Weathering, atmospheric deposition and vegetation uptake: Role for ecosystem sensitivity to acid deposition and critical load

Abstract

Critical loads of acidity represent the maximum acceptable atmospheric deposition for an ecosystem type. Two hundred and forty-one ecosystem types have been defined in France using pedologic, geologic and vegetation data. Weathering rate plays the most important part in soil buffering capacity, but for poor weatherable soils, non-marine atmospheric deposition represents up to 80% of base-cation inputs. Base-cation vegetation uptake decreases significantly the buffering capacity in case of high-productivity forests. Ecosystems combining low weathering rate and low non-marine base-cation deposition with high biomass productivity are the most sensitive to acidification.

Full text

Weathering, atmospheric deposition and vegetation uptake:
role for ecosystem sensitivity to acid deposition and critical load
David Moncoulona, Anne Probst a,∗, Jean-Paul Partyb
aCNRS–IRD–UPS, LMTG, 14, av. Édouard-Belin, 31400 Toulouse, France
bSol Conseil, 251, route de la Wantzenau, Robertsau, 67000 Strasbourg, France
Abstract
Critical loads of acidity represent the maximum acceptable atmospheric deposition for an ecosystem type. Two hundred and
forty-one ecosystem types have been defined in France using pedologic, geologic and vegetation data. Weathering rate plays
the most important part in soil buffering capacity, but for poor weatherable soils, non-marine atmospheric deposition represents
up to 80% of base-cation inputs. Base-cation vegetation uptake decreases significantly the buffering capacity in case of high-
productivity forests. Ecosystems combining low weathering rate and low non-marine base-cation deposition with high biomass
productivity are the most sensitive to acidification.
Résumé
Altération, dépôts atmosphériques et prélèvement par la végétation : rôle dans la sensibilité des écosystèmes aux dé-
pôts acides et charges critiques. Les charges critiques d’acidité représentent le dépôt atmosphérique maximal admissible pour
un écosystème. Deux cent quarante et un types d’écosystèmes ont été définis en France à partir de données pédologiques, géo-
logiques et de végétation. L’altération joue un rôle prépondérant contre l’acidification, mais pour les sols faiblement altérables,
les dépôts atmosphériques non marins peuvent représenter jusqu’à 80 % des apports de cations basiques. Le prélèvement de
cations par la végétation contribue significativement à diminuer le pouvoir tampon des sols pour les forêts à forte productivité.
Les écosystèmes combinant faible altération et faibles dépôts de cations non marins ainsi qu’une forte productivité sont les plus
sensibles à l’acidification.
Keywords: acidification; ecosystems; weathering; atmospheric deposition; critical load; France
Mots-clés : acidification ; écosystèmes ; altération ; dépôt atmosphérique ; charge critique ; France
*Corresponding author.
E-mail address: aprobst@lmtg.obs-mip.fr (A. Probst).

Version française abrégée
Les dépôts atmosphériques acides de soufre et
d’azote ont fortement augmenté en Europe entre 1960
et 1980. Depuis, les émissions de soufre ont été ré-
duites de 70% alors que les émissions d’azote sont
constantes ou en légère augmentation [1,5]. L’étude
de ces phénomènes en France a montré leur impact
sur la santé des écosystèmes forestiers et aquatiques
[9,10,16,17]. En forêt, l’acidification se traduit par
une augmentation de [H+]et[Al
3+] dans la solution
de sol, la désaturation du complexe d’échange et le
lessivage des cations basiques [19]. La convention de
l’ONU sur la pollution atmosphérique transfrontière à
longue distance (1979) a défini les charges critiques
comme « l’estimation quantitative de l’exposition à un
ou plusieurs polluants, en dessous de laquelle des ef-
fets néfastes significatifs sur des éléments sensibles
précis de l’environnement n’apparaissent pas, en l’état
actuel des connaissances » [11]. La France, en tant que
pays signataire du dernier protocole (Göteborg, 1999),
s’est engagée à calculer les charges critiques spéci-
fiques à ses écosystèmes et à les fournir aux instances
chargées de la coordination au niveau européen [13,
14,18]. Ces données constituent un élément important
dans la négociation sur la réduction des émissions de
polluants. La présente étude vise à déterminer la sen-
sibilité des écosystèmes de la France métropolitaine
vis-à-vis des dépôts atmosphériques acides par le cal-
cul des charges critiques en soufre et azote. Dans ce
but, une nouvelle classification des écosystèmes est
créée à partir de données géologiques, pédologiques et
de végétation. L’altération, les dépôts atmosphériques
et les prélèvements par la végétation sont déterminés
et l’importance relative de ces paramètres est mise en
évidence.
L’équation de calcul des charges critiques en soufre
est la suivante [6] :
CLmax(S)=BCweath +BC∗
dep −BCuptake −ANCle,crit
Pour la charge critique en azote acidifiant (si les dépôts
de soufre sont nuls) :
CLmax(N)=CL(S+N)
=CLmax(S)+Nimm +Nuptake
où le suffixe weath est le taux d’altération, le le les-
sivage, où ∗
dep désigne les dépôts totaux non ma-
rins, uptake le prélèvement par la végétation, et où
crit signifie critique, imm immobilisation. Tous les
flux sont en equivha−1an−1. BC(cations basiques) =
Mg2++Ca2++K++Na2+;BC
∗
dep =Mg2++Ca2+
+K++Na2+−Cl−.
Les deux principales méthodes pour le calcul de la
limite critique, ANCle,crit sont [6] :
(1) [H+]et[Al
3+] critiques dans l’eau de drainage
ANCle,crit [Al]=−Q[H+]crit +[Al3+]crit
(2) [Al]
[BC]critique dans l’eau de drainage
ANCle,crit [Al/BC]
=−Q[H+]crit −[Al]
[BC]crit
×1,5BC∗
dep +BCweath −BCuptake
Le drainage Qest déterminé en combinant les
cartes des écosystèmes et des pluies efficaces, via une
relation entre pluies efficaces et eau de drainage sur
12 stations du réseau français de surveillance des fo-
rêts (RENECOFOR) [12,21].[H+]crit et [Al3+]crit sont
fixés respectivement à 25 µequiv l−1(pH =4,6) et à
200 µequivl−1[3,13,16].[Al]
[BC]crit est fixé à 1,2 [12].
La première méthode utilise des concentrations cri-
tiques fixées de protons et d’aluminium et est donc
essentiellement dépendante du drainage. La seconde
méthode prend en compte un bilan de flux de cations
en solution, qui limite la toxicité de l’aluminium à
pH faible. Prenant en compte un effet tampon lié au
milieu, la seconde méthode est utilisée à l’échelle na-
tionale.
Les dépôts atmosphériques de cations basiques
BC∗
dep, corrigés des apports marins [4], ont été déter-
minés à partir de l’extrapolationde données de dépôts
hors couvert forestier du réseau RENECOFOR [2] sur
une grille de 10 ×10 km. Pour estimer les dépôts secs
et humides sous couvert forestier, et en l’absence de
données spatialisées, un coefficient a été appliqué aux
dépôts hors couvert. Il est estimé à partir de mesures
in situ [21] de pluviolessivats, qui sont ici considérés
comme équivalents aux dépôts totaux sous couvert fo-
restier. Les prélèvements par la végétation BCuptake et
Nuptake sont calculés à partir des données de l’Inven-
taire forestier national sur la production de biomasse
par les forêts [8] et du contenu de l’espèce dominante
en cations et azote [12]. Le taux d’altération en cations

basiques BCweath est calculé en appliquant le modèle
numérique PROFILE [20] aux sols des Vosges, des
Ardennes et de 12 stations du réseau RENECOFOR.
Ces résultats sont ensuite extrapolés à l’ensemble des
sols de la carte des sols de France, en se basant sur leur
teneur en argile [12]. L’immobilisation d’azote est es-
timée à 150 equivha−1an−1pour les zones de plaine
et à 300 equivha−1an−1pour les zones de montagne
[15].
Les classifications d’écosystèmes existantes, es-
sentiellement basées sur des données de végétation,
manquent d’informations pédologiques et géologiques
pour être appliquées au calcul des charges critiques
[12]. La création d’une nouvelle classification a donc
été nécessaire. La carte des sols et matériaux paren-
taux de l’INRA est regroupée en 31 types [7,12], puis
croisée avec la carte de végétation potentielle. La carte
résultante est ensuite croisée avec la carte d’occupa-
tiondusol[12]. Seuls les polygones forestiers ou de
plus de 85 % de prairies sont conservés. Parmi les 241
types d’écosystèmes de la classification finale, cinq
écosystèmes non calcaires et représentatifs des zones
sensibles à l’acidification ont été choisis pour présen-
ter le calcul des charges critiques (Tableau 1). Les
charges critiques de soufre ont été calculées et carto-
graphiées à l’échelle de la France (Fig. 1).
Le taux d’altération du sol est le premier indi-
cateur du pouvoir tampon vis-à-vis de l’acidifica-
tion. Selon les types de sol, l’altération produit 30
à30×103equiv ha−1an−1. Les écosystèmes sur
sols développés sur roches calcaires et volcaniques
ont un pouvoir tampon élevé vis-à-vis de l’acidifi-
cation (BCweath >1500 equivha−1an−1). Les autres
écosystèmes (51,5% de la surface étudiée) sont po-
tentiellement sensibles à l’acidification. Les surfaces
les plus sensibles (BCweath <200 equivha−1an−1)
correspondent aux sables et grès acides (Tableau 1,
sites 3, 4 et 5). Pour les granites (site 2), les schistes
et les quartzites, BCweath est compris entre 200 et
500 equivha−1an−1. Les formations détritiques ont
des taux d’altération compris entre 500 et 1000 equiv·
ha−1an−1. Les dépôts atmosphériques de cations ba-
siques nonmarins varient de 112 à 2181 equivha−1·
an−1. Ils peuvent apporter un pouvoir tampon non né-
gligeable sur des sols où l’altération est faible. Ce phé-
nomène est important, au vu de nosrésultats au sud du
Massif central, dans les massifs cristallins des Alpes
du Nord et sur les granites vosgiens. Les prélèvements
par la végétation varient de 67 à 1222 equivha−1an−1
pour les cations et de 57 à 1324 equivha−1an−1pour
l’azote. De forts prélèvements de cations diminuent
le pouvoir tampon du sol. Au contraire, les prélève-
ments d’azote diminuent l’impact acidifiant. BCuptake
et Nuptake sont importants pour les espèces dominantes
Pinus pinaster A., Picea excelsa L. pour les coni-
fères, Fagus sylvatica L.oulegenreQuercus pour
les feuillus. Les régions à forte productivité sont sur-
tout le Sud-Ouest (côte atlantique), le Massif central
et le Nord-Est de la France. L’influence du choix de
l’espèce forestière n’est pas négligeable sur des sites
sensibles à l’acidification.
Les charges critiques calculées sur les écosystèmes
français mettent en évidence des zones considérées
sensibles aux dépôts acidifiants de soufre et d’azote :
Landes, schistes de Bretagne, Vendée et Normandie,
formations détritiques en Sologne et Île-de-France,
grès vosgiens, granites du Nord-Est du Massif central.
Les zones de dépassement de charge critique sont dé-
terminées par comparaison avec les données de dépôts
atmosphériques acides [2]. Ces zones sont principale-
ment : la côte atlantique (Landeset Pyrénées), les grès
vosgiens et les schistes de Bretagne et de Normandie.
Ce sont ces écosystèmes « sensibles», qui sont utili-
sés pour la modélisation dynamiquede la réponse aux
baisses de dépôts atmosphériques acidifiants.
1. Introduction
Between 1960 and 1980, long-range transbound-
ary deposition of atmospheric acidifying pollutants
(SO2,NO
x,andNH
3) has dramatically increased, fol-
lowing the rise of industrial activity, agriculture and
traffic development in Europe. Since 1980, sulphur
emissions have been reduced up to 70%, but nitro-
gen emissions are constant or slightly increasing [1,
5]. Acid atmospheric deposition on soil induces the
increase of [H+]and[Al
3+] in soil solution and the
leaching of base cations [19]. Toxicity of Al3+at
low pH is responsible for some ecosystem damage.
In France, the effects of acidification on surface wa-
ter [10,16] and forest [9,17] have been investigated
since the 1980s, mainly in the Vosges Mountains and
the Ardennes. Within the framework of the United Na-
tions Economic Commission for Europe ‘Convention
on Long-Range Transboundary Air Pollution’ (1979),
31 countries signed the 1999 Gothenburg protocol to

reduce acidification and eutrophicationby 2010. Since
1991, critical load maps are used as a basis for inter-
national negotiations on air pollution abatement strate-
gies [6]. The critical load is “the quantitative estimate
of an exposure to one or more pollutants below which
significant harmful effects on specified sensitive ele-
ments of the environments do not occur according to
the present knowledge” [11]. As a signatory country,
France has provided critical load results to the Euro-
pean coordination centre database in 1995, 1999, and
2003 [13,14,18]. These data play an important part in
the negotiation on pollutant emission reduction. Crit-
ical load input parameters were derived from a sim-
plified ecosystem classification, which consisted of 31
types of forest ecosystems, and were mainly focused
on the determination of weathering rates and acid neu-
tralising capacity.
The present study aims at determining the ecosys-
tem sensitivity against acid atmospheric deposition in
metropolitan France. For this purpose, a new ecosys-
tem classification was created from geological, pedo-
logical and vegetation parameters. Soil weathering
rate, atmospheric deposition, and base-cation removal
in vegetation are determined and their relative weight
is discussed in the calculations of critical loads of sul-
phur and nitrogen.
2. Method
2.1. Critical-load calculation
Critical loads for sulphur (SO2) and nitrogen (NOx
and NHx) are derived from a simplified steady-state
mass balance (Eq. (1)) applied on the soil top-layer
leachate flux [6]:
Hle +Alle +BCle +NH4,le
(1)
=SO4,le +NO3,le +Clle +HCO3,le +RCOOle
Leaching of acid neutralising capacity (ANCle)isde-
fined as (Eq. (2)):
(2)ANCle =HCO3,le +RCOOle −Hle −Alle
At the steady state, after all simplifications (Eqs. (3)–
(6)):
(3)BCle =BCweath +BC∗
dep −BCuptake
(4)Clle =Cl∗
dep
(5)SO4,le =Sdep
(6)Nle =NO3,le =Ndep −Nuptake −Nimm
Critical load for S, CLmax(S), thus corresponds to the
critical Sdep (Eq. (7)) with no N deposition:
CLmax(S)=BCweath +BC∗
dep −BCuptake
(7)−ANCle,crit
Critical load for acid nitrogen CLmax(N) corresponds
to the critical deposition of nitrogen assuming S depo-
sition is zero (Eq. (8)):
CLmax(N)=CL(S+N)
(8)=CLmax(S)+Nimm +Nuptake
Subscript weath stands for weathering, le for leach-
ing, ∗
dep for total non-marine deposition, uptake for
vegetation uptake, crit for critical. All fluxes are in
equivha−1yr−1. BC(base cations) =Mg2++Ca2++
K++Na2+;BC
∗
dep =Mg2++Ca2++K++Na2+−
Cl−.N
imm stands for nitrogen immobilisation.
2.1.1. Critical acid neutralising capacity
determination (ANCle,crit)
Two methods for critical ANCle,crit calculation
are compared (Eq. (2)), assuming that HCO3,le and
RCOOle can be neglected for forest soils [6]:
(1) critical [H+]and[Al
3+] in drainage water
(Eq. (9)):
(9)ANCle,crit [Al]=−Q[H+]crit +[Al3+]crit
(2) critical ratio [Al]
[BC]in drainage water (Eq. (10)):
ANCle,crit [Al/BC]
=−Q[H+])crit −[Al]
[BC]crit
(10)×1.5(BC∗
dep +BCweath −BCuptake)
Q, the drainage, is determined after combining the
ecosystem and drainage water maps, according to the
relationship between efficient rain water and drainage
calculated on 12 stations of the RENECOFOR net-
work (French forest survey network) [12,21].Ac-
cording to the French forest sensitivity, [H+]crit and
[Al3+]crit are set respectively to 25 µequivl−1(pH =

4.6) and 200 µequiv l−1[3,13,16].[Al]
[BC]crit is set to 1.2;
this value has been determined on soil solution sam-
ples from 17 sites of the RENECOFOR network [12].
The first method uses constant critical concen-
tration of protons and aluminium and is essentially
drainage-dependent. The second method takes into ac-
count the base-cation fluxes in soil solution, which
decrease Al toxicity. In the present study, to integrate
the ecosystem buffering capacity, the second method
is used at the national scale.
2.1.2. Atmospheric non-marine deposition (BC∗
dep)
Base-cation deposition, BC∗
dep (Eq. (11)), has been
determined at the national scale, using extrapolation of
the RENECOFOR network data [2] on a 10 ×10 km
grid. These deposition fluxes are determined from
long-term bulk concentration measurements and in-
terpolated long-term rainfall associated volumes. All
relevant data have been sea-salt corrected, on the ba-
sis of the ratio between base cations and sodium,
considering that Na deposition is 100% originating
from sea salts (Eq. (12)). Marine cation deposition is
not associated with an increase in buffering capacity,
since their deposition is always accompanied with the
strong acid anions Cl−and SO2−
4[4]. The concen-
tration ratios in seawater used for sea-salt correction
are: [Cl]
[Na]sea =1.80; [K]
[Na]sea =0.036; [Ca]
[Na]sea =0.038,
and [Mg]
[Na]sea =0.12 (gl−1/gl−1). Marine salinity is
35 g kg−1.
(11)
BC∗
dep =Ca∗
dep +K∗
dep +Mg∗
dep +Na∗
dep −Cl∗
dep
C∗
dep =Cdep −Nadep[C]
[Na]sea (12)with C =Ca,Mg,KorCl
For forest ecosystems, total (wet+dry) deposition
must be considered, because it represents a significant
higher input to soils than the bulk ‘open-field’ depo-
sition [16]. As a default value, throughfall deposition
is considered as an estimation of total (wet +dry) de-
position under forest cover. Since throughfall data are
not available at the national scale [21], a coefficient is
applied to the bulk deposition data from the RENECO-
FOR network, by calculating the ratio between bulk
and throughfall deposition on long-term investigated
sites [21]. This ratio depends on tree species and loca-
tion of the site.
2.1.3. Base-cation and nitrogen vegetation uptake
(BCuptake and Nuptake)
Base-cation and nitrogen uptake (N and BCuptake)
represent the net removal of elements by the vege-
tation (Eqs. (13) and (14)) on a long term. At the
steady state, it is dependent on forest management.Be-
cause annual forest uptake data was not available, the
method of the CCE guidance manual [6] was applied:
(13)BCuptake =Biomassgrowth [BC]biomass
(14)Nuptake =Biomassgrowth [N]biomass
Biomassgrowth is determined using data on regional
forest productivity and forest coverage [8]. [BC]biomass
and [N]biomass is the natural content of base cations in
the biomass, which depends on tree species [12,15].
2.1.4. Base-cation weathering rate (BCweath)
To approximate weathering fluxes, the PROFILE
model [20] was applied on French soil conditions.
It uses experimental kinetic laws to calculate min-
eral dissolution at the steady state, according to soil
properties, in a multi-layer soil. Soil mineral com-
position was determined with normative calculations
using the chemical composition of forest soils from
the Vosges Mountains, the Ardennes and 12 sites from
the RENECOFOR network, representative of the acid
sensitive area [12]. From these results, a relationship
between soil clay content and weathering rate was
determined for the 20–40 cm horizon of these soils
(Eq. (15))[12]:
(15)
BCweath =0.1666(AS20–40)(n=12,R
2=0.94)
S20–40 is the sum of the base cations (mequiv/100 g) in
this horizon and BCweath is the base-cation weathering
rate.
After calibration on 102 sites of the RENECO-
FOR network, weathering rates were thus extrapo-
lated using the French soil clay content at the national
scale [12].
2.1.5. Nitrogen immobilisation (Nimm)
Nitrogen immobilisation in the soils was set up
for plain and mountain areas, respectively to 150 and
300 equivha−1yr−1[15], regarding French environ-
mental conditions.

Table 1
Description of the five selected ecosystems. Input data and critical loads for S and N for the five selected ecosystems. All fluxes are in
equiv ha−1yr−1
Tableau 1
Description des cinq écosystèmes sélectionnés. Données d’entrée et charges critiques en S et N pour les cinq écosystèmes sélectionnés. Tous
les flux sont exprimés en equiv ha−1yr−1
Ecosystem No. 12 3 4 5
Location Massif Central Massif Central Paris Basin Vosges Mountains Landes
Bedrock Volcanic rocks Granite Tertiary sands Sandstone Eolian sand
Soil Andosol Dystric cambisol Podzoluvisol Podzol Podzol
Vegetation Fagus sylvatica
L. Quercus humilis
Miller Carpinus betulus
L. & Quercus
Fagus
sylvatica L. Pinus pinaster
Aiton
Q(myr
−1) 0.6 0.4 0.125 0.275 0.35
BC∗
dep 1011 1507 210 815 600
BCuptake 320 319 171 697 500
BCweath 2000 250 30 30 30
Ni300 150 150 150 150
Nuptake 346 139 152 755 423
BCdep +BCweath −BCuptake 2691 1438 69 148 130
– ANCle,crit [Al/BC] 4994 2688 155 335 321
CLmax(S) with ANCle,crit [Al/BC] 7685 4126 224 483 451
CLmax(N) with ANCle,crit [Al/BC] 8331 4415 526 1388 1024
2.2. Ecosystem classification and choice of five
representative sites
Existing ecosystem classifications, mainly based on
vegetation data, are not adapted to critical-load cal-
culation, which mostly depends on the soil buffering
capacity [12]. To estimate critical loads, a new ecosys-
tem classification, which combines soil, bedrock, and
vegetation data must be defined at the national scale.
The soil map, which represents 21 types of soils, was
combined with the parent material map, composed of
31 types of bedrock, using a Geographic Information
System [7,12]. Thirty-one combinations between soils
and bedrocks were distinguished. A potential vegeta-
tion map (representing the steady-state long term veg-
etation cover), which is composedof 18 types of vege-
tation [12], is then overlaid with the soil-bedrock map.
The resulting map is further combined with the current
land use map. Forest polygons or polygons with more
than 85% grassland, considered as ‘natural’ ecosys-
tems, are finally kept. This new classification consists
of 241 ecosystem types. The ‘natural ecosystem’ area
represents 179429 km2, i.e., 33% of the French terri-
tory. Among these types, in the present study,five dif-
ferent non-calcareous ecosystems were chosen to rep-
resent critical load calculation on French acid sensitive
areas (Table 1). Site 1 presents high weathering and
high non-marine base-cation atmospheric deposition.
Site 2 is characterized by high non-marine base-cation
deposition. Site 3 is characterized by low weather-
ing rate, low drainage and low non-marine base-cation
deposition. Site 4 exhibits low weathering rate, high
non-marine base-cation deposition and high vegeta-
tion uptake. Site 5 is characterized by low weathering
rate, high marine base-cation deposition, and high veg-
etation uptake.
3. Results and discussion
Results of the detailed calculation of critical loads
for the five selected sites are described in Table 1.
The critical loads for French forest and grassland
ecosystem classification are presented in Fig. 1.Ac-
cording to the guidance manual [6], five sensitiv-
ity classes are defined: 0–200 equiv ha−1yr−1; 200–
500 equivha−1yr−1; 500–1000 equiv ha−1yr−1;
1000–2000 equiv ha−1yr−1, and the less sensitive,
class 5, is >2000 equivha−1yr−1.

Fig. 1. Critical loads of sulphur for French forest and grassland ecosystems. Location of the five selected sites.
Fig. 1. Charges critiques de soufre pour les écosystèmes forestiers et prairiaux français. Localisation des cinq sites sélectionnés.
3.1. Weathering rates (BCweath,Eq.(7))
For French soils, BCweath ranges between 30 and
30 ×103equivha−1yr−1. For calcareous ecosystems
(46% of forests and grasslands), BCweath exceeds
2000 equivha−1yr−1, because of the high percent-
age of calcium carbonate in soils. With such a nat-
ural buffering capacity, these sites are not sensitive to
acidification. Volcanic soils developed on basalt (Ta-
ble 1, site 1), granodiorite, gabbro or diorite (2.5% of
the studied area) present also high weathering rates
(1500–2000 equivha−1yr−1). The presence of ap-
atite (>1%), of volcanic glasses, and the high per-
cent of anorthite (>25%) in the volcanic soils ex-
plain their efficient buffering capacity against acidi-
fication. The other ecosystems are more sensitive to
acidification: 51.5% of the French forests and grass-
lands, i.e., 92 405 km2. The most sensitive ecosystems
(BCweath <200 equivha−1yr−1) are located on acid
sands, Tertiary sands and sandstone (Table 1, sites 3,
4, and 5), with a high percentage of unweatheredmin-
erals (80–95% of quartz). BCweath ranges between 200
and 500 equivha−1yr−1on granite (Table 1, site 2),
schists or quartzites. High concentrations of orthose
(K-feldspar), Na or Ca plagioclases, and traces of
other minerals (such as apatite) slightly increase the
base-cation weathering rate. Detrital formations in the
central part of France present intermediate weathering
rate (500–1000 equivha−1yr−1). BCweath is the first
indicator of soil buffering capacity against acidifica-
tion in the long term.

3.2. Base-cation atmospheric deposition (BC∗
dep,
Eq. (7))
For French ecosystems, BC∗
dep ranges between
112 equivha−1yr−1and 2181 equivha−1yr−1.In
the hard acid sedimentary and metamorphic rocks of
the Alps, the granite of the Massif Central (Table 1,
site 2) and the Vosges Mountains (Table 1, site 4), at-
mospheric deposition represents more than 80% of the
base-cation input in the mass balance. In the Vosges
mountains, BC∗
dep (800–1500 equiv ha−1yr−1) mostly
originates long-range transport of emissions from the
nearby industrial areas in France and Germany. In
these areas, the increase of sulphur deposition is often
accompanied with an increase of base-cation deposi-
tion, which contributes to the soil buffering capacity
[17]. In the southern part of the Massif Central and
the Alp mountains, Ca is supposed to originate the
calcareous dust from the close Mediterranean ecosys-
tems and the high Ca concentrations in the Saharan
air masses [2]. In the central part of France (Table 1,
site 3), low atmospheric base-cation deposition does
not increase the buffering capacity of poor weathered
soils.
3.3. Vegetation uptake (BCuptake,Eq.(7))
BCuptake ranges between 67 and 1222 equiv ha−1·
yr−1.N
uptake ranges between 57 and 1324 equivha−1·
yr−1. High removal of base cations and nitrogen (Ta-
ble 1, site 4 and 5) has to do with the important bio-
mass productivity of beech forests (Fagus sylvatica L.)
in northeastern France and pine forests (Pinus pinaster
Aiton) in the coastal Atlantic area. At the national
scale, species associated with high BCuptake are Pinus
pinaster Aiton, Picea excelsa L. (for coniferous), Fa-
gus sylvatica L. and the Quercus genus (for deciduous
trees). On the opposite, grasslands, marshes, and other
non-forest areas exhibit low productivity. It is impor-
tant to notice that, in the long-term, the flux of element
uptake must only be considered for harvested forests.
On the other sites, at steady state, uptake by vegetation
tends towards zero, i.e. uptake theoretically equals de-
position by inner forest cycle. The influence of tree
species and forest management on ecosystem sensitiv-
ity to acidification is therefore not negligible on acid
sensitive sites.
3.4. French critical load map (Fig. 1)
The most sensitive areas to S deposition (CLmax(S)
<1000 equiv ha−1yr−1) are located in the Landes for-
est and marshes, schists of Brittany and Normandy,
detrital formations and Tertiary sands in Centre-North
of France, sandstone in the Vosges mountains and
granite in the Northeast of the Massif Central. The
areas where critical loads are rather low (1000 <
CLmax(S) <2000 equiv ha−1yr−1) are located in the
granitic region of the Massif Central and the Vos-
ges mountains, in the metamorphic bedrocks of the
Alps and the Pyrenees. The critical load of the nitro-
gen map [18] reflects the same sensitive areas, mainly
in the Landes forest, in Brittany, in the Centre-North
of France and in the Vosges Mountains. The Massif
Central, the Ardennes, and the Vosges Mountains are
less sensitive to acid deposition by comparison with
the previous maps of critical loads [13,14], because
the high non-marine atmospheric deposition of base
cations has been taken into account. On the other hand,
the maritime area is now more sensitive, because at-
mospheric inputs are mostly originating marine salts.
3.5. Exceedances
The current open-field deposition of sulphur
(SO42−) in France ranges between 200 and 750 equiv
ha−1yr−1[2]. As already explained, throughfall base-
cation deposition data were used for critical load cal-
culation. In the same way, to calculate the exceedances
of critical loads, throughfall S deposition data should
ideally be considered. Nevertheless, because these
data are lacking, no throughfall deposition map can
be used. Thus, as a first approximation,the difference
between current sulphur open-field deposition and
critical load of sulphur can be reasonably calculated.
The highest exceedances are estimated for the Atlantic
coast (Landes and Atlantic Pyrenees), the sandstones
in the Vosges Mountains, the schist of Brittany and
Normandy. Other areas, where exceedances may oc-
cur are granite in the North of the Massif Central
(Limousin and Beaujolais), and in the Vosges moun-
tains. However,one must keep in mind that these data
must be considered as the minima exceedances, since
the considered sulphur deposition is underestimated.

4. Conclusion
A new French ecosystem classification was set up
to calculate critical loads. It reflects the large diversity
of French soils, bedrock types, and vegetation. Two
hundred and forty-one ecosystem types have been de-
fined for the French territory.
Critical load values were calculated by taking into
account the base cations fluxes for critical limit calcu-
lation in leachate water. Weathering rate controls the
soil-buffering capacity against acidification. Soils with
high weathering rate, developed on calcareous or vol-
canic rocks, are protected against acidification. On the
other hand, poor weathered soils are the most sensi-
tive to acidification: sandstones in the Vosges Moun-
tains, aeolian sands in the Landes of Gascogne, acid
granite and schist in Brittany, Vendée, and Normandy,
detrital formations and Tertiary sands in the central
part of France. Nevertheless, one important result of
this study is that the high atmospheric deposition of
non-marine cations on sensitive areas increases the
ecosystem-buffering capacity. In the Vosges Moun-
tains, the crystalline part of the Alp Mountains and
the Southeast of the Massif Central, base-cation at-
mospheric deposition produces 80% of the total input
of base cations to the soil.
Nowadays, atmospheric deposition of sulphur is
decreasing, while nitrogen deposition is rather con-
stant. Areas where acid deposition exceeds or has ex-
ceeded critical load will begin to recover. Neverthe-
less, non-marine base-cation atmospheric deposition,
which originates pollution, will also reduce along with
the sites buffering capacity. On these sites, the recov-
ery time will thus depend on the ecosystem properties
(soil, bedrock, vegetation, climate), but also on the
atmospheric deposition trends. The next step of this
study will therefore be a dynamic modelling [18] to
predict recovery times for acidified ecosystems, tak-
ing all the fluxes into account.
Acknowledgements
The authors particularly thank the ADEME (‘Agen-
ce de l’environnement et de la maîtrise de l’énergie’,
France) for supporting our works and the RENECO-
FOR/ONF Staff for providing open-field atmospheric
deposition data.
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