Increasing nitrogen addition rates suppressed long-term litter decomposition in a temperate meadow steppe

Plant litter decomposition is critical for the carbon (C) balance and nutrient turnover in terrestrial ecosystems and sensitive to the ongoing anthropogenic nitrogen (N) input. Previous studies evaluating the N effect on litter decomposition relied mostly on short-term experiments (< 2 years), which probably masked the real N effect on litter decomposition. Therefore, long-lasting experiments are imperative for the overall evaluation of the litter decomposition dynamics under N enrichment. We conducted a long-term (4-year) N addition experiment with N levels from 0 – 50 g N m -2 yr -1 to examine the potential abiotic and biotic factors in regulating the decomposition process of litterfall from the dominant species Leymus chinensis. The long-term experiment exhibited a consistent decrease of decomposition rate with increasing N addition rates, providing strong evidence showing the inhibitory effect of N addition on decomposition. The N-induced alterations in soil environment (acidification and nutrient stoichiometry), microbial activity (microbial biomass and enzyme activity), changes of litter quality (residual lignin and nutrient content) and plant community (aboveground productivity and species richness) jointly contributed to the lowered decomposition. During the whole decomposition process, the changes of litter quality, including accumulation of lignin and the concentrations of nutrient, were mainly driven by the soil environment and microbial activity in this N-enriched environment. The findings help clarify how increasing N input rates affect long-term litter decomposition, and improve the mechanistic understanding of the linkages between ecosystem N enrichment and C cycling.