Anthropogenic N Deposition Increases Soil C Storage by Decreasing the Extent of Litter Decay: Analysis of Field Observations with an Ecosystem Model |
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Authors: | Kyle A Whittinghill William S Currie Donald R Zak Andrew J Burton Kurt S Pregitzer |
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Institution: | (1) School of Natural Resources and Environment, University of Michigan, Ann Arbor, Michigan 48109, USA;(2) School of Forest Resources and Environmental Sciences, Michigan Technological University, Houghton, Michigan 49931, USA;(3) College of Natural Resources, University of Idaho, Moscow, Idaho 83844, USA;(4) Earth Systems Research Center, Institute for Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824, USA |
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Abstract: | Recent meta-analyses of experimental studies simulating increased anthropogenic nitrogen (N) deposition in forests reveal
greater soil carbon (C) storage under elevated levels of atmospheric N deposition. However, these effects have not yet been
included in ecosystem-scale models of soil C and N cycling and it is unclear whether increased soil C storage results from
slower decomposition rates or a reduced extent of decomposition (for example, an increase in the amount of litter entering
slowly decaying humus pools). To test these alternatives, we conducted a meta-analysis of litter decomposition data. We then
used the results from our meta-analysis to model C and N cycling in four sugar maple forests in Michigan using an ecosystem
process model (TRACE). We compared model results testing our alternative hypotheses to field data on soil C storage from a
17-year N deposition experiment. Using data from published litter decomposition studies in forests, we determined that, on
average, exogenous N inputs decreased lignin decomposition rates by 30% and increased cellulose decomposition by 9%. In the
same set of litter decomposition studies increased exogenous N availability increased the amount of litter entering slowly
decaying humus pools in a manner significantly related to the lignocellulose index of decaying litter. Incorporating changes
to decomposition rates in TRACE did not accurately reproduce greater soil C storage observed in our field study with experimentally
elevated N deposition. However, when changes in the extent of decomposition were incorporated in TRACE, the model produced
increased soil C storage by increasing the amount of litter entering the humus pool and accurately represented C storage in
plant and soil pools under experimental N deposition. Our modeling results and meta-analysis indicate that the extent of litter
decay as humus is formed, rather than slower rates of litter decay, is likely responsible for the accumulation of organic
matter, and hence soil C storage, under experimental N deposition. This effect should be incorporated in regional to global-scale
models simulating the C balance of forest ecosystems in regions receiving elevated N deposition. |
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