Nitrogen deposition effects on soil organic matter chemistry are linked to variation in enzymes, ecosystems and size fractions

Recent research has dramatically advanced our understanding of soil organic matter chemistry and the role of N in some organic matter transformations, but the effects of N deposition on soil C dynamics remain difficult to anticipate. We examined soil organic matter chemistry and enzyme kinetics in three size fractions ([250 lm, 63–250 lm, and \63 lm) following 6 years of simulated atmospheric N deposition in two ecosystems with contrasting litter biochemistry (sugar maple, Acer saccharum—basswood, Tilia americana and black oak, Quercus velutina—white oak, Q. alba). Ambient and simulated (80-kg NO3 –N ha year) atmospheric N deposition were studied in three replicate stands in each ecosystem. We found striking, ecosystem-specific effects of N deposition on soil organic matter chemistry using pyrolysis gas chromatography/mass spectrometry. First, furfural, the dominant pyrolysis product of polysaccharides, was significantly decreased by simulated N deposition in the sugar maple–basswood ecosystem (15.9 vs. 5.0%) but was increased by N deposition in the black oak–white oak ecosystem (8.8 vs. 24.0%). Second, simulated atmospheric N deposition increased the ratio of total lignin derivatives to total polysaccharides in the [250 lm fraction of the sugar maple–basswood ecosystem from 0.9 to 3.3 but there were no changes in other size classes or in the black oak–white oak ecosystem. Third, simulated N deposition increased the ratio of lignin derivatives to N-bearing compounds in the 63–250 and [250 lm fractions in both ecosystems but not in the \63 lm fraction. Relationships between enzyme kinetics and organic matter chemistry were strongest in the particulate fractions ([63 lm) where there were multiple correlations between oxidative enzyme activities and concentrations of lignin derivatives and between glycanolytic enzyme activities and concentrations of carbohydrates. Within silt-clay fractions (\63 lm), these enzyme-substrate correlations were attenuated by interactions with particle surfaces. Our results demonstrate that variation in enzyme activity resulting from atmospheric N deposition is directly linked to Electronic supplementary material The online version of this article (doi:10.1007/s10533-008-9257-9) contains supplementary material, which is available to authorized users. A. S. Grandy (&) Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824, USA e-mail: [email protected] R. L. Sinsabaugh M. Stursova Department of Biology, University of New Mexico, Albuquerque, NM, USA J. C. Neff Department of Geological Sciences, University of Colorado, Boulder, CO, USA D. R. Zak School of Natural Resources & Environment and the Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI, USA 123 Biogeochemistry (2008) 91:37–49 DOI 10.1007/s10533-008-9257-9