The effects of snow-N deposition and snowmelt dynamics on soil-N cycling in marginal terraced grasslands in the French Alps
Résumé
Atmospheric nitrogen (N) deposition increasingly impacts remote ecosystems. At high altitudes, snow is a key carrier of water and nutrients from the atmosphere to the soil. Medium-sized subalpine grassland terraces are characteristic of agricultural landscapes in the French Alps and influence spatial and temporal snow pack variables. At the Lautaret Pass, we investigated snow and soil characteristics along mesotopographic gradients across the terraces before and during snowmelt. Total N concentrations in the snowpack did not vary spatially and were dominated by organic N forms either brought by dry deposition trapped by the snow, or due to snow-microbial immobilization and turnover. As expected, snowpack depth, total N deposited with snow and snowmelt followed the terrace toposequence; more snow-N accumulated towards the bank over longer periods. However, direct effects of snow-N on soil-N cycling seem unlikely since the amount of nitrogen released into the soil from the snowpack was very small relative to soil-N pools and N mineralization rates. Nevertheless, some snow-N reached the soil at thaw where it underwent biotic and abiotic processes. In situ soil-N mineralization rates did not vary along the terrace toposequence but soil-N cycling was indirectly affected by the snowpack. Indeed, N mineralization responded to the snowmelt dynamic via induced temporal changes in soil characteristics (i.e. moisture and T°) which cascaded down to affect N-related microbial activities and soil pH. Soil-NH4 and DON accumulated towards the bank during snowmelt while soil-NO3 followed a pulse-release pattern. At the end of the snowmelt season, organic substrate limitation might be accountable for the decrease in N mineralization in general, and in NH4 + production in particular. Possibly, during snowmelt, other biotic or abiotic processes (nitrification, denitrification, plant uptake, leaching) were involved in the transformation and transfer of snow and soil-N pools. Finally, subalpine soils at the Lautaret Pass during snowmelt experienced strong biotic and abiotic changes and switched between a source and a sink of N.