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 mu m, 63-250 mu m, and < 63 mu m) 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 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 mu m 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 mu m fractions in both ecosystems but not in the < 63 mu m fraction. Relationships between enzyme kinetics and organic matter chemistry were strongest in the particulate fractions (> 63 mu m) 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 mu m), 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.