『Abstract
　In arctic and alpine ecosystems, soil nitrogen (N) dynamics can differ markedly between winter and summer months, and nitrogen losses can be measurable during the spring and fall transitions. To explore the effect of seasonality on biogeochemical processes in a temperate alpine environment, we used a combination of field incubations (year-round) and ¹⁵N tracer additions (late fall, early spring, summer) to characterize soil N dynamics in a wet and dry meadow in the Sierra Nevada, California. The snowmelt to early summer season marked a period of high ¹⁵N uptake and turnover in the two soils, coincident with the increase in microbial N pools at the start of snowmelt (wet and dry meadow); an increase in net N mineralization and net nitrification as snowmelt progressed (wet meadow only); and measurable net production of ¹⁵N-NH4⁺ in mid-summer (wet and dry meadow). Whereas fluctuations in microbial biomass were generally synchronous between the wet and dry meadow soils, only wet meadow soils appeared to mineralize N in response to declines in the microbial N pool. Net N mineralization and net nitrification rates in the dry meadow soil were negligible on all but one sampling date, in spite of periodic decreases in biomass of up to 60％. Across both sites, high ¹⁵N recoveries in microbial biomass N, rapid ¹⁵N-NH4⁺ turnover, and low or negative net ¹⁵N-NH4⁺ fluxes suggested tight cycling of N,particularly in the late fall and early spring.

Keywords: Alpine; Winter; Snowmelt; Seasonal transitions; Nitrification; Microbial N dynamics』

Introduction
Methods
　Study sites
　Net N mineralization and nitrification
　¹⁵N uptake
　¹⁵N turnover
　Trace gas flux
　Statistical analyses
Results
　Soil microclimate
　Microbial N dynamics and net N mineralization
　¹⁵N recovery and uptake rates
　¹⁵N turnover
　Trace gas flux
Discussion
　Overwinter N mineralization
　Spring transition - snowmelt
　Fall transition
　Nitrogen export
　Conclusions
Acknowledgments
References