Long-term atmospheric nitrogen (N) deposition has been shown to reduce leaf litter and lignin decomposition in forest soils, leading to an accumulation of soil carbon. Reduced decomposition has been accompanied by altered structure and function of fungal communities, the primary decomposers in forest ecosystems; however, a mechanistic understanding of fungal responses to chronic N enrichment is lacking. A reduction in soil and litter manganese (Mn) concentrations under N enrichment (i.e., Mn limitation) may explain these observations, because Mn is a cofactor and regulator of lignin-decay enzymes produced by fungi. We conducted a six-month incubation experiment in which we applied Mn amendments to soils from the Harvard Forest Chronic Nitrogen Amendment Study (Petersham, MA, USA) to evaluate the role of Mn availability in decomposition dynamics of chronically N-enriched soils. After six months of incubation, we measured litter mass loss, the percent change in litter lignin, and the potential activities of lignin-decay enzymes (peroxidase and phenol oxidase). We also characterized fungal community composition by ITS2 metabarcoding of the litter fungal community.
Results/Conclusions
We found a significant positive correlation between Mn availability and lignin-decay enzyme activities in litter (P < 0.0001; R2 = 0.48). Using partial least squares regression (PLSR), we confirmed that Mn was the single main factor increasing the activities of lignin-decay enzymes, rather than a hidden or joint effect of other co-varying nutrients. In addition, we observed an increase in the relative abundance of fungi classified as ‘weak’ decomposers (e.g., yeasts) under long-term N enrichment, and a reversal of this response with Mn amendment. By reducing the relative abundance of ‘weak’ decomposers, Mn may promote fungal communities better adapted to decompose lignin. We conclude that Mn limitation is an important control on decomposition dynamics under long-term atmospheric N deposition, and may represent a mechanism that explains reduced decomposition and soil carbon accumulation under this global change factor.