SEASONAL PARTITIONING OF NITROGEN BY PLANTS AND SOIL MICROORGANISMS IN AN ALPINE ECOSYSTEM

The seasonal dynamics of plant N assimilation and microbial N immobilization were studied in an alpine ecosystem to evaluate temporal patterns of plant and microbial N partitioning and the potential for plant vs. microbial competition 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 by 15N uptake. Microbial N pools were low during the first half of the growing season (9.5 g N/m2 on 1 June) and increased late in the season, from 11.4 g N/m2 on 1 August 1991 to 38.6 g N/m2 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 μg N·g−1 resin·mo−1). Maximum N availability as indicated by net N mineralization rates occurred in August (0.54 g N·m−2·mo−1). Plants took up 96.1%, and soil microorganisms took up 3.9% of the 15N recovered from 12-d field incubations of 15NH4+ 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 microbial competition for N may have been limited by: (1) constraints on microbial growth from the seasonal alpine freeze–thaw cycles, and (2) influences of roots on N cycling by soil microorganisms. The alternation between plant N uptake early in the season and microbial N uptake late in the season may enhance N retention in this N-limited ecosystem.