Seasonal partitioning of nitrogen by plants and soil microorganisms in an alpine ecosystem

The seasonal dynamics of plant N assimilation and microbial N immobilizatio n were studied in an alpine ecosystem to evaluate temporal patterns of plan t and microbial N partitioning and the potential for plant vs. microbial co mpetition for N. Plant N uptake was higher in the first half of the growing season than later in the season, as indicated by changes in biomass N and hv N-15 uptake Microbial N pools were low during the first half of the grow ing season (9.5 g N/m(2) on 1 June) and increased late in the season, from 11.4 g N/m(2) on 1 August 1991 to 38.6 g N/m(2) on 14 October 1991. Two different measures of N availability were highest in the midseason. Ion exchange resin bag N uptake was greatest in July (86.0 mu g . N . g(-1) re sin . mo(-1). Maximum N availability as indicated by net N mineralization r ates occurred in August (0.54 g N . m(-2) . mo(-1)). Plants took up 96.1% a nd soil microorganisms took up 3.9% of the N-15 recovered from 12-d field i ncubations of (NH4+)-N-15 in June; the corresponding percentages were 92.6% and 7.4% in August 1991. Thus, plants acquired N early in the season when they were actively growing, and the highest net microbial N immobilization occurred later in the season, after plant senescence. The potential for mic robial competition for N may have been limited by: (1) constraints on micro bial growth from the seasonal alpine freeze-thaw cycles, and (2) influences of roots on N cycling by soil microorganisms. The alternation between plan t N uptake early in the season and microbial N uptake late in the season ma y enhance N retention in this N-limited ecosystem.