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Seasonal partitioning of nitrogen by
plants and soil microorganisms in an alpine ecosystem
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Charles H. Jaeger III, Russel K. Monson,
Melany C. Fisk and Steven K. Schmidt
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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 15 N uptake. Microbial N pools
were low during the first half of the growing 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 mg N·g 21 resin · mo 21 ). Maximum N avail-ability
as indicated by net N mineralization rates occurred in August (0.54 g
N· m 22 ·mo 21 ). Plants took up 96.1%, and soil microorganisms
took up 3.9% of the 15 N recovered from 12-d field incubations of 15 NH4
1 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 freezethaw 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.
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Ecology (1999) Vol. 80 No. 6: 1883-1891
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