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Rapid cycling of organic nitrogen in taiga forest ecosystems

TitleRapid cycling of organic nitrogen in taiga forest ecosystems
Publication TypeJournal Article
Year of Publication2007
AuthorsKielland, K, McFarland, JW, Ruess, RW

We examined the dynamics of organic nitrogen (N)turnover in situ across a primary successional sequencein interior Alaska, USA, in an attempt tounderstand the magnitude of these fluxes in cold,seasonally frozen soils. Through a combination ofsoil extraction procedures and measurements of13C-enriched CO2 efflux from soils amended in thefield with 13C-labeled amino acids, we were able totrace the fate of this N form. Amino acid turnoverin situ at soil temperatures of 10C or below showthat amino acids represent a highly dynamic soil Npool with turnover times of approximately 3–6 h.The rapid turnover of free amino acids is associatedwith high soil proteolytic activity, which in turn istightly correlated with soil protein concentration.Moreover, these estimates of soil amino acid turnoverin the field correspond well with measurementsof amino acid turnover under equivalenttemperatures in the laboratory. The gross flux ofamino acid-N over the growing season greatly exceededthe annual vegetation N requirement, suggestingthat microbial biomass represent asignificant sink for this organic N. Depending onthe strength of this sink, N flow via free soil aminoacids can potentially account for the entire N demandof vegetation in the absence of net N mineralization.These relationships underscore theimportant biogeochemical role of labile DON fractionsin high-latitude forest ecosystems.