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Les cycles biogéochimiques dans les écosystèmes forestiers tempérés. The biogeochemical cycles in temperate forest ecosystems
Abstract
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.
... The enrichment with Si determined in the analyzed profiles may be also connected with the source of biocycling. Annual turnover of Si in temperate climates in relation to the diversity of vegetation and soil type varies from 3 to 7 kg ha -1 year -1 in coniferous forests to 30 to 45 kg ha -1 year -1 in deciduous forests (Bartoli, 1986;Bartoli & Souchier, 1978). Moreover, Watteau et al., (2001) observed significant amounts of Si stored within the cells of beech leaves and roots in the form of polyphenolic substance coatings and associations with the pectocellulosic matrix. ...
Progression of weathering processes and pedogenesis result in changes in mineral and chemical composition of soils in comparison with non-wethered regolith. The optimal method of analyzing geochemical transformations caused by weathering, which occur together with pedogenic processes, is known as mass balance analysis. This method enables determining the absolute value of an element's enrichment or depletion in weathered soil material via the concentration of the element in parent material. The aim of the study is to analyze changes in the chemical composition of mica schists regolith on the example of the Zlote Mountains and to estimate the soil-forming processes contribution to changes in the chemical composition of the analyzed soil-saprolite profiles. Obtained results indicate significant depletion of Mg, Ca and Na in soils due to leaching of alkaline substances. Another process leading to transformation of the chemical composition in soils of sandy texture is podzolization which causes depletion of Fe, Al and Mn in the upper part of regolith. On the other hand, biocycling is a process which can lead to an enrichment of the regolith with certain elements (P, K). Some changes of the chemical composition in analyzed profiles can be also attributed to the occurrence of geomorphic processes and aeolian supply.
... En revanche, seulement quelques études ont été conduites sur les forêts tropicales, européennes ou nord américaines (Lucas et al., 1993;Bartoli, , 1986Markewitz and Richter, 1998). Au cours des derniers 60 millions d'années, il apparaît une remarquable concordance dans le temps entre la radiation des diatomées et l'évolution des formations herbacées. ...
The aim of this experimental study is, in a first time, the characterization of reactivity and physico-chemical properties of micrometrics biogenic opals called phytoliths for several terrestrial plants species from differents environments. In this regard, several complementary technics were used, such as solubility measurements, dissolution kinetics, electrophoretic measurements and potentiometric titrations, that allowed better constraining the acid-base surface reactions and thermodynamic and kinetic parameters of phytoliths interactions with aqueous solutions. In the second part of this work, we studied terrestrial plant litter interaction with aqueous solutions via litter degradation experiments in batch and mixed-flow reactors. Obtained results demonstrated high aqueous reactivity of both phytoliths and plant litter that exceeds dissolution rate of common soil minerals by 1-2 orders of magnitude. Quantitative evaluation of plant litter and phytoliths control on Si release from these solid reservoirs demonstrates strong possible impact of plants on biogeochemical cycle of silicon in almost all terrestrial environments.
... En revanche, seulement quelques études ont été conduites sur les forêts tropicales, européennes ou nord américaines (Lucas et al., 1993;Bartoli, , 1986Markewitz and Richter, 1998). Au cours des derniers 60 millions d'années, il apparaît une remarquable concordance dans le temps entre la radiation des diatomées et l'évolution des formations herbacées. ...
Cette étude expérimentale a consisté dans un premier temps en la caractérisation de la réactivité et des propriétés physico-chimiques de microparticules d’opales biogènes appelées phytolithes, pour plusieurs espèces de plantes terrestres provenant de milieux différents. Pour ce faire, une approche combinant plusieurs techniques complémentaires telles que, mesures de solubilité, cinétiques de dissolution, mesures électrophorétiques et titrages acido-basiques, a été menée sur ces phytolithes ce qui a permis de mieux quantifier le cycle biogéochimique du silicium (Si) à travers les plantes terrestres. Dans un second temps et afin de mieux contraindre ce cycle biogéochimique, cette étude a aussi porté sur les interactions de litières de plantes terrestres avec les solutions aqueuses en combinant cette fois des expériences de dégradation de litières dans des réacteurs fermés et dans des réacteurs à circulation. Les résultats démontrent une très forte réactivité des phytolithes et des litières de plantes vis-à-vis du relargage de Si, qui sont au minimum de un à deux ordres de grandeur plus élevés que les minéraux. La quantification du contrôle des phytolithes et des litières de plantes sur le relargage de Si à partir de ces réservoirs solides, démontre la possibilité d’un fort impact des plantes sur le cycle biogéochimique du silicium dans la plupart des environnements terrestres.
... A consequence of this study is that the amount of suspended phytoliths in rivers is a good indicator of both phytolith production and soil erosion. High phytolith loads should be found where vegetation is highly productive such as the intertropical low-elevation forests (e.g., Lucas et al. 1993;Alexandre et al. 1997;Wu¨st and Bustin 2003), and grassy covers (e.g., Meunier et al. 1999;Runge 1999;Alexandre et al. 2000), while temperate and high elevation forests do not produce significant amount of phytoliths (Bartoli, 1986;Bremond et al., 2004b). The second factor which is required for finding significant amount of phytoliths in the particulate load of rivers is that the proportion of material originated from the erosion of soil must be high compared with the load originated from the erosion of parent rocks (devoid of phytoliths). ...
Particulate biogenic silica (BSi) carried by rivers to estuaries and marine sediments is generally assumed to be primarily composed of diatoms. Phytoliths-biogenic opal formed in plants-are found in some marine sediments where they are interpreted to be the result of atmospheric and river inputs. In this study, we evaluate the contribution of phytoliths to the suspended load of rivers of the Nyong basin (Cameroon). BSi (2 mu m to 2 mm fraction) in the soils and the rivers range respectively, from 0.9 to 3.9 wt% and from 1.3 to 4 wt%. About 90% of the BSi pool in both soils and river suspended load are composed of phytoliths. Thecamoebians and fresh water diatoms are minor components. The concentrations of BSi and the phytolith assemblages show great similarities between the waters and the soil samples. This result implies that the erosion of top soils is the major source of the suspended load, in good agreement with the transport-limited weathering regime of the study basin.
Résumé
Le petit bassin versant du Strengbach (Haut-Rhin, Vosges) a été choisi pour établir des bilans hydrogéochimiques en relation avec le dépérissement forestier (Programme DEFORPA). La description du bassin et le but des recherches sont donnés dans (14). Les premiers résultats géochimiques concernent les eaux de pluie, de source et du ruisseau. L’acidité des eaux de pluie (pH moyen= 4.5) est déjà neutralisée dans les eaux de sources et du ruisseau mais les anions d’acides forts dominent la charge anionique des eaux de surface. Le pouvoir tampon, déjà très faible, est affecté pendant la fonte des neiges et à la reprise des écoulements à 1’automne.Les concentrations élevées en sulfate dans le ruisseau peuvent être expliquées par d’importants dépôts secs de soufre. Ce que confirment les premières analyses de pluviolessivats.Le contraire s’observe dans le bassin non forestier du Ringelbach où l’alcalinité domine la charge anionique des eaux de surface drainant aussi une roche mère granitique.
Summary
The Strengbach basin (Haut-Rhin, Vosges massif) has been chosen to study hydrochemical balances in surface waters in relation with forest decline (DEFORPA Program). The caracteristics and the equipment of the basin are described in part I. The first results concern rain, spring and river waters. The acidity of rain waters (mean pH=4.5) is already neutralized in spring and stream waters but the strong acid anions always dominate the anionic charge of surface waters: sulfate represents 60 % of the stream anionic charge, while the alkalinity remains low (44 μeq/1). The buffering capacity is then very weak and is affected during snowmelt and with the first increase in automn runoff. The high concentration of sulfate in stream water can be explained by important dry deposits of sulfur: the first analyses of chemical composition of throughfall confirm this hypothesis. The contrary is observed in the non forested Ringelbach basin where alkalinity dominates the anionic charge of the stream waters also draining granitic bedrock.
The soil genesis in the equatorial areas is greatly controlled by the biological activity. The ferrallitic soil profiles depend on the biological cycling of the elements that acts on the mineral-solution reactions. Most of the secondary minerals suffer a permanent disequilibrium by precipitation-dissolution, and reflect the local physico-chemical conditions. The podzols develop when the organo-metallic compounds formed in the topsoil are leached out of the system before their microbial degradation. This depends on the water dynamics in the system.
In the Vosges Mountains (NE of France), integrated plot-catchment studies have been carried out since 1985 in the Strengbach basin to study the influence of acid atmospheric inputs on surface water quality and element budgets. In this paper, available mid-term time series (1985–1991) have been considered to detect obvious trends, if any, in surface water chemistry and element budgets. Air quality data showed a slight decline for SO2, whereas NO2 slightly increased over the period, but these trends are not very significant. This is in agreement with increased N concentration (mainly as NH
4+) and with the stability of SO
42− in open field precipitation. Because of a significant decrease in rainfall amount over the period, only inputs of NH
4+ increased significantly whereas H+ and SO
42+ inputs declined. In spring and streamwaters, pH and dissolved Si concentration increased mainly as a result of a reduced flow. Na+, K+, Cl− and HCO-3~− concentrations remained stable whereas Ca2+, Mg2+ and SO
42+ concentrations declined significantly. Only NO
3− concentration increased significantly in springwaters. The catchment budgets revealed significant losses of base cations, Si and SO
42−. These losses decreased over the period. Nitrogen was retained in the ecosystem. However, a longer record is needed to determine whether or not changes in surface water chemistry have resulted from short-term flow reductions or long-term changes in input-output ion budgets. This is specially true with N because the decline in SO
42− output was accompanied by N accumulation.
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.
The characteristics of surface and soil waters draining an old declining spruce stand, and their chemical behaviour under acid input influence within a small catchment are described using open field precipitations, throughfalls, soil solutions, spring and streamwater data. Element fluxes are calculated in the different ecosystem compartments, and the proton mass balance has been assessed. The results show the importance of acid occult deposits and, more generally, of acid atmospheric inputs, in explaining present soil acidification. This acidification is characterized by the base cation deficiencies observed in spruce thereby showing symptoms of decline. However, stream waters and spring waters are not yet acidified owing to the neutralization of acid inputs by weathering in the regolith. Nevertheless, streamwater chemistry is dominated by strong acid anions which are not characteristic of waters draining granitic bedrock in pristine areas.
La genèse des sols en milieu équatorial présente une forte composante biologique. La structure générale des profils ferrallitiques s'explique par le recyclage biologique des principaux éléments intervenant dans les équilibres minéraux-solutions, et la plupart des minéraux secondaires des sols ferrallitiques sont en rééquilibrage constant avec les conditions du milieu. La genèse des podzols est liée à une exportation précoce des composés organo-métalliques formés dans les horizons de surface, dépendante de la dynamique de l'eau à l'échelle des systèmes. (Résumé d'auteur)
In most soils of the humid tropics, kaolinitic topsoil horizons overlie more gibbsitic horizons. This arrangement cannot be produced simply by leaching. Quantitative measurement of the turnover of chemical elements in the litterfall in an Amazonian ecosystem indicates that the forest cycles a significant amount of elements, particularly silicon. As a result, fluids that percolate through topsoil horizons already contain dissolved silicon. This effect keeps silicon from being leached down and may account for the stability of kaolinite in the soil upper horizons. The soil mineral composition is thus maintained by biological activity.
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