Soil nitrogen dynamics during snow melt in moist acidic tussock tundra soils

Soil nitrogen (N) is an important limiting nutrient in Arctic ecosystems. Studying the partitioning and availability of N during spring snow melt—a crucial time at which soils thaw and the stage is set for plant growth—will improve our ability to predict global change effects on these ecosystems. Here we examine soil N partitioning and availability in a manipulative experiment in which the timing of snow melt is accelerated. Using soil cores extracted in water and K₂SO₄ as well as measurements of soil pore water in microlysimeters, we monitored total dissolved N (TDN), microbial biomass N (MBN), and three forms of labile N: NH₄⁺, NO₃^–, and total free amino acids (TFAA) over the course of spring snowmelt in moist acidic tussock tundra soils. We also used a plant-soil-microbe simulation model to predict the change in these variables during snow melt. Our results show a drop in microbial C:N ratio during soil thaw as well as drops in the TN:MBN and labile N:MBN ratio, suggesting that the microbial community assimilates a substantial amount of N from the soil solution at the time of soil thaw. Concurrent with microbial changes at the time of soil thaw, labile N pools (NH₄⁺, NO₃^–, and TFAA) drop from their highest recorded levels for the season to much lower levels. This drop in labile N availability was altered by the timing of snow melt, occurring earlier in accelerated snow melt plots. The drop in soil N and decline in microbial C:N were also present in our simulation model. Taken together, these results suggest that N mineralized during the winter becomes available in the soil solution when soils thaw and is then rapidly taken up by microbes and possibly plants as well. The timing of this N transfer process appears tightly linked to climatic conditions and thus may be shifted in time with future changes in climate seasonality.