Nitrogen addition increases carbon storage in soils, but not in trees, in an eastern U.S. deciduous forest

Background/Question/Methods

Forests ecosystems in most industrialized and agricultural regions receive elevated rates of atmospheric nitrogen (N) deposition from air pollution, and there has been a lively debate in the scientific literature concerning the extent to which this elevated N deposition enhances carbon (C)  storage in forests.  To evaluate the effects of excess N deposition on C and N cycling, we experimentally added N (as NH₄NO₃) to naturally-occurring, single-species plots of five different tree species that are common in the Northern Hardwood forests of northeastern North America: sugar maple (Acer saccharum Marsh), American beech (Fagus grandifolia Ehrh.), yellow birch (Betula alleghaniensis Britton), eastern hemlock (Tsuga canadensis (L.) Carr) and northern red oak (Quercus rubra L.).   The experiment was performed in the Catskill Mountains of southeastern New York State, USA, and used a paired-plot design with six replicate plots per species.

Results/Conclusions

After six years of treatment, most species showed only minor increases in foliar N concentrations in N-treated plots. No significant effects of the N treatment were observed on woody biomass increment or aboveground net primary production (ANPP) for any species.   In the oak plots, the N treatment increased acorn production in mast years.  The N treatment caused a significant increase in C stock, N stock and C:N ratio in the forest floor, with the largest effect in the hemlock plots.   Nitrate leaching increased significantly in treated plots compared to controls.   These results suggest that the ANPP of these forests is not limited by N availability, but that excess N may cause accumulations of C in the forest floor, particularly in hemlock stands, perhaps through inhibition of decomposition rates or by altering phenolic chemistry of the litter.   The magnitude, and sometimes the direction, of the N treatment responses varied among species, suggesting that predictions of forest responses to elevated N deposition should take into account spatial and temporal variation in tree species composition.