Plant litter decomposition represents one of the largest terrestrial carbon (C) fluxes and an important link between ecosystem C and nitrogen (N) cycles. The activity of litter-decomposing microbes is sensitive to N availability, and much work has investigated the effect of anthropogenic N fertilization on the decomposition process. Previous efforts to synthesize decomposition responses to N across experiments, however, assume litter mass is lost according to first-order kinetics throughout decomposition. This approach may obscure potentially opposing effects of N on different stages of the litter decomposition process. In this analysis, we compared the fit of single, double, and asymptotic exponential, as well as Weibull decomposition models to litter decay series generated from globally distributed N fertilization experiments and evaluated the influence of N enrichment on the shape of the litter decomposition curve. We further used the higher-order models to separately evaluate the effect of N addition on early vs. late stages of decomposition and identify the conditions that explain variation in the N fertilization response.
Results/Conclusions
Our dataset included 334 paired (control and N-fertilized) decomposition sequences from 53 published papers (548 unique decomposition curves). Studies were globally distributed but concentrated in the northern hemisphere, particularly North America and Asia. N fertilization imparted divergent effects on early vs. late-stage decomposition, that depended on litter and soil chemical characteristics. These findings contrast those of previous syntheses that assumed a constant rate of substrate loss throughout decomposition and found limited effects of N fertilization on the decomposition process. Specifically, N fertilization increased the initial rate of litter decomposition, but also increased the fraction of litter that decomposed slowly at late stages. Fertilization more strongly enhanced early-stage decomposition in substrates with high initial calcium (Ca) concentrations and more strongly slowed late-stage decomposition in substrates with high initial lignin concentrations. Fertilization also more strongly inhibited late-stage decay in substrates with early N stimulation, adding further evidence that microbial necromass production in early-stage decomposition is an important conduit for the formation of slow-cycling organic matter. This analysis provides an important link in our understanding of N effects on the interface between litter decomposition and soil C formation and facilitates efforts to represent coupled C and N cycles in biogeochemical models.