PARENT SESSION
Oral Session #15: Ecosystem Ecology. Presiding: J. Jenkins.
Monday, August 6, 2001. 1:00 PM to 4:45 PM. Hall of Ideas F.

The role of organic N in the nitrogen economy of floodplain balsam poplar stands in interior Alaska.

McFarland, Jack¹, Ruess, Roger ¹, Kielland, Knut ¹, Doyle, Allen ², ^{1 2}

ABSTRACT- Here we present the results from the first in a series of experiments designed to quantify rates of organic vs. inorganic N cycling and uptake by vegetation across a range of temperate and boreal forest ecosystems. Using a multiple stable isotope (¹³C and ¹⁵N) design, that allowed us to simultaneously assess root uptake of ammonium and glycine, we evaluated the contribution of organic N to the nitrogen economy of vegetation in a deciduous boreal forest characterized as predominantly ectomycorrhizal. Specifically, we investigated the compartmentalization of inorganic and organic nitrogen within the soil and evaluated the competition for these N sources between plants and microorganisms. Approximately 60-70% of the ¹⁵N applied to soils in our experiment was retained within the soil pool. Soil microorganisms represented the largest sink for ¹⁵N , ranging from 12 to 64% recovery depending on the treatment and sampling period. In contrast, total recovery of ¹⁵N in fine root biomass was small, averaging 1.39 and 1.64% after two weeks for glycine and NH₄⁺ respectively. Our results fall short of identifying the ultimate fate of these N sources in the long-term competition between plants and microbes for nitrogen. However, we present strong evidence that plants in this boreal forest ecosystem, assimilate simple organic N substrates with the same efficiency as they do inorganic N. Moreover, we contribute to a growing consensus that soil microorganisms play an important role in N cycling processes both as mediators of N availability to plants and as regulators for ecosystem N retention

KEY WORDS: organic N, Populus balsamifera, Alaska, ¹⁵N