Anticipated permafrost thaw in upcoming decades may exert significant impacts on forest soil nitrogen (N) dynamics. The rate of soil N mineralization (Nmin) plays a crucial role in determining soil N availability. Nevertheless, our understanding remains limited regarding how biotic and abiotic factors influence the Nmin of forest soil in response to permafrost thaw. In this study, we investigated the implications of permafrost thaw on Nmin within a hemiboreal forest based on a field investigation along the degree of permafrost thaw, having monitored permafrost conditions for eight years. The results indicate that permafrost thaw markedly decreased Nmin values. Furthermore, Nmin demonstrated positive associations with soil substrates (namely, soil organic carbon and soil total nitrogen), microbial biomass carbon and nitrogen, and soil moisture content. The decline in Nmin due to permafrost thaw was primarily attributed to the diminished quality and quantity of soil substrates rather than alterations in plant community composition. Collectively, our results underscore the pivotal role of soil substrate and microbial biomass in guiding forest soil N transformations in the face of climate-induced permafrost thaw.

The aim of this work was to assess the biogeochemical role of riparian soils in the High Arctic to determine to what extent these soils may act as sources or sinks of carbon (C) and nitrogen (N). To do so, we compared two riparian areas that varied in riparian vegetation coverage and soil physical perturbation (i.e., thermo-erosion gully) in NE Greenland (74 degrees N) during late summer. Microbial soil respiration (0.4-3.2 mu mol CO2 m(-2) s(-1)) was similar to values previously found across vegetation types in the same area and increased with higher temperatures, soil column depth and soil organic C degradation. Riparian soils had low nitrate concentrations (0.02-0.64 mu g N-NO3- g(-1)), negligible net nitrification rates and negative net N mineralization rates (-0.58 to 0.33 mu g N g(-1) day(-1)), thus indicating efficient microbial N uptake due to low N availability. We did not find any effects of physical perturbation on soil respiration or on N processing, but the dissolved fraction of organic matter in the soil was one order of magnitude lower on the disturbed site. Overall, our results suggest that riparian soils are small N sources to high-Arctic streams and that a depleted dissolved organic C pool in disturbed soils may decrease exports to the adjacent streams under climate change projection.