『Abstract
　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 (＞250μm, 63-250μm, and ＜63μm) 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^-1 year^-1) atmospheric N deposition were studied in three replicate stands in each ecosystem. We found striking, ecosystem-specific 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 ＞250μm 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 ＞250μm fractions in both ecosystems but not in the ＜63μm fraction. Relationships between enzyme kinetics and organic matter chemistry were strongest in the particulate fractions (＞63μm) 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 (＜63μm), 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 changes in soil organic matter chemistry, particularly those that occur within coarse soil size fractions.

Keywords: Nitrogen deposition; Enzymes; Carbon structure; Pyrolysis gas chromatography/mass spectrometry; Soil organic matter』

Introduction
Methods
　Experimental site
　Soil fractionation
　SOM characterization
　Enzyme analyses
　Data analysis
Results
　SOM concentration and distribution
　SOM composition
　Enzyme kinetics and relationships to SOM
Discussion
　N deposition and SOM chemistry: a conceptual model
Summary and conclusions
Acknowledgments
References