In arctic and alpine ecosystems, soil nitrogen (N) dynamics differ markedly between winter and summer months: winter processes are often dominated by net N mineralization, while summer processes are dominated by net N immobilization.  However, in alpine environments where winter soil temperatures stay near 0 ºC, net N mineralization can be expected to occur year-round, and N losses can be measurable during the spring and fall transitions. We used a combination of field incubations (year-round) and ¹⁵N tracer additions (late fall; early spring; summer) to characterize seasonal N dynamics, and to measure short-term ¹⁵N uptake and turnover, in a wet and dry alpine meadow in the Sierra Nevada, California.  We were particularly interested in the fall and spring transitions because they have been shown to be periods of high activity elsewhere.

Results/Conclusions   Soil N processes in wet and dry meadow soils varied seasonally, with the greatest ¹⁵N uptake and turnover occurring during spring snowmelt and into the summer.  Microbial N pools declined 4- to 10-fold in the late fall at both sites but recovered by the start of snowmelt, in the late spring.  Overwinter N mineralization and nitrification rates were low in the wet meadow soil, but accounted for 60-70% of annual net N production when the spring snowmelt period was included.  Net N mineralization and net nitrification remained measurable until late fall, when wet meadow soils shifted to immobilization.  In contrast, net N mineralization and net nitrification rates in the dry meadow soil were negligible on all but one sampling date. Rapid ¹⁵N-NH₄⁺ uptake by microbes appeared to result in high N retention at both sites on all sampling dates.  Plant uptake of ¹⁵N-NH₄⁺ was likewise consistent across sampling dates, accounting for approximately 10-15% of total ¹⁵N recovery at the wet meadow site, and 5-10% at the dry meadow site.  Seasonal variation in ¹⁵N turnover was evident at both sites, with net production of ¹⁵N-NH₄⁺ measured only in mid-summer.  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.