Human activity has increased the amount of N entering terrestrial ecosystems from atmospheric NO3- deposition. High levels of inorganic N are known to suppress the expression of phenol oxidase, an important lignin-degrading enzyme produced by white-rot fungi. We hypothesized that chronic NO3- additions would decrease the flow of C through the heterotrophic soil food web by inhibiting phenol oxidase and the depolymerization of lignocellulose. This would likely reduce the availability of C from lignocellulose for metabolism by the microbial community. We tested this hypothesis in a mature northern hardwood forest in northern Michigan, which has received experimental atmospheric N deposition (30 kg NO3--N ha(-1) y(-1)) for nine years. In a laboratory study, we amended soils with C-13-labeled vanillin, a monophenolic product of lignin depolymerization, and C-13-labeled cellobiose, a disaccharide product of cellulose degradation. We then traced the flow of C-13 through the microbial community and into soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial respiration. We simultaneously measured the activity of enzymes responsible for lignin (phenol oxidase and peroxidase) and cellobiose (beta-glucosidase) degradation. Nitrogen deposition reduced phenol oxidase activity by 83% and peroxidase activity by 74% when compared to control soils. In addition, soil C increased by 76%, whereas microbial biomass decreased by 68% in NO3- amended soils. C-13 cellobiose in bacterial or fungal PLFAs was unaffected by NO3- deposition; however, the incorporation of C-13 vanillin in fungal PLFAs extracted from NO3- amended soil was 82% higher than in the control treatment. The recovery of C-13 vanillin and C-13 cellobiose in SOC, DOC, microbial biomass, and respiration was not different between control and NO3- amended treatments. Chronic NO3- deposition has stemmed the flow of C through the heterotrophic soil food web by inhibiting the activity of ligninolytic enzymes, but it increased the assimilation of vanillin into fungal PLFAs. (C) 2004 Elsevier Ltd. All rights reserved.
