Evaluation of the petrogenic source of nitrogen in an alpine watershed

Declining snow cover duration and increasing summer temperature observed in recent decades have impacted key processes of the critical zone at high elevation such as rock weathering, soil microbial activity and primary productivity. For example, we observed a disproportionate greening (i.e. a long-term increase of vegetation cover) of sparsely vegetated, late snow melting sites that has yet to be explained. Factors locally alleviating nutrient limitation inherent to high elevation barren landscapes could drive spatially heterogenous soil development and greening, which calls for a thorough evaluation of nutrient budgets in those environments, with a focus here on nitrogen. Our study site is the “vallon de Roche Noire”, a highly instrumented watershed located near the Lautaret pass (2000 m a.s.l.), where atmospheric deposition and streamwater monitoring give us good overall constraints on the nitrogen budget, and where recent efforts to detect greenness changes at high spatial resolution (30m) have allowed us to identify sparsely vegetated, late snow melting sites as “hotspots of greening”. We use here a combination fieldwork, laboratory analyses and experimental incubations to evaluate the potential geogenic source of nitrogen and its contribution to the overall nitrogen budget of the watershed. In particuliar, we use the isotopic signature of the rock leachates, determined from the experimental incubations, and from atmospheric deposition to infer their respective contributions to the outlet.