Long-term movement of 15N tracers into fine woody debris under chronically elevated N inputs

William S. Currie, Knute J. Nadelhoffer, Benjamin Colman

    Research output: Contribution to journalArticlepeer-review

    Abstract

    Two key questions in the study of large-scale C (carbon) and N (nitrogen) cycling in temperate forests are how N cycling in soil detritus controls ecosystem-level retention of elevated N deposition, and whether elevated N deposition is likely to cause increases in C pools. The large C:N ratios in woody detritus make it a potentially important contributor to N retention, if N immobilization increases, and a potentially important contributor to C sequestration, if pool sizes increase. We studied N concentrations, C:N ratios, and pool sizes of N and biomass in fine woody debris (FWD < 5 cm diam.) 12 years into a long-term N-amendment study in two contrasting forests, a naturally-regenerated forest dominated by Quercus spp., and a 63-yr old plantation of Pinus resinosa. We also quantitatively recovered 15N tracers (originally applied as 15NH4 and 15NO3) in FWD, eight years following their application in the same study, in both ambient and N-amended plots. We used these data to test predictions of tracer redistributions made by a biogeochemical process model that included 15N. Results from the N pool-size analysis and the 15N tracer-recovery analysis indicated that under elevated N inputs of 5 g N m-2 yr-1 (as NH4NO3) over the decadal time period, only 0.15%-0.76% of the elevated N inputs were recovered in FWD of N-amended plots relative to ambient. Any increase in N immobilization in wood appeared to be minimal, in agreement with model predictions. Under N amendments, pool sizes of C in FWD were not significantly different from ambient, whereas pool sizes of N were marginally higher. Patterns of 15NH4 vs. 15NO3 recovery, treatment differences, and forest-type differences suggested that plant uptake, rather than detrital immobilization, was the dominant mechanism of 15N tracer movement into FWD. This result indicates that plant-soil cycling operating over a decadal time scale or longer controls C:N ratios and N pool sizes in woody debris.

    Original languageEnglish
    Pages (from-to)313-323
    Number of pages11
    JournalPlant and Soil
    Volume238
    Issue number2
    DOIs
    StatePublished - 2002

    Keywords

    • Immobilization
    • Nitrogen retention
    • Nutrient cycling
    • Tracer
    • Uptake
    • Woody debris

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