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SOIL DETRITAL PROCESSES CONTROLLING THE MOVEMENT OF 15 N TRACERS TO FOREST VEGETATION
Author(s) -
Currie William S.,
Nadelhoffer Knute J.,
Aber John D.
Publication year - 1999
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/1051-0761(1999)009[0087:sdpctm]2.0.co;2
Subject(s) - biogeochemical cycle , environmental science , deciduous , ecosystem , plant litter , vegetation (pathology) , ecology , forest ecology , soil water , litter , soil science , biology , medicine , pathology
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, 15 N has been added as a tracer to more closely study the fates and redistributions of NH 4 + and NO 3 − at the ecosystem level. We developed TRACE, a biogeochemical process model based on previous models, to interpret ecosystem‐level 15 N field data following applications of 15 N‐enriched NO 3 − or NH 4 + at the Harvard Forest, Massachusetts, USA. We simulated the forms, masses, atom%, and timing of 15 N 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 15 N in ambient plots, while limited enough to allow much greater uptake of 15 N by vegetation in fertilized plots. Ectorganic horizons, including litter and humified matter, were key in retaining 15 N inputs. We found that fine root uptake and turnover could not account for the rapid movement of 15 N into soil pools; direct assimilation into soil pools was required for both NH 4 + and NO 3 − 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.