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

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 N-15 tracers (originally applied as (NH4)-N-15 and (NO3)-N-15) 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 N-15. Results from the N pool-size analysis and the N-15 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 (NH4)-N-15 vs. (NO3)-N-15 recovery, treatment differences, and forest-type differences suggested that plant uptake, rather than detrital immobilization, was the dominant mechanism of N-15 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.