Long-term atmospheric nitrogen (N) deposition has been shown to reduce leaf litter and lignin decomposition in northern forest soils, leading to an accumulation of soil carbon (C). A concomitant loss of soil and litter manganese (Mn) following N deposition may explain this observation. Manganese (Mn) is a cofactor of critical lignin-decay enzymes produced by saprotrophic fungi, and its availability thereby influences decomposition rates. However, the extent of Mn loss across N addition experiments is poorly characterized; thus, it is unclear whether Mn limitation is a generalizable driver of reduced litter decay. Two studies were conducted to address this hypothesis: 1) a cross-site analysis of C stocks and Mn availability across three long-term simulated N deposition experiments in the northeastern U.S.; and 2) a Mn addition experiment to evaluate whether increasing Mn availability enhances decomposition of chronically N-amended litter. Soil and leaf litter were collected from the Harvard Forest Chronic Nitrogen Amendment Experiment (Petersham, MA), Cary Institute of Ecosystem Studies (Millbrook, NY) and Bear Brook Watershed in Maine (Beddington, ME). Samples were analyzed for total C and micro/macronutrient concentrations. Soil cores were incubated for three months, whereafter total litter mass loss and oxidative enzyme activities were evaluated.
Preliminary analyses of field data suggest that Mn loss and C accumulation may not be generalizable across all long-term N addition experiments. Following three months of incubation, decomposition was reduced with increasing N fertilization levels (i.e. leaf litter source) in samples from Harvard Forest, but not the Cary Institute (p < 0.001; p = 0.876, respectively). Decomposition rates and oxidative enzyme activity generally increased with increasing Mn availability; however, this trend was only significant for oxidative enzymes in samples from the Cary Institute (p = 0.0489). It has been suggested that litter decay rates are more strongly affected by Mn availability in late-stage decomposition; therefore, a scheduled six-month harvest of this experiment may elucidate remaining questions about the role of Mn in chronically N-fertilized systems.