Anne Poszwa

A comparison between Ca and Sr cycling in forest ecosystems

In favourable conditions, the 87Sr/86Sr isotope ratios of the Sr delivered by rain and soil mineral weathering differ. Assuming that Ca and Sr behave similarly in forest ecosystems, several authors have used the 87Sr/86Sr variation in forest compartments to calculate the contribution of rain and mineral weathering to Ca fluxes and pools. However, there are a number of experimental reports showing that Ca and Sr may behave differently in the soil and in the plant. We have tested this Ca–Sr analogy in the field by measuring the variation of Sr and Ca concentrations, fluxes and pools in spruce, beech and maple stands on granite, sandstone and limestone. Results show that (1) variations of Ca and Sr concentrations are generally correlated at each level of the ecosystems. (2) In spruce on acid soils, a preferential uptake of Ca over Sr occurs (Aubure spruce Sr/Ca = 0.8×10−3; soil exchangeable Sr/Ca between 2 and 6×10−3). On calcareous soils, a preferential uptake of Sr over Ca by spruce may occur. (3) In spruce and beech on acid and calcareous soils, a preferential translocation of Ca over Sr from roots to leaves occurs ((Sr/Ca) in leaves was between 10 and 90% of that in roots). (4) The biological cycling of Ca and Sr leads to an enrichment of the upper soil layers in Ca and Sr. Compared to Sr, Ca accumulates in the upper layer of acid soils because Ca cycling through litterfall is favoured over Sr cycling, and possibly because of the selectivity of acid organic exchangers for Ca.

Do deep tree roots provide nutrients to the tropical rainforest

The contribution of deep tree roots to the nutrition of a tropicalrainforest were studied along an edaphic transect in French Guyana. Soil typeswere mapped in relation to the texture of the upper horizons and the depth ofoccurrence of the loamy saprolite. The position of mature individuals of fourcommon species, differing by they rooting depth, was identified and tree leaveswere analysed for major nutrients and strontium (Sr) isotopic ratios.On average, the range of leaf isotopic ratio (87Sr/86Sr= 0.714–0.716) was narrow compared to that of bulk soils(87Sr/86Sr = 0.72–0.77). Steep gradients ofincreasing 87Sr/86Sr in roots with soil depth were foundin all investigated profiles, which indicated that the flux of Sr deposited inrain and leached from the litter layer was tightly retained in the upper soillayers. Over the whole of the site, as well as within each soil unit, tree87Sr/86Sr ratios were very similar whatever the species,and close to litter and near-surface roots 87Sr/86Srratios, suggesting no or very little Sr contribution from deep tree roots.Variations of Ca and Sr concentrations in leaves were strongly correlated butnot with leaf 87Sr/86Sr ratios. These results support thetheory that Sr and Ca uptake and cycling are mostly superficial in tropicalrainforests.

Evidence of Current Soil Acidification in Spruce Stands in the Vosges Mountains, North-Eastern France

To demonstrate directly soil acidification under spruce stands in the Strengbach catchment, soils from two adjacent stands aged 40 and 90 years were sampled intensively in 1990 and resampled in 1996. Soils already were very acid in 1990. Between 1990 and 1996, soils had experienced a significant decrease in exchangeable Ca2+ and Mg2+ at all depths at the two sites except in the 0-10 cm layer, for which base saturation remained constant. Losses of Ca2+ calculated from a budget study at the same sites and from the comparison of exchangeable stores were similar. In contrast, the loss of Mg2+ from the exchange complex was higher than that computed from the field budget. Various reasons, including most probably uncertainties linked to the extremely low levels of Mg in the ecosystem, may explain this discrepancy. Since 1987, a general decrease of the (Ca2+ + Mg2+)/Al3+ ratio in soil solution was observed at both sites. These results are consistent with present day acidification of soils poor in weatherable minerals under spruce stands in the Vosges Mountains.

A Retrospective Isotopic Study of Spruce Decline in the Vosges Mountains (France)

The objective of this study was to assess the time variation of mineral and water stress levels across the life of a declining, Mg-deficient, spruce stand, in order to clarify the factors that caused the decline. Since 1985, strong soil acidification linked to a large leaching of nitrate and base cations was measured at the study site. In 1994, 5 trees were felled and tree rings were measured and analysed for Ca, Mg, K, Sr, 13C12C and 87Sr/86Sr isotopic ratios. Strontium pools and fluxes as well as root Sr isotope ratio in relation to depth were also measured. Wood chemical concentrations and isotope ratios were strongly related to the dominance status of each tree. On average during the study period, the 87Sr/86Sr ratio of spruce wood decreased. Using a mechanistic model computing long term variations of 87Sr/86Sr ratio in trees and soils, we reproduced the observed trend by simulating soil acidification – increasing Sr drainage from the whole profile, and particularly from the organic horizon –, and root uptake becoming more superficial with time. Between 1952 and 1976, tree ring Δ 13C decreased strongly and continuously, which, in addition to other factors, might be related to an increase in water stress. Thus, a decrease in rooting depth, possibly related to soil acidification, appeared as a possible cause for the long term increase in water stress. The extreme drought event of 1976 appears to have revealed and triggered the decline.

Long-Term Effects of High Nitrogen Loads on Cation and Carbon Riverine Export in Agricultural Catchments

The intensification of agriculture in recent decades has resulted in extremely high nitrogen inputs to ecosystems. One effect has been H(+) release through NH(4)(+) oxidation in soils, which increases rock weathering and leads to acidification processes such as base-cation leaching from the soil exchange complex. This study investigated the evolution of cation concentrations over the past 50 years in rivers from the Armorican crystalline shield (Brittany, western France). On a regional scale, acidification has resulted in increased base-cation riverine exports (Ca(2+), Mg(2+), Na(+), K(+)) correlated with the increased NO(3)(-) concentration. The estimated cation increase is 0.7 mmol(+)/L for Ca(2+) + Mg(2+) and 0.85 mmol(+)/L for total cations. According to mass balance, cation loss represents >30% of the base-cation exchange capacity of soils. Long-term acidification thus contributes to a decline in soil productivity. Estimates of the total organic nitrogen annually produced worldwide indicate that acidification may also constitute an additional carbon source in crystalline catchments if compensated by liming practices.

Variations of plant and soil 87Sr/86Sr along the slope of a tropical inselberg.

Abstract• From the summit downslope a granitic inselberg in French Guiana, soils and vegetation evolve from bare granite covered by cyanobacteria, to a savannah-type vegetation on thin patchy sandy accumulations, then to a low forest on shallow young soils and to a high forest on deep highly weathered ultisols.• We have used element budgets and Sr isotopic variations in soils and plants to investigate the mineral nutrient supply sources of the different plant communities.• Granite and atmospheric deposition have 87Sr/86Sr ratios of 1.3 and 0.71, respectively. The 87Sr/86Sr ratio of cyanobacteria (0.72) suggests granite weathering by cyanobacteria crusts. The 87Sr/86Sr ratio of the savannah-type vegetation is 0.73 and varies between 0.75 and 0.76 in the low and high forest leaf litter regardless of soil depth, age and degree of impoverishment.• These almost constant ratio suggest that forest Sr comes from rainwater and from the summit of this inselberg, where it is released and redistributed along the slope, by surface flow, lateral redistribution of litter, and mineral particles. However, because of its very low content in the rock and soils, Ca is supplied to plants by atmospheric deposition.Résumé• Du sommet vers la base d’un inselberg granitique (Nouragues, Guyane Française), les sols et la végétation évoluent depuis des savanes sur des ilots sableux entre les affleurements rocheux couverts de cyanobactéries, vers une forêt basse sur sols peu épais, riches en minéraux altérables puis une forêt haute sur sols très profonds et altérés.• Les variations isotopiques du strontium des sols et des plantes ont été mesurées pour rechercher les sources de nutriments des différentes communautés végétales.• Les rapports 87Sr/86Sr du granite et des dépôts atmosphériques sont respectivement de 1,3 et 0,71. Le rapport 87Sr/86Sr des cyanobacteries (0,72) suggère une libération de Sr par altération du granite. Le rapport 87Sr/86Sr de la savanne est de 0,73 et varie entre 0,75 et 0,76 dans les litières de forêt basse et haute, quelle que soit la profondeur, et la richesse en minéraux altérables des sols.• La faiblesse et l’homogénéité surprenante de ces rapports suggèrent une alimentation en Sr des forêts essentiellement à partir de dépôts atmosphériques et des sols de la partie haute de l’inselberg, via des écoulements de surface, des redistributions latérales de litière et de particules minerales lors de crises érosives. Cependant, en raison de l’extrême pauvreté de la roche et des sols en calcium, le Ca des communautés végétales provient de la pluie.

Isotopic approach to trace water and nutrient dynamics in forest soils

Forest soil fertility is generally estimated by studying the water and nutrient cycles essential to living organisms (i.e. biogeochemical cycles). Use of geochemical and/or isotopic tracers is an innovative, complementary approach to more traditional studies. Experimental procedures and results of some recent studies in forest ecosystems using natural tracers (e.g. 18O, 13C, 26Mg) or artificially supplied tracers (e.g. enrichment in Sr, Rb, 15N, 44Ca, 26Mg, 32P) are presented. Results are discussed in terms of relevance for using these tools to identify sources, to estimate residence times of water and elements, and to trace nutrient fluxes, of both organic and mineral origins, that are internal or external to the ecosystem.