par Drouet, Thomas 
;Herbauts, Jacques ;Gruber, Wolf ;Demaiffe, Daniel
Référence Geoderma, 126, 3-4, page (203-223)
Publication Publié, 2005-06
;Herbauts, Jacques ;Gruber, Wolf ;Demaiffe, Daniel Référence Geoderma, 126, 3-4, page (203-223)
Publication Publié, 2005-06
Article révisé par les pairs
| Résumé : | The two main sources of Ca in forest ecosystems are mineral weathering release and atmospheric inputs. We use the 87Sr/86Sr isotopic ratio (Sr is used as a proxy for Ca) to determine the Ca contribution from atmospheric deposition input in two forest ecosystems (beech stands) growing on soils formed from parent materials with contrasting total Ca contents and isotopic ratios: Pleistocene loess in Central Belgium (acid leached soil) and Lower Devonian shales in the Ardenne Massif of High Belgium (ochreous brown earth). The 87Sr/86Sr ratio and the Ca and Sr contents were measured in bulk precipitation, in vegetation (beechwood growth rings and leaves) and in main soil horizons (total, acid-extractable and labile pools). The relative contributions of atmospheric input and soil mineral weathering to vegetation were calculated using mixing equations. Calculations based on Sr isotope ratios of bulk precipitation (end-member 1; 87Sr/ 86Sr close to ocean water: 0.7090), 0.1 M HCl-extractable soil fraction (end-member 2) and beech wood (mixing compartment) indicate that about 43% (Central Belgium) and 39% (Ardennes) of Sr uptake originates from atmospheric inputs. For Ca, the contributions of atmospheric input are 75% and 78%, respectively. These estimations are, however, very sensitive to the choice of the appropriate 87Sr/86Sr and Sr/Ca ratios for the weathering end-member. The isotopic composition of the mineral source is estimated from the soil mineralogical composition and the relative weatherability of the Sr-bearing minerals. Due to soil processes (mineral weathering, element recycling) and geomorphological events (addition of allochtonous minerals to the autochtonous pool inherited from the local bedrock), the distribution of minerals in both soil profiles is heterogeneous and varies from horizon to horizon. Which horizons are relevant and which soil pool (total soil, acid-extractable fraction or labile pool) should be selected for isotopic measurement of the weathering end-member, is therefore open to uncertainty. The choice of the appropriate 87Sr/86Sr and Sr/Ca ratios for the weathering end-member is discussed. Our results emphasize the importance of the atmospheric contribution of Ca for tree mineral nutrition in the studied forest stands, suggesting that these ecosystems on acid soils are sensitive to chemical changes in the atmospheric environment, e.g. acid depositions associated with decreasing input of atmospheric cations. Our results support therefore the conclusion of Hedin et al. [Hedin, L.O., Granat, L., Likens, G.E., Bulshand, T.A., Galloway, J.N., Butler, T., Rodhe, H., 1994. Steep declines in atmospheric base cations in regions of Europe and North America. Nature, 367: 351-354], suggesting that the atmospheric base cation trends in Europe and North America are ecologically relevant on the scale of decades for poorly buffered ecosystems. This is a serious possibility of depletion of the soil available cation pool of the High Belgium beech forest, which grows on a bedrock with extremely low levels of total calcium. © 2004 Elsevier B.V. All rights reserved. |
