Litter and soil characteristics mediate the buffering effect of snow cover on litter decomposition

In cold biomes, snow cover mitigates the harsh winter soil conditions, thereby enhancing overwinter decomposition of organic matter which controls the availability of nutrients for plants and microbial uptake at the beginning of the growing season. Yet, how this buffering effect is modulated by litter traits, soil characteristics and herbivory remains poorly studied. We conducted a litter-bag experiment in four types of subalpine grasslands, two of which being naturally free of snow during most of the winter. Litter bags were filled either by grasses or forbs sampled in plots submitted the preceding summer to grasshopper grazing treatments. Snow cover strongly increased the decomposition of forbs, but not of grasses. Litter quality (low C:N and polyphenols triggering a priming effect) and soil ammonium content were correlated with litter decomposition rate in the presence of snow only, whereas soil organic matter content was positively associated with decomposition rate under both snow regimes. Herbivory did not affect decomposition. Our findings may be explained by the functional differences between copiotrophic and oligotrophic microbes, copiotrophs being more sensitive to harsh abiotic conditions than oligotrophs. As long as copiotrophs are favored by high litter quality and nutrient-rich soils (high ammonium content), litter decomposition is enhanced in the absence of snow only.