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Long-Term Simulated Atmospheric Nitrogen Deposition Alters Leaf and Fine Root Decomposition
Citation:
Xia, M., A. Talhelm, AND K. Pregitzer. Long-Term Simulated Atmospheric Nitrogen Deposition Alters Leaf and Fine Root Decomposition. ECOSYSTEMS. Springer, New York, NY, , 1-14, (2017).
Impact/Purpose:
This manuscript describes an experiment testing whether a simulated nitrogen deposition treatment had differential effects on the decomposition of leaf litter versus fine roots in northern hardwood forests in Michigan. The authors observed that N additions resulted in slower decomposition of fine roots. This finding is novel because fine root decomposition is not well studied and the effects of nitrogen deposition on fine root decomposition have not often been tested. If this finding is integrated into carbon cycle models, estimates of the amount of forest carbon sequestration caused by nitrogen deposition may increase.
Description:
Atmospheric nitrogen deposition has been suggested to increase forest carbon sequestration across much of the Northern Hemisphere; slower organic matter decomposition could contribute to this increase. At four sugar maple (Acer saccharum)-dominated northern hardwood forests, we previously observed that 10 years of chronic simulated nitrogen deposition (30 kg N ha-1 yr-1) increased soil organic carbon. Over three years at these sites, we investigated the effects of nitrogen additions on decomposition of two substrates with documented differences in biochemistry: leaf litter (more labile) and fine roots (more recalcitrant). Further, we combined decomposition rates with annual leaf and fine root litter production to estimate how nitrogen additions altered the accumulation of soil organic matter. Nitrogen additions marginally stimulated early-stage decomposition of leaf litter, a substrate with little acid-insoluble material (e.g., lignin).
In contrast, nitrogen additions inhibited the late stage decomposition of fine roots, a substrate with high amount of acid insoluble material and a change consistent with observed decreases in lignin-degrading enzyme activities with nitrogen additions at these sites. At the ecosystem scale, the slower fine root decomposition led to additional root mass retention (g m-2), which explained 5, 48, and 52 % of previously-documented soil carbon accumulation due to nitrogen additions. Our results demonstrated that nitrogen deposition had contrasting effects on leaf litter and fine root decomposition because of differences in litter chemistry. Although previous nitrogen deposition studies have focused on leaf litter, this work suggests that slower fine root decomposition is a major driver of soil carbon accumulation with nitrogen deposition.