Microbial activity in Arctic tundra ecosystems continues through the winter and is an important component of the annual C budget. This activity is sensitive to climatic variation, particularly snow depth because that regulates soil temperature. The influence of winter conditions on soil N cycling is poorly understood. In this study, we used intact core incubations sampled periodically through the winter and following growing season to measure net N mineralization and nitrification in dry heath and in moist tussock tundra under ambient and experimentally increased snow depths (by use of a snowfence). In dry heath, we sampled soils under Dryas octopetela or Arctostaphylos alpine, while in tussock tundra, we sampled Eriophorum vaginatum tussocks and Sphagnum dominated areas between tussocks. Our objectives were to: (1) examine how different winter snow regimes influenced year-round N dynamics in the two tundra types, and (2) evaluate how these responses are affected by dominant species present in each system. In tussock tundra, soils with increased winter snow cover had high net N mineralization rates during the fall and winter, followed by immobilization during thaw. In contrast, N mineralization only occurred during the autumn in soils with ambient snow cover. During the growing season when N immobilization dominated in areas with ambient snow cover, soils with increased winter snow cover had positive net mineralization and nitrification rates. In dry heath tundra, soils with increased snow depth had high late winter net N mineralization rates, but these rates were: (a) comparable to early winter rates in soils under Arctostaphylos plants with ambient snow cover; (b) greater in soils under Arctostaphylos plants than in soils under Dryas plants; and (c) less than the rates found in tussock tundra. Our findings suggest under ambient snow conditions, low soil temperatures limit soil N mineralization, but that deeper snow conditions with the associated warmer winter soil temperatures dramatically increase over-winter N mineralization and thereby alter the amount and timing of plant-available N in tundra ecosystems. (C) 2003 Elsevier Ltd. All rights reserved.
