Soil detrital processes controlling the movement of 15N tracers to forest vegetation

Controlled field experiments to study the effects of heightened atmospheric inputs of nitrogen (N) to forests typically demonstrate that most N enters nonextractable pools in soil, while some N is taken up by vegetation, and varying amounts are exported. In a few experimental manipulations of N inputs to forests, N-15 has been added as a tracer to more closely study the fates and redistributions of NH4- and NO3- at the ecosystem level. We developed TRACE, a biogeochemical process model based on previous models, to interpret ecosystem-level N-15 field data following applications of N-15-enriched NO3- or NH4+ at the Harvard Forest, Massachusetts, USA. We simulated the forms, masses, atom%, and timing of N-15 applications in ambient and chronically fertilized plots over two growing seasons in coniferous and deciduous forest stands. Incorporating principles of stable-isotope redistributions, such as mass balance and pool dilution, into the process model provided a strong means of comparing alternative model formulations against field data. TRACE explicitly illustrated the manner in which rates of gross N turnover in soils could be high enough to provide strong sinks for N-15 in ambient plots, while limited enough to allow much greater uptake of N-15 by vegetation in fertilized plots. Ectorganic horizons, including litter and humified matter, were key in retaining N-15 inputs. We found that fine root uptake and turnover could not account for the rapid movement of N-15 into soil pools; direct assimilation into soil pools was required for both NH4- and NO3- in both deciduous and coniferous forests. Such high rates of N assimilation could not be accounted for by microbial biomass production using detrital C as the substrate. These findings have far-ranging implications for understanding the reciprocal effects of N deposition on forest C budgets, and forest C cycling on ecosystem N retention.