percolation of the rainwater. In smaller pores, the water is retained

as capillary water. The plants are able to take up capillary water that is stored in

Figure 2 Void size dis-

tribution in a clayey fer-

ralsol. 1e, void index of

the pore size mode (ratio

of pore volume of the pore

sizemode tototalsolid vol-

ume); IW, immobile wa-

ter; WA, water available

for root uptake. Adapted

after Grimaldi et al (1993).

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142 LUCAS

poresthatare>−0.1to0.3µm, a limit corresponding to thewilting point. In pores

smaller than 0.1 µm, advection is negligible, and the water can be considered

immobile; elements dissolved in the water contained in these pores are exchanged

by diffusion with the advective water in larger pores. The capacity of soil material

to retain water (holding capacity) depends on the abundance of small pores, which

is greater in clay-rich than in sandy materials. The holding capacity, coupled with

the uptake by roots between rain events, provides a buffer to the moisture fronts

that percolate deeper. In clay ferralsols from Amazonia exposed to an average

2100 mm of rainfall, free water is never sampled in tension-free tray lysimeters

below a depth of 80 cm (Lucas et al 1996). In the upper decimeters of these soils,

the physical state of water is quite variable, alternating from free to capillary water

depending on the rain events. In deeper zones, water slowly percolates downward

as capillary water. Because there is a general correlation between water content

and the abundance and activity of microorganisms, to the extent that soil aeration

remains nonlimiting (Robert & Chenu1992), hydric pulses in the upper decimeters

are correlated with pulses of biological activity.

This correlation has a bearing on the signiﬁcance of soil solutions that are sam-

pled in situ in the weathering proﬁles. Most commonly, soil solutions are sampled

using tension-free lysimeters or porous cups (Litaor 1988, Grossmann & Udluft

1991). The lysimeters sample freely percolating water situated in pores larger

than ∼30µm, and the porous cups sample water situated in pores >−1.5 µm.

The water in smaller pores is inaccessible to these sampling devices, although it

may represent the greater part of the soil water in clayey materials. This water

surrounds mineral particles and interacts with them. Its chemistry may be sig-

niﬁcantly different from that of the sampled water, because the thickness of the

diffuse layer at the mineral-water interface is signiﬁcant when compared with the

distance between any point in the solution and the mineral surface (Fripiat et al

1971).

Plant-Induced Precipitation of Minerals

Plants can locally induce the precipitation of minerals, either in the root-

rhizosphere system or in their aerial parts, that is, stems, leaves, and ﬂower

parts. The precipitation of clay (kaolinite) in root cells accompanying decomposi-

tion of cell tissues wasdemonstrated by Callot et al (1984, 1992). Theprecipitation

occurs when the secondary cell wallis still intact, which rules out the hypothesis of

a physical displacement of clay particles from the surrounding soil. The Si and Al

needed for precipitation may come from the soil solution and from the plant tissue

itself. Jaillard (1987) and Jaillard et al (1991) documented that the precipitation

of calcite in the root-rhizosphere system is an important process in areas subject

to alternately humid and dry conditions such as Mediterranean climates. The cal-

cite precipitation can occur inside or outside the plant tissue. When outside, it is

usually closely related to associated fungal activity. These processes can result in

the genesis of carbonate-indurated soil horizons called calcretes, which can serve

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THE ROLE OF PLANTS IN WEATHERING 143

as paleoecological indicators (Wright et al 1995, Becze-De´ak et al 1997). The

precipitation of iron and manganese is frequently observed in the rhizosphere in

hydromorphic soils. Migration of Fe and Mn and localization of the phases are

related to the variations of EHin the region surrounding the roots due to the dif-

fusion of O2from the root cells (Chen et al 1980) and to the microbial activity

in the rhizosphere. FeIII is precipitated mainly as goethite and lepidocrocite, and

FeII mobility may be regulated by biologically precipitated green rust (G´enin et al

1998). Mn oxides occur as quite small crystals that are difﬁcult to identify. The

oxidation of MnII and the precipitation of oxides containing MnIV, which are very

slow in laboratory conditions, are catalyzed in soils by bacteria or fungi (Herbillon

1994).

In plant tissues, biomineralization is related to the accumulation of elements

taken up by roots from the soil solution. Cystoliths are minute carbonate con-

cretions occurring in the aerial parts of plants. Phytoliths are small opal (SiO2)

concretions precipitated within the cells, cell walls, and intercellular spaces. Their

size and shape often are indicative of the plant family or genus that yielded them.

Because some phytoliths can be relatively stable in soils for thousands of years,

they may yield information on changes in vegetation and pedogenic processes

during late Quaternary time. Because they are more soluble than most siliceous

soil minerals, they constitute a source of labile Si in the upper part of the soil

cover (Alexandre et al 1994, Meunier et al 1999), which signiﬁcantly affects the

chemical composition of the soil solution as well as the cycling of Si. Watteau

et al (2000) observed in beech leaves and roots that Si is stored within the cells

as coatings of polyphenolic substances and between and within the cell walls in a

close association with the pecto-cellulosic matrix.

Uptake of Elements by Plants

Plant tissues are mainly composed of H, C, and O, but they contain several other

elements in signiﬁcant amounts. The average mineral composition of plants is

given in Table 1. Among the naturally occurring elements, only 17 are truly

essential for the growth of all plants. The incorporation of H, C, and O mainly

occurs during photosynthesis, and these are generally not considered to be mineral

nutrients. The other elements are mostly taken up from the soil solution by roots.

N, P, S, K, Mg, and Ca are found in relatively high concentrations in plant tissues.

They are considered majorelements or macronutrients. Fe, Mn, Zn, Cu, B, Mo, Cl,

and Ni are found in low concentrations in plant tissues and are considered minor

elements or micronutrients. Na, Si, Co, I, and V are needed by only some plant

species. Si has a special status because it is strictly essential only for some plant

species,butitisbeneﬁcial formanyothers (Epstein1994). All of theother elements

are found in plant tissue in small quantities. Some of them, such as Al, although

potentially toxic to plants, may be found in signiﬁcant concentrations because of

their abundance in acid soil solutions. The main source for most elements is the

soil solution. For some elements, such as N, S, and P or Cl, Na, and Ca in coastal

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144 LUCAS

TABLE 1 Typical composition of plantsa

Dry matter Dry matter

Element (mg g−1) Element (ppm)

C∼440 Fe 20–600

O∼440 B 2–800

H∼60 Mn 10–600

K 5–80 Zn 5–250

Ca <60 Cu 5–50

Mg <50bNi 0.05–5

P<50bCo 0.05–10

S<15 Mo 0.1–10

Na <10

Cl <8

Si <100

Al <10

aData are from Callot et al 1982 and Epstein 1994.

bValue usually <5mgg

−1dry weight.

areas, a signiﬁcant amount may come from rainfall and dry deposition of aerosols.

Direct leaf absorption may also occur and was postulated by Alfani et al (1996)

for Cu and Pb.

Becauseof theireconomicimportance, the uptakeandcycling ofmacronutrients

and many trace elements are well documented—in natural ecosystems as well as

in cultivated areas, in both tropical and temperate areas (Likens et al 1977, Mengel

& Kirkby 1987, Attiwill & Adams 1993, Marschner 1995). The uptake of toxic,

harmful, or environmentally hazardous elements such as heavy metals has been

the focus of numerous studies. Comparatively few studies have been done of the

cycling of those elements that constitute most of the secondary minerals and thus

are most involved in the results of weathering: Si, Al, and, to a lesser extent, Fe,

Mn, and Ti.

Si uptake is metabolically regulated and can be independent of the plant’s trans-

piration rate (Barber & Shone 1966, Leo & Barghoorn 1976). Si is precipitated

in phytoliths, although many plants do not have readily observable phytoliths yet

contain relatively large amounts of amorphous silica. Precipitation occurs near

transpiration termini, but it can also occur in the xylem vessels and in the endo-

dermis of roots (Raven 1983). Terrestrial plants contain Si in appreciable concen-

trations, ranging from a fraction of 1% of the dry matter to several percent and in

some plants >−10%. Si concentrations in leaf litterfall in tropical forests range

from 0.05 to 25 mg g−1dry weight. The lower value (0.05 mg g−1) is from a forest

in Malaysia growing on limestone, a Si-depleted rock. Most values range between

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THE ROLE OF PLANTS IN WEATHERING 145

4and8mgg

−1(Klinge & Rodrigues 1968, Gautam-Basak & Proctor 1983, Lucas

et al 1993, Cornu et al 1995).

PlantscontainAl in lower concentrations thanSi. Alistoxic for plants when ab-

sorbed in excess, but Al tolerance varies greatly between species and even between

varieties of the same species. Al uptake is metabolically regulated by exclusion

mechanisms from root uptake and by active excretion (Andersson 1988). Al can

be precipitated with Si in phytoliths (Bartoli & Wilding 1980). Al concentrations

in leaf litterfall in tropical forests range from 0.12 to 6.2 mg g−1dry weight, and

most values range between 0.2 and 0.8 mg g−1(Webb et al 1969, F¨olster & de Las

Salas 1976, Gautam-Basak & Proctor 1983, Lucas et al 1993, Cornu 1995).

Element Cycling, Chemical Composition of the Soil Solution,

and Interaction with Soil Minerals

The elements that have accumulated in plant tissues constitute a reservoir of those

elements in and above the soil proﬁle. Assuming that the amount and composition

of the vegetation cover are in steady state, all of the elements taken up annually

from the soil return annually to the top of the soil proﬁle. This return occurs mainly

by means of litterfall or decaying plants; however, foliar leaching by rainfall

(especially during leaf senescence in autumn) and stem ﬂow (the leaching of

elements along the branches and trunks by the downward ﬂowing water) can also

besigniﬁcant. If themassvegetationincreaseswith time orif vegetationis cropped,

there is a net loss of elements from the soil volume inﬂuenced by the roots.

Litterfall is decomposed within the organic horizons in the upper part of the

soil, so that most of the mineral elements contained in plant tissues are released

into the soil solution that percolates downward. At a few centimeters in depth,

the soil solution may be potentially enriched with Si, Al, Fe, and other elements

that control the stability of secondary minerals. Direct interaction of an element

with a soil mineral is signiﬁcant if the considered element is a constituent of the

mineral and if its concentration in the soil solution is sufﬁcient with regard to

thermodynamic equilibrium. Grimaldi (1987, 1988) described, for example, the

control of Al in catchment waters in French Guyana by precipitation with

a low rate of a secondary mineral, likely gibbsite. Three main types of plant

cycling are possible (Figure 3). If the element is not a constituent of any mineral

of the considered soil, the biogeochemical cycling of this element involves only

atmosphere, soil solution, and plants (Figure 3A). This happens to N and S in most

soils and to Ca, Mg, or K in highly leached soils where all primary minerals have

been weathered and no secondary minerals contain these elements. If the element

is a constituent of minerals of the considered soil, but its concentration in the

soil solution remains below saturation with any secondary minerals, plant cycling

only increases the dissolution of the element-bearing minerals (Figure 3B). This

happens, for example, to Ca, Mg, or K inmany soils. If the elementis a constituent

of minerals of the considered soil and its concentration in the soil solution reaches

saturation with secondary minerals, plant cycling directly interacts with the result

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146 LUCAS

Figure 3 Plant cycling interaction with soil minerals. For explanations of the processes

illustrated in parts A,B, and C, see text.

of pedogenesis (Figure 3C). This may happen for Si, Al, or Fe, because of the low

solubility of oxides or clay minerals.

Table 2 gives values, in kilograms per hectares per year, of measured annual

turnover resulting from litterfall in some tropical and temperate forest ecosys-

tems. The values for Si are high, comparable with the values for Ca, which is

the main macronutrient. The values for Al and Fe are lower but signiﬁcant with

respect to the solubility of these elements in water. Assuming that all of the

elements contained in the annual litterfall are dissolved in the water that annually

percolates through the litter, we can obtain average concentrations in the soil so-

lution beneath the litter. The water that annually percolates through the litter was

considered equal to the trough ﬂow, which is annual rainfall minus rain intercep-

tion by the canopy. The results of these calculations are given in Table 3. Assu-

ming that the elements contained in the litter are released in the soil solution

as free ions, these waters are plotted in the solubility diagrams of Figure 1.

With such anassumption, the waters are supersaturated with Al andSi with respect

to gibbsite and kaolinite for a pH >−4.5 at 25◦C (Figure 1). These waters are also

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THE ROLE OF PLANTS IN WEATHERING 147

TABLE 2 Annual turnover caused by the litterfall in some forest ecosystems

Turnover (Kg/ha year−1) of element:

Type of forest Referencea

ecosystem (location) Si Al Fe Mn Ti Ca

Rainforest, ferrasols (Carajas, Brazil) 9 2 2.6 1 0.05

Rainforest, ferrasols (Manaus, Brazil) 33.3 3.9 1.2 0.79 0.22 36.7

Rainforest, ferrasols (Manaus, Brazil) 48.6 6.7 0.4

Rainforest, podzols (Manaus, Brazil) 21.1 4.2 0.28

Deciduous forest, cambisols (Vosges, France) 22 ±7 1.5

Coniferous forest, podzols (Vosges, France) 5 ±2

Deciduous forest, cambisols (Vosges, France) 35.5

Deciduous forest, alﬁsols (Vosges, France) 45

Mixed forest, cambisols (Vosges, France) 21

Coniferous forest, alﬁsols (Vosges, France) 22.5

Bamboo forest, volcanic ash soil (Island of Reunion) 970–1380

aA, Rose et al 1993; B, Lucas et al 1993; C and D, Cornu 1995; E and F, Bartoli 1986; G to J, Bartoli & Souchier 1978; K, Meunier et al 1999.

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148 LUCAS

TABLE 3 Calculated average composition of the soil solution beneath litter

Composition (mM) of element in forest ecosystema[annual trough ﬂow (mm)]

Element A(1530) B(1790b)C (1790b)D (1790b)E (910) F(910)

Si 2.1 ×10−26.7 ×10−29.7 ×10−24.2 ×10−28.6 ×10−22.0 ×10−2

Al 4.8 ×10−38.1 ×10−31.4 ×10−28.7 ×10−36.1 ×10−3

Fe 3.0 ×10−31.2 ×10−34.0 ×10−42.8 ×10−4

aLetters above columns correspond to ecosystems in Table 2.

bCalculated by the method of Roche (1990).

saturated with respect to amorphous ferric hydroxide [Fe(OH)3] in the same range

of pH.

These considerations (a) assume that all of the elements contained in the lit-

ter are released into the soil solution as free ions and (b) ignore the role of the

complexing compounds released by biological activity. In fact, biominerals such

as phytoliths are partially resistant to dissolution within the litter. In a study of

Hawaiian ecosystems, Kelly et al (1998) considered that a pasture ecosystem

maintains lower amounts of more soluble silica than the forest because phytoliths

produced by grasses are more resistant to weathering. Watteau et al (2000), study-

ing the release of Si from decomposing beech leaves, observed that Si associated

with polyphenolic substances is more labile than Si associated with the pecto-

cellulosic membranes. In a rainforest ferralsol from the Congo (Alexandre et al

1994, 1997), phytoliths are selectively dissolved in the litter. Some resistant phy-

toliths are translocated downward into the underlying horizons where they are

slowlydissolved. The measured amount of Si stored in that proﬁle is high (∼11 ×

103kg ha−1in the ﬁrst 30 cm) and progressively decreases with depth. Higher

values for Si storage as soil phytoliths (45 ×103to 104 ×103kg ha−1) were

found in soil from the Island of R´eunion (Meunier et al 1999). Assuming that only

5%–10% of the phytoliths from the litterfall are dissolved annually in the litter,

the concentrations of Si in Table 3 can be divided by 10–20, giving concentrations

that are still sufﬁcient to maintain supersaturation for kaolinite in the solutions

percolating beneath the litter.

When complexing agents are released in the soil water by the biota, they com-

bine with dissolved Al, Fe, or other metallic elements to form organometallic com-

plexes that either remain in solution or are adsorbed onto the surfaces of minerals

or organic matter. These complexed metals are no longer available to precipitate

as secondary minerals. The stability constants of organic complexes with Fe and

Al are relatively high (Bufﬂe 1988). Cornu et al (1998) calculated that >−90%

of the dissolved Al and Fe in tropical ferralsol and podzols was present as organic

complexes in the soil solutions. The existence of dissolved organic complexes is

thus able to keep the soil solutions undersaturated with secondary minerals, even

when total Al, Si, and Fe concentrations are relatively high (Stevenson & Vance

1989, Duchaufour 1990).

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THE ROLE OF PLANTS IN WEATHERING 149

The stability of secondary minerals in the weathering mantle depends on inter-

actions between the biological cycling of elements, the rates of release of elements

within the litter and translocation of biogenic minerals downward in the soil pro-

ﬁle, the complexing capacity of soil solutions, and the dynamics of water within

the weathering mantle. These interactions are described below in the case study

of an exhaustively investigated weathering cover in central Amazonia.

PLANT-CONTROL OF WEATHERING COVER:

EXAMPLE OF AN AMAZONIAN SOIL SYSTEM

The Ferralsol-Podzol Systems

Ferralsols are highly weathered, yellow to red, clayey to sandy-clay soils. The

organic residues decompose quickly on the topsoil, and the litter forms a thin hori-

zon. Most elements are leached, including Si, and there is a relative accumulation

of Al and Fe. The remaining primary minerals are those that are the most resistant

to weathering, mainly quartz. The secondary minerals that accumulate are mainly

1:1 clays (kaolinite), Al-hydroxides (gibbsite), Fe-oxi-hydroxides (goethite) and

Fe-sesquioxydes (hematite). Podzols are highly weathered, bleached, sandy soils.

The organic residues decompose slowly on the topsoil, and the litter forms a thick,

peat-like horizon. All elements are leached out of the upper horizons, including Al

and Fe, which migrate as organometallic complexes. The upper horizons consist,

of residual white sand, mainly quartz and other resistant minerals. Organic matter,

Al, and Fe can accumulate at depth.

The weathering mantle of the Central Amazon Basin, 80 km north of

Manaus, Brazil, has been described in detail by Chauvel et al (1987) and Lucas

et al (1996). The geomorphic landscape consists of low plateaus dissected by ﬂat-

bottomed valleys; the topographic relief between plateau and valley bottom is

typically 30 to 40 m. The underlying rocks are sandy-clayey, Cretaceous-Tertiary

sediments that consist mainly of quartz and kaolinite. The annual mean rain-

fall is 2100 mm, and the annual mean temperature is 26◦C. The soils on the

plateaus and on the upper parts of the slopes are clayey, deeply weathered ferral-

sols that support a typical rainforest ﬂora. On the lower parts of gradual slopes,

the soils are thick, sandy podzols. Along the slopes, the ferralsols and the pod-

zols grade into one another. As the soils become progressively sandier down-

slope, the trees become smaller, and the species diversity becomes lower. The

long-term evolution of the soil mantle is a progressive replacement of the clayey-

ferralsol weathering mantle by a sandy podzolic mantle. As a slope develops

backward at the expense of a plateau, the podzols progress upslope at the ex-

pense of ferralsols (Lucas et al 1987). This forms a transformation system (Boulet

et al 1997). Under natural forest, the surface runoff is very weak (Franken &

Leopoldo 1984). The mechanical surﬁcial erosion is small with regard to geo-

chemical leaching, and the evolution of the whole system is mainly geochemi-

cal. Stallard & Edmond (1983) stressed that the weathering rates in the lower parts

of the Amazon Basin are extremely slow, which applies to the studied area. The

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150 LUCAS

old, thick weathering cover (>30 m) contains a very small amount of base cations

(Irion 1984), and more than 98% of the constitutive minerals are residual quartz

and secondary minerals (kaolinite, gibbsite, hematite, goethite). This weathering

cover thus conﬁgures an outstanding geochemical model for studying the con-

ditions of the stability of the secondary minerals and particularly the effect of

vegetation.

The plateau ferralsols consist of a 3- to 8-m-thick kaolinite-rich horizon overly-

ing a more gibbsitic ∼3-m-thick horizon, followed by an ∼8-m-thick progressive

transition to saprolite. The depth to the unweathered sediment is unknown. Water

percolates vertically downward to the deep-water table, which is at ∼20 to ∼30 m

below the surface. From the saprolite to the top of the gibbsitic horizon, quartz

grains show features of increasingly strong corrosion. The quartz abundance de-

creases,from∼55% in clayey layersand80% in sandy layers intheparent saprolite

to 5% at ∼2 m below the surface. The ferralsols develop by progressive dissolu-

tion of quartz and neo-formation of kaolinite. Petrographic evidence attests that

gibbsite is currently formed in the more gibbsitic horizon, underneath the thick

kaolinitic upper horizon (Irion 1984, Lucas et al 1986).

The kaolinite in the proﬁleis heterogeneous (Lucas at al 1986, Giral-Kacmarc´ık

et al 1998). In the saprolite, kaolinites are hexagonally shaped and arranged in

well-formed booklets. They have low Fe substitution and high crystallinity; the

Hinckley Index values, which are different in each particle size fraction, range

from 1.1 to 1.4, and infrared (IR) spectra have well-deﬁned bands at 3672 and

938 cm−1, which indicate well-ordered lattice. All of these features progressively

change in an upward direction. In the uppermost part of the proﬁle, kaolinites

are highly rounded. They have higher Fe substitution and low crystallinity; the

Hinckley Index values range from 0.4 to 0.9, and the 3672- and 938-cm−1bands

are lacking in IR spectra, which indicate disordered lattice. The well-deﬁned

upward decrease in kaolinite crystallinity indicates that kaolinite is progressively

modiﬁed in response to changing microenvironments as weathering fronts move

progressively downward.

The δ18O values of kaolinites along the proﬁle are in the range of the δ18O

values of kaolinites in equilibrium with present-day percolating waters. Variations

inδ18O valuesof kaolinites fromdifferent-sizedfractions atasingle depthrepresent

climatic variations on a timescale yet to be determined, but perhaps one that is

seasonal and on the order of centuries or millennia (Giral-Kacmarcik et al 1998).

The kinetics of precipitation and dissolution of secondary minerals were also

estimated from petrographic features (Callot et al 1992) and from experiments

in which mineral separates (kaolinite, gibbsite, and amorphous silica) enclosed

within a permeable membrane were buried in the upper horizons (Cornu et al

1995). Kaolinite was precipitated in the cells of decomposing roots. Dissolution

ofkaoliniteand/orprecipitation of new minerals in the buriedmembranewas noted

within 6 months of burial. The reactivity of minerals is thus very rapid with regard

to the duration of pedogenesis, and kaolinite is indeed in dynamic equilibrium in

the uppermost part of the proﬁle. The studied weathering mantle represents a case

study regarding the stability of the kaolinite in the upper horizons.

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THE ROLE OF PLANTS IN WEATHERING 151

Various explanations for the persistence of kaolinite in the upper horizons have

already been proposed: uplift of deep material by the fauna (Nye 1955), rapid

percolation of the rainwater through the kaolinitic horizons so that the water-

mineral interaction does not reach equilibrium (Kronberg et al 1982), or decreased

activity of water in the topsoil during the dry seasons displacing the kaolinite-

gibbsite equilibrium (Tardy & Novikoff 1988). These hypotheses do not apply to

the studied soil however. Faunal activity is restricted to the uppermost part of the

thick kaolinitic horizons. Percolating free water is absorbed in the ﬁrst 80 cm,

and soil solution slowly percolates deeper as capillary water at a rate estimated at

3–4 m year−1by isotopic measurements (Rozanski et al 1991, Giral-Kacmarc´ık

et al 1998). If, during exceptionally dry periods, soil suction reaches 15 ×105Pa,

which is the limit for plant wilting, maximum water activity coefﬁcient changes

would range from 1 to 0.989, which correspond to a very small (+0.55%) shift in

the dissolved [Si(OH)4] concentration at the gibbsite-kaolinite equilibrium.

The podzols that develop downslope consist of a quartz sand horizon with a

thickness ranging from 1 to >−4 m, overlying more clayey layers inherited from

the parent sediments. Organic matter accumulates at the interface between sandy

and more clayey material. The sandy horizons have a high hydraulic conductivity.

Biogeochemistry of the System

To investigate the processes that determine whether the products of weathering

become either stable kaolinitic ferralsol or podzols, the biogeochemical cycling

of elements was exhaustively determined in both environments. The litterfall was

sampled in a 2-year experiment (Table 2, line B) and a 1-year experiment (Table 2,

lines C and D). The soil water was sampled by tension-free cylindrical and tray

lysimeters installed beneath the litter and at 10, 20, 40, and 80 cm. The groundwa-

ter was sampled in boreholes at the water table. The annual balance of dissolved

Si, Fe, and Al that is transferred by rain, throughfall, stem ﬂow, soil water, and

groundwater was determined throughout a hydrological year (Chauvel et al 1989,

Cornu et al 1997a). The springwaters were sampled in small streams issuing from

two well-deﬁned areas, a ferralsol area and a podzol area. The dissolved fraction

in springwater was separated by tangential ultraﬁltration with a threshold at 5 kDa

(Eyrolle 1994). The speciation of Si, Fe, and Al and the state of equilibrium of the

solutions with respect to the dominant minerals (kaolinite, gibbsite, and quartz)

were determined by using the MINTEQ (Center for Exposure Assessment Model-

ing1993)program.Thereleaseofelementsbydecomposinglitterwasinvestigated

insituthroughlitter-bag experiments (Cornu et al 1997b).Thechemistryofthesoil

solutions was studied within the soil by using vermiculite and chelating resins as

test minerals (Righi et al 1990); vermiculite undergoes de-alumination in the pres-

ence of complexing agents, and forms hydroxy-Al vermiculites in simply acidic

systems, the chelating resins exchange cations from the soil solution. The main

results of these studies are summarized below.

The main source of Si, Al, Fe, and macronutrients in the topsoil is litterfall.

Inputs from rain, dust, and biological release in the canopy are small but not

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152 LUCAS

negligible. As seen above (Table 2), the input of Si and Al by litterfall is high

in both ferralsol and podzol forests. The concentrations of these elements in soil

solution exceeded what was necessary to reach saturation with kaolinite, although

moreSi was recycledinthe ferralsol forest than inthepodzol forest. In the ferralsol,

the total Si input in the topsoil (40–50 kg/ha year−1) was about three- to fourfold

greaterthan the Sileachedout of thesystem by deepdrainage(11–17 kg/ha year−1).

When the water percolates through the litter in both environments, it becomes

enriched with dissolved Si, Al, and Fe. The annual balance, however, indicates a

deﬁcit with regard to the annual input from throughfall and litterfall. That means

that a part of the Si and Al released by the litter is not transferred into the soil as

dissolved compounds in free water but as translocated particles (phytoliths) and

dissolved compounds in capillary water, which was not sampled by the lysimeter.

In the uppermost 40 cm of the soils, the concentrations in nutrients in the soil

waters in both the ferralsol and the podzol are of the same order of magnitude.

Uptake of nutrients by roots is high in the uppermost 20 cm of soil. The Si and

Al concentrations in that part of the proﬁle are high but vary greatly with time.

The Al concentrations are correlated with concentrations of NO3−or SO42−and

are higher in the hours after a rainfall, which indicates that the release of Al is

related to pulses of microbial activity. In the ferralsol soil solutions, the Si and Al

concentrations range from 26 to 54 µM and 4.1 to 20 µM, respectively, and the pH

ranges from 3.9 to 4.8. In the ferralsol water table, the Si and Al concentrations

range from 39 to 64 µM and 1.9 to 4.8 µM, respectively, and the pH ranges from

4.6 to 5. In the podzol soil solutions, the Si concentrations are slightly higher,

and the Fe and Al concentrations are considerably higher than in the ferralsol soil

solution, although these latter elements are less abundant in the podzol material

than in the ferralsol. The pH ranges from 3.2 to 4.6. In both environments, most of

the dissolved Al and Fe are present as organometallic complexes, and the waters

are undersaturated with kaolinite, gibbsite, and quartz. Nevertheless, the in situ

experiment with test minerals enclosed in permeable bags revealed differences

between the two environments. The ferralsols are characterized by a simple acidic

system, whereas podzols are more affected by organic complexation, especially in

the rainy periods. The waters sampled in the ferralsols are thus not representative

oftheprevailingprocesses in the horizons. This may be understood by considering

the water dynamics in the horizons.

Plant-Sustained Stability of Kaolinite

In the topsoil, the peak of biological activity that follows rainfall events enhances

the mineralization of the litter, increases acidity, and produces complexing organic

compounds that attack minerals aggressively. This results in high concentrations

of Si, Al, and Fe in the soil solutions. Most Al and Fe occur as organic complexes,

and the soil solution is undersaturated with kaolinite.

In the ferralsols, the dissolved organometallic Fe and Al complexes cannot

migrate deeply into the proﬁle, because they are adsorbed on surfaces of minerals

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THE ROLE OF PLANTS IN WEATHERING 153

Figure 4 Processes con-

trolling Al transfer in the

topsoil. Dashed arrow, oc-

curs in both environments;

black arrow: occurs in fer-

ralsols; white arrow: oc-

curs mainly in podzols.

with high exchange capacities, until the complexes are biologically decomposed

(Figure 4). Metals are then released to the soil solution, which reaches saturation

withkaolinite. The bimodal distributionof pores (Figure2)favorsabufferingeffect

between the percolating water and the water actually in contact with minerals.

Kaolinite thus remains in the topsoil because (a) the recycling of Si by biological

pumping provides high concentrations of Si in the soil solutions and (b) organic

compounds, inclusive of those that form Al complexes, are not able to migrate

through the clayey materials, so that Al is not leached out. The reason gibbsite

precipitates at greater depths is still unclear. It may be due to one or more of the

following processes: acidiﬁcation by CO2produced in place by root respiration

(Grimaldi & Pedro 1996) or CO2accumulated after transferring from the upper

horizons; increased Al concentrations in the soil solution because of exclusion

mechanisms of Al from root uptake; transfer of Al by inorganic complexes (Taylor

1988); or shifting ofthermodynamic equilibrium due tochanges in nanopore fabric

and abundance. Water inﬁltrating the surface takes some years to reach the deep

water table and longer to emerge in springs. The long residence time of water in

the weathering mantle provides enough time for it to approach Si saturation for

quartz. The spring waters that ﬂow out of the system are thus clear (colorless),

poor in organic matter, Fe, and Al, and relatively enriched in Si (Table 4).

Genesis of Podzols versus Ferrasols

In the sandy uppermost horizons of the podzols, the low moisture-storage capacity

results in transient waterlogging followed by dry conditions. This heterogeneous

moisture regime combined with soil acidity inhibits a complete mineralization of

organic matter and favors the production of organic compounds. The resulting

dissolved organo-Fe and organo-Al complexes are rapidly leached out of the sys-

tem, because of the high hydraulic conductivity, the low hydraulic buffering ca-

pacity, and the low exchange capacities of the surfaces of the sandy materials.

Because of the low residence times of the percolating waters and because the rate

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154 LUCAS

TABLE 4 Concentrations of elements in spring watersa,b

Concentration (mg liter−1) in soil type area

Ferralsol (state of element) Podzol (state of element)

Element Dissolved Bulk Dissolved Bulk

Si 1.7 1.9 0.8 0.91

Al 0.12 0.23 0.31 0.63

Fe 0.6 0.06 0.08 0.28

MO 0.42 1.2 4.4 22

aAdapted from Lucas et al 1996.

bThe dissolved fraction was separated by tangential ultraﬁltration with a threshold at 5 kDa.

of Si recycling by plantsis lower than in ferralsol, the Siconcentrations in the solu-

tions remain lower than in the ferralsols solutions. The spring water thus has high

concentrations of dissolved organic matter, Fe, and Al and lower concentrations

of Si than ferralsol spring waters (Table 4).

The soil proﬁle, that is, the product of the pedogenic process, is thus entirely

controlled by biological pumping and system hydrodynamics (Figure 5). The bio-

logical pumping allows the maintenance of a Si stock in the upper horizons, but the

stability of the kaolinite is ensured only where the organo-Al complexes cannot be

leached out of the topsoil. In such places, the ﬁnal result of weathering is a clayey,

kaolinitic material. The soil solutions slowly drain downward, quartz is dissolved,

Si is leached, and there is a relative accumulation of Al and Fe. Where topsoil

solutions can drain rapidly out of the system, all of the elements are leached at

a high rate, and a relative accumulation of residual quartz grains develops. This

process creates positive feedback by increasing the hydraulic conductivity of the

Figure 5 Dynamic of the podzol-ferrasol soil system. OM, organic matter.

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THE ROLE OF PLANTS IN WEATHERING 155

Figure6 Comparison of thedynamics of Amazonianferralsol (A), Amazonianpodzol (B),

eastern-France cambisol or alﬁsol (C), and eastern-France podzols (D). Phytolith storage

is given for the 0- to 50-cm horizons. Illustrations are based on data from Cornu (1995),

Lucas et al (1996), Bartoli & Souchier (1978), Bartoli (1986), and Berthelin et al (1990).

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156 LUCAS

soil material and favoring a heterogeneous moisture regime in the topsoil. The on-

set of podzolization is likely to be favored by a sandy parent material that ensures

easy leaching downward, ultimately toward the forest stream. Downslope alluvial

deposits are particularly conducive to this scenario. After this beginning, positive

feedback leads to the development of podzols at the expense of the ferralsol cover.

A sandy parent material, however, is not necessary to initiate such a process. In

French Guyana, podzols develop at the expense of sandy-clay ferralsols, the whole

system being developed on the same saprolite issued from basement rock (Boulet

et al 1997).

Theresults described abovemustbecomparedwith those obtained in temperate

areas by Bartoli & Souchier (1978) and Bartoli (1986) regarding plant cycling of

Si (Table 2) and by Berthelin et al (1990) regarding soil processes. Most of these

resultsconcernsoilsformedongraniteorsandstoneintheVosges (eastern France).

Results are summarized in Figure 6. Cambisols or alﬁsols have high Si plant cy-

cling, moderate weathering of minerals, low residence time of phytoliths, and low

Si leaching out of the system. Organic matter is fully mineralized. Dissolved or-

ganic compounds cannot migrate through horizons, and acidity of the soil solution

is mainly due to nitrates. Free cations in the soil solution are mainly Ca2+,Mg

and K+if soil is formed on base-rich parent material, and Al3+if soil is formed on

base-poorparent material. Podzols havelowSibiological cycling, highweathering

of minerals, and high Si leaching from the system. Organic matter is incompletely

mineralized. Dissolved organic compounds migrate freely through eluvial hori-

zons, and acidity of the soil solution is mainly due to complexing organic acids.

Cations in the soil solution form organometallic complexes that migrate toward

lower depths. The role of plant cycling and the biogeochemistry of the systems

are, thus, close to those related to Amazonian soils. The Si storage as phytoliths is,

however, much higher in tropical ferralsols than in temperate soils. Another differ-

enceisthat Si leaching from the system ishigherinthe temperate podzol than in the

moreclayey temperatesoil, which is theopposite of whatoccurs in Amazonia. This

may be due to the low residence time of percolating water in temperate alﬁsols or

cambisolsand to permanencyof easily weatherable minerals in temperate podzols.

The change in soil dynamics after a change of vegetation may give rapidly

noticeable results when soil is already sandy. The evolution of nonpodzolic sandy

soilsto podzols (by removalof clay and Feoxidesandbleaching) in French Guyana

occurred during a few years after forest clearing and cropping. The same was

observed in France after substitution of deciduous forest by coniferous (Y Lucas,

unpublished data).

CONCLUSIONS

Biologicalactivity,mainly related to plant growth,maymodifyrates and especially

products of weathering relative to those that would result from abiotic processes.

Plants have signiﬁcant actions at the scale of microsites, mainly due to peculiar

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THE ROLE OF PLANTS IN WEATHERING 157

chemistry in the rhizosphere, and at the scale of bulk soil, mainly due to plant

cycling of elements, control of water dynamics, and plant-induced release of acid

and complexing compounds into the soil solution. The effect of plants is well

expressed when considering most of the secondary minerals (e.g. kaolinite, gibb-

site, goethite, hematite) for which the soil solution is likely to reach saturation. It

is likely less signiﬁcant with respect to primary minerals bearing base cations for

which the soil and weathering solutions usually remain far for equilibrium (Drever

1994).

The results of weathering on Si-Al secondary minerals, the main constituents

of the weathering mantles, depend on the balance between two main processes:

(a) the stability of the clay minerals, sustained by the plant Si cycling; and (b) the

leachingof plant-induced organo-Al compounds.Theseresults are evidentinequa-

torialareaswhere old weathering mantles are highly affectedbybiologicalactivity,

and they can also be observed in temperate areas. A change of vegetation may sig-

niﬁcantly change the stability of secondary minerals owing to changes in water

dynamics as well as in the nature of the litterfall and the resultant changes in

Si cycling. Other minerals, such as iron-bearing minerals, are also affected by

these processes. It is thus important to continue to obtain more data on the cy-

cling of mineral-forming elements and to better understand the biogeochemical

cycles.

From a global and geologic point of view, the cycling of elements, mainly

Si, by plants is a key factor in the stability of secondary minerals; in most

places, the weathering mantle can be regarded as a dynamic-equilibrium sys-

tem sustained by plants. The plants maintain not only the nutrients they need

for growing but also a mineral substrate favorable to life. These considerations

raise questions for long-term results of changes in the vegetation, for example

replacement of high-recycling deciduous forest by low-recycling coniferous for-

est or crops. The expression of these changes in the soil proﬁle, however, de-

pends on the soil type. The results of the replacement of forest by crops on

the mineral composition of the soil may be apparently insigniﬁcant on clayey

soils, but rapidly noticeable on sandy soils. The processes by which plants control

the weathering mantle have thus to be considered when studying global changes

through geological times as well as during present time. They must be included

in predictive models of soil water chemistry, stream water chemistry, and soil

genesis.

ACKNOWLEDGMENTS

The author would like to gratefully acknowledge ﬁnancial support through the

French National Research Programs, Programme d’Etude de la G´eosph`ere In-

tertropicale (PEGI) and Programme de Recherche Sols Erosion (PROSE). The au-

thorswouldalsoliketoacknowledgehelpful reviews from SM Savinandunknown

reviewers.

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158 LUCAS

Visit the Annual Reviews home page at www.AnnualReviews.org

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Annual Review of Earth and Planetary Science

Volume 29, 2001

CONTENTS

BREAKTHROUGHS IN OUR KNOWLEDGE AND

UNDERSTANDING OF THE EARTH AND PLANETS, G Schubert 1

HUMAN IMPACTS ON ATMOSPHERIC CHEMISTY, PJ Crutzen, J

Lelieveld 17

INNER-CORE ANISOTROPY AND ROTATION, Jeroen Tromp 47

PARTIAL MELTING EXPERIMENTS ON PERIDOTITE AND

ORIGIN OF MID-OCEAN RIDGE BASALT, Ikuo Kushiro 71

TECTONIC EVOLUTION OF THE JAPANESE ISLAND ARC

SYSTEM, Asahiko Taira 109

THE ROLE OF PLANTS IN CONTROLLING RATES AND

PRODUCTS OF WEATHERING: Importance of Biological Pumping, Y

ucas 135

RUSTY RELICS OF EARTH HISTORY: Iron(III) Oxides, Isotopes, and

Surficial Environments, Crayton Yapp 165

USING SPRINGS TO STUDY GROUNDWATER FLOW AND

ACTIVE GEOLOGIC PROCESSES, Michael Manga 201

GROUND PENETRATING RADAR FOR ENVIRONMENTAL

APPLICATIONS, Rosemary Knight 229

DATING MODERN DELTAS: Progress, Problems, and Prognostics,

Jean-Daniel Stanley 257

RHEOLOGICAL PROPERTIES OF WATER ICE--APPLICATIONS TO

SATELLITES OF THE OUTER PLANETS, WB Durham, LA Stern 295

THE LATE ORDOVICIAN MASS EXTINCTION, Peter M Sheehan 331

HYDROGEN IN THE DEEP EARTH, Quentin Williams, Russell J.

Hemley 365

PHYSICS OF PARTIALLY SATURATED POROUS MEDIA: Residual

Saturation and Seismic-Wave Propagation, Xun Li, Lirong Zhong, Laura

J Pyrak-Nolte 419

RESPONSE OF LATE CARBONIFEROUS AND EARLY PERMIAN

PLANT COMMUNITIES TO CLIMATE CHANGE, William A

DiMichele, Hermann W Pfefferkorn, Robert A Gastaldo 461

GIANT DIKE SWARMS: Earth, Venus, and Mars, RE Ernst, EB

Grosfils, D Mège 489

THE CARBON BUDGET IN SOILS, Ronald Amundson 535

CONTINUOUS FREE OSCILLATIONS: Atmosphere-Solid Earth

Coupling, Toshiro Tanimoto 563

Annu. Rev. Earth Planet. Sci. 2001.29:135-163. Downloaded from arjournals.annualreviews.org

by University of British Columbia Library on 03/03/10. For personal use only.

Plant-soil feedback across spatiotemporal scales from immediate effects to legacy

Article

Dec 2023
SOIL BIOL BIOCHEM

Jan Frouz

Silicon fertiliser application increases the terrestrial ecosystem carbon pool at the global scale

Article

Feb 2024
GEODERMA

Soil phosphorus and biomass carbon co-determine plantation soil organic carbon density: a case study in western Beijing, China

Article

Full-text available

Aug 2023

Context. Studies of afforestation have traditionally neglected the influences of plant microhabitats on the growth and carbon sink capacities of planted forests. Aims. We investigated the potential mechanisms related to the relationship of afforestation elevation to soil organic carbon density (SOCD). Methods. The carbon density of three plantation ecosystems and barren land soils were evaluated at two elevations in the Donglingshan Mountains of Beijing, with structural equation modelling and variation partitioning analyses used to identify the environmental factors that influenced the carbon densities of plantation ecosystems. Key results. Afforestation elevation was related to the vegetation phenology of plantation forests. Specifically, growth periods at higher elevations were delayed relative to those at lower elevations, while different growth periods affected growth rate of diameter at breast height (R DBH), in addition to the carbon and nitrogen contents of ground surface litters. Consequently, lower elevation afforestation reduced the carbon sink capacity of coniferous plantation ecosystems in the study area. Lower plantation elevations were associated with significantly reduced R DBH values of Pinus tabuliformis. Further, biomass carbon density (BCD) and SOCD of Larix principis-rupprechtii plantations were significantly lower due to decreased elevations. Soil nitrogen concentrations, litter nitrogen density (LND), soil phosphorus concentrations, and BCD were the primary drivers of plantation SOCD. Conclusions. Overall, different plantation elevations were associated with different vegetation phenologies and R DBH values, which further affected LND and BCD, thereby ultimately affecting variation of SOCD. Implications. This study provides important insights into the selection of afforestation plots to maximise plantation carbon sequestration capacities.

A new model of bauxitization in quartzitic landscapes: A case study from the Southern Espinhaço Range (Brazil)

Article

Jul 2023

Lithology plays a fundamental role in rock weathering and erosion, and in landscape evolution. When weathering‐ and erosion‐prone lithologies are protected from erosion by more resilient rock types (e.g., quartzites and banded iron formations) unusual weathering products result. At the Southern Espinhaço Range, Minas Gerais, Brazil, bauxitic weathering profiles are found in a unique geomorphological–lithological–climatic setting. Resistant quartzites acted as a barrier against erosion of interbedded hematite‐phyllite lenses, channelling solution flows and facilitating the formation of deep weathering profiles. The long‐term exposure of the hematite‐phyllites under alternating wet and dry tropical climates favoured widespread bauxitization. Here we investigate the geochemical, mineralogical, geochronological and micromorphological signatures of scaffolded bauxites in order to reconstruct their evolutionary history. Our data suggest that recurrent aluminium and iron mobilization within the profiles were mainly driven by mineral dissolution‐reprecipitation mediated by bioturbation and the influx of vegetation‐derived organic species. (U–Th)/He geochronology of Al‐goethite reveals that bauxitization started at least since the Lower Miocene, with important intensification of weathering in the Upper Miocene and Lower Pleistocene. The adjacent resilient quartzites acted as scaffolds for bauxitization and supported the preservation of more erodible weathering profiles developed over phyllites. Surface waters that could not infiltrate into the impermeable adjacent quartzites preferentially infiltrated into the more weathereable phyllites, enhancing their porosity and permeability, further enhancing weathering. The evolutionary history of Southern Espinhaço Range bauxites suggests a new model of bauxitization in ancient land surfaces evolution underlain by quartzites, where erosion‐prone lithologies are scaffolded by resilient quartzites and survive long‐term weathering with minimum erosion, producing bauxites.

Do Southeast Asia's paleo‐Antarctic trees cool the planet?

Article

Full-text available

Jun 2023
NEW PHYTOL

Many tree genera in the Malesian uplands have Southern Hemisphere origins, often supported by austral fossil records. Weathering the vast bedrock exposures in the everwet Malesian tropics may have consumed sufficient atmospheric CO2 to contribute significantly to global cooling over the past 15 Myr. However, there has been no discussion of how the distinctive regional tree assemblages may have enhanced weathering and contributed to this process. We postulate that Gondwanan‐sourced tree lineages that can dominate higher‐elevation forests played an overlooked role in the Neogene CO2 drawdown that led to the Ice Ages and the current, now‐precarious climate state. Moreover, several historically abundant conifers in Araucariaceae and Podocarpaceae are likely to have made an outsized contribution through soil acidification that increases weathering. If the widespread destruction of Malesian lowland forests continues to spread into the uplands, the losses will threaten unique austral plant assemblages and, if our hypothesis is correct, a carbon sequestration engine that could contribute to cooler planetary conditions far into the future. Immediate effects include the spread of heat islands, significant losses of biomass carbon and forest‐dependent biodiversity, erosion of watershed values, and the destruction of tens of millions of years of evolutionary history.

Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)

Article

Full-text available

Jun 2023

Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the weathering of granitic rocks in two 6 m deep weathering profiles in Mediterranean and humid climate zones along the Chilean Coastal Cordillera. We used geochemical and mineralogical methods such as (micro-) X-ray fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron microprobe (EMP) mapping to elucidate the transformations involved during weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing (WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III) oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the Mediterranean climate zone is significantly controlled by WIF. It generates a surface–subsurface connectivity that allows fluid infiltration to great depth and hence promotes a deep weathering zone. Moreover, the water supply to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense formation of secondary minerals in the upper section of the weathering profile. This intense formation arises from pronounced dissolution of primary minerals due to the high water infiltration (high precipitation rate) into the subsurface. The secondary minerals, in turn, impede the infiltration of fluids to great depth and thus mitigate the intensity of primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary mineral weathering in the upper regolith are controlled by positive and negative feedbacks between the formation of secondary minerals and the infiltration of fluids.

Expanding the Spatial Reach and Human Impacts of Critical Zone Science

Article

Full-text available

Mar 2024

Two major barriers hinder the holistic understanding of subsurface critical zone (CZ) evolution and its impacts: (a) an inability to measure, define, and share information and (b) a societal structure that inhibits inclusivity and creativity. In contrast to the aboveground portion of the CZ, which is visible and measurable, the bottom boundary is difficult to access and quantify. In the context of these barriers, we aim to expand the spatial reach of the CZ by highlighting existing and effective tools for research as well as the “human reach” of CZ science by expanding who performs such science and who it benefits. We do so by exploring the diversity of vocabularies and techniques used in relevant disciplines, defining terminology, and prioritizing research questions that can be addressed. Specifically, we explore geochemical, geomorphological, geophysical, and ecological measurements and modeling tools to estimate CZ base and thickness. We also outline the importance of and approaches to developing a diverse CZ workforce that looks like and harnesses the creativity of the society it serves, addressing historical legacies of exclusion. Looking forward, we suggest that to grow CZ science, we must broaden the physical spaces studied and their relationships with inhabitants, measure the “deep” CZ and make data accessible, and address the bottlenecks of scaling and data‐model integration. What is needed—and what we have tried to outline—are common and fundamental structures that can be applied anywhere and used by the diversity of researchers involved in investigating and recording CZ processes from a myriad of perspectives.

The Anthropogenic Salt Cycle

Article

Oct 2023

Modeling Cosmogenic Nuclides in Transiently Evolving Topography and Chemically Weathering Soils

Article

Full-text available

Oct 2023

Terrestrial cosmogenic nuclides (TCN) are widely employed to infer denudation rates in mountainous landscapes. The calculation of an inferred denudation rate (Dinf) from TCN concentrations is typically performed under the assumptions that denudation rates were steady during TCN accumulation and that soil chemical weathering negligibly impacted soil mineral abundances. In many landscapes, however, denudation rates were not steady and soil composition was significantly impacted by chemical weathering, which complicates interpretation of TCN concentrations. We present a landscape evolution model that computes transient changes in topography, soil thickness, soil mineralogy, and soil TCN concentrations. We used this model to investigate TCN responses in transient landscapes by imposing idealized perturbations in tectonically (rock uplift rate) and climatically sensitive parameters (soil production efficiency, hillslope transport efficiency, and mineral dissolution rate) on initially steady‐state landscapes. These experiments revealed key insights about TCN responses in transient landscapes. (a) Accounting for soil chemical erosion is necessary to accurately calculate Dinf. (b) Responses of Dinf to tectonic perturbations differ from those to climatic perturbations, suggesting that spatial and temporal patterns in Dinf are signatures of perturbation type and magnitude. (c) If soil chemical erosion is accounted for, basin‐averaged Dinf inferred from TCN in stream sediment closely tracks actual basin‐averaged denudation rate, showing that Dinf is a reasonable proxy for actual denudation rate, even in many transient landscapes. (d) Response times of Dinf to perturbations increase with hillslope length, implying that response times should be sensitive to the climatic, biological, and lithologic processes that control hillslope length.

Taxonomy and genesis of soils in contrasting ecosystems of Southern Western Ghats, India

Article

Jul 2023
CATENA

Plant impact on the biogeochemical cycle of silicon and related weathering processes

Article

Full-text available

Jan 1997

Observations de concentrations argileuses dans les structures racinaires au cours des transformations pedologiques.

Article

Jan 1984

Importance de l'hydrolyse acide dans les systèmes pédologiques des régions tropicales humides. Rôle de la forêt et conséquences sur la genèse des sables blancs tropicaux.

Article

Jan 1996

Differences in decomposition of carbonate and silicate minerals as influenced by humic acid

Article

Jan 1991

K.H. Tan

Biochemical decomposition of primary minerals was investigated by shaking calcite, hornblende and microcline with fulvic (FA) or humic acid (HA) at pH 7.0 and 2.5 for 0 to 885 hrs. Silicon, Al, Fe, K and Ca released were determined in the extracts by atomic absorption spectroscopy. Changes in mineral structure were studied by X-ray diffraction analysis, petrographic and scanning electron microscopy. The results indicated that the dissolution of Si and Al from the silicate minerals yielded a parabolic curve. The dissolution of Ca and Fe from hornblende, and K from microcline was also curvilinear in regression. -from Author

Membranes and Transport Systems in Plants: An Overview

Article

Jun 1994

Donald P. Briskin

Membranes define the outer boundary of the living protoplast and the internal compartmentation of plant cells. From a structural point of view, membranes consist of a lipid bilayer and proteins essential for functions such as solute transport, signal transduction, and numerous metabolic reactions. While membranes can represent a significant barrier to the free movement of many solutes, those with sufficient lipid solubility may move across membranes by dissolving into the lipid bilayer. However, selective membrane transport is generally observed for hydrophilic solutes such as mineral nutrients and cell metabolites. Such selective transport requires an input of metabolic energy, and in plants this occurs via the production of proton electrochemical gradients across the membrane by substrate- (ex. ATP, PPi) driven proton pumps. Selective solute transport is then mediated by membrane-associated secondary transport systems which utilize the proton electrochemical gradient to drive the transport process. This review of membrane structure and transport system function provides a background for a further examination of herbicide interactions with plant membranes.

(The regional organization of the soil mantle in Brazil: chronological differentiation).

Article

Jan 1984

B. Volkoff

Relief, geology, climate, vegetation allow dividing Brazil into 4 main regions. In each region, areas can be characterized by distinct pedologic types (latosols, lateritic or red-yellow podzolic soils, fersiallitic soils, cambisols). The latosol areas are related to 2 pediplaned surfaces with old pedologic covers. They quite differ from other areas that show a young topography and recent pedologic cover. -from English summary

Les cycles biogéochimiques dans les écosystèmes forestiers tempérés. The biogeochemical cycles in temperate forest ecosystems

Article

Jan 1986

François Bartoli

Cet article de synthèse sur les cycles biogéochimiques des éléments minéraux dans les écosystèmes forestiers tempérés tente de faire le point des travaux de recherche effectués, en essayant tout particulièrement de saisir l'incidence des cycles biogéochimiques sur le mode de dissolution des minéraux des sols, les préconcentrations géochimiques et les formes de transfert des éléments minéraux dans les rivières.

Litter fall and mineralization in three tropical evergreen forest stands, Colombia

Article

Jan 1976
Acta Cient Venez

Mineral Nutrition of Higher Plants

Article

Jan 1986

H. Marschner

Hydrology of catchment areas of Central-Amazonian forest streams

Chapter

Jan 1984

Interest in the hydrological cycle of the Amazon Basin has recently increased due to the possibility of change in the water balance as a result of intensive deforestation. In this chapter we will demonstrate our work on water and nutrient cycling in selected catchment areas in the central Amazon Basin near Manaus. This research work began in 1976 with the installation of the first catchment area in the forest reserve Ducke, a watershed called ‘Barro Branco’, and continued in 1978 with a much larger undisturbed catchment area in the forest reserve Km 60 of the INPA, the watershed called ‘Bacia Modelo’. In 1980 two other experimental watersheds were installed in the neighbourhood of the ‘Bacia Modelo’; one of them is used as a buffer watershed and the other as an experimental watershed mainly for forestry.