The effects of snow-N deposition and snowmelt dynamics on soil-N cycling in marginal terraced grasslands in the French Alps

Abstract

International audienceAtmospheric 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

Nutrient and mercury deposition and storage in an alpine snowpack of the Sierra Nevada, USA

“…5). Two possible sources could cause higher concentrations of organic N in snowpack compared to wet deposition: snowpack microbial conversion of inorganic N to organic N and dry deposition of organic N during stormfree periods (Clement et al, 2012;Jones, 1999;Williams et al, 2001). Our measurements do not allow for differentiation between the two possible sources of snowpack organic N; however snow pit profile sampling shows coinciding decreases in inorganic N and increases in organic N with snow pit depth and therefore age (Fig.…”

Effect of altered snow conditions on decomposition in three subalpine plant communities

“…A denser and thicker snow layer inside a ski slope can be considered a source of more water relative to the natural surroundings. Hence, more intensive mineralnutrient leaching from litter and superficial runoff is expected during melting time [16]. This implies an increasing shortage of available nutrients for microbes during snow melt.…”

“…While alpine ecosystems are characterised by nutrient-poor soils [57], the seasonal release of meltwater and nutrients, especially nitrogen, along with drastic soil temperature changes may influence the nutrient rates of underlying soils and soil processes [16]. However, the effect of nitrogen cumulated by snow cover on soil nitrogen cycling and decomposition is still ambiguous [11,16]. Future research, including microbial assessment, nutrient cycling tracing, and quantification of runoff and leaching, is necessary to clarify these processes during and after the melting period.…”

“…Surprisingly, only a limited number of papers have focused on the effects of anthropogenic impact on snow cover and decomposition in mountain environments [16][17][18]. The straightforward evidence of the influence of artificially altered snow conditions on mountain ecosystems comes from studies on ski slopes because ski slope operations unequivocally change snow characteristics [19][20][21].…”

Climate change impacts on the Alpine ecosystem: an overview with focus on the soil

“…The role of snow in the climate system includes strong positive feedbacks related to albedo and other weaker feedbacks related to moisture storage, latent heat and insulation of the underlying surface (Déry, Brown 2007;Qingbai et al 2015). The lack of snow cover reduces the degree of insulation and results in cold soil temperatures, extensive soil freezing and increase in freeze/ thaw cycles, influencing soil microbiological activity and SOM (Soil Organic Matter) mineralization (Edwards, Cresser 1992;Groffman et al 2001;Clement et al 2012;Welc et al 2014). Glaciers are among the most important elements of the cryosphere and many works have been written in the last years which studied glacier variations in relation to global warming (Le Roy et al 2015).…”