Biogeochemistry of a temperate forest nitrogen gradient

Background/Question/Methods

Wide natural gradients of soil nitrogen (N) can be used to examine fundamental relationships between plant-soil-microbial N cycling and hydrologic N loss in forest ecosystems.  Such natural soil N gradients can also be used to test the applicability of N saturation theory as a general framework for understanding ecosystem N dynamics in both polluted and unpolluted forests.  We measured plant production, N uptake and return in litterfall, soil gross and net N mineralization rates, and hydrologic N losses of nine Douglas-fir forests growing across an exceptionally wide soil N gradient in the Oregon Coast Range. 

Results/Conclusions

Surface mineral soil N (0 - 10 cm) ranged nearly 3-fold from 0.29 - 0.78 %N, and in contrast to predictions of N saturation theory, was linearly related to 10-fold variation in net N mineralization from 8 - 82 kg N ha^-1yr^-1.  Net N mineralization was unrelated to soil C:N, soil texture, precipitation and temperature differences among sites.  Net nitrification was negatively related to soil pH, and accounted for < 20% of net N mineralization at low N sites, increasing to 85 - 100% of net N mineralization at intermediate and high N sites.  The ratio of net:gross N mineralization and nitrification increased along the gradient, indicating progressive saturation of microbial N demands at high soil N.  Aboveground N uptake by plants increased asymptotically with net N mineralization to a peak of ~ 35 kg N ha^-1yr^-1.  Aboveground net primary production per unit net N mineralization varied inversely with soil N, suggesting progressive saturation of plant N demands at high soil N.  Hydrologic N losses were dominated by dissolved organic N at low N sites, with increased nitrate loss causing a shift to dominance by nitrate at high N sites, particularly where net nitrification exceeded plant N demands.  With the exception of N mineralization patterns, our results broadly support the application of the N saturation model developed from studies of anthropogenic N deposition to understand N cycling and saturation of plant and microbial sinks along natural soil N gradients.  This convergence of behavior in unpolluted and polluted forest N cycles suggests that where future reductions in deposition to polluted sites do occur, symptoms of N saturation are most likely to persist where soil N content remains elevated.