Quantifying the indirect effects of nitrogen deposition on grassland litter chemical traits

Litter chemical traits are one of the dominant controls on litter decomposition. Increasing atmospheric nitrogen (N) deposition is expected to alter litter chemical traits at the community level in both direct (altering intraspecific chemistry) and indirect ways (changing species abundance and composition). Compared to intraspecific changes, the role of changes in species composition in driving the responses of litter chemical traits to N enrichment has been seldom quantitatively addressed. We quantified the relative contribution of intraspecific changes versus changes in community composition on litter traits and how this would be influenced by the magnitude of N deposition by taking advantage of a long-term field N addition experiment in a semi-arid grassland with a wide range of N addition rates. Nitrogen deposition altered plant species abundance by facilitating the dominance of one species with a nutrient acquisitive strategy, producing higher quality litter and being more responsive to N addition at the intraspecific level. Overall, changes in species composition, intraspecific changes and their interaction all led to higher litter quality (higher N and lower lignin, cellulose and hemicellulose concentrations) under N deposition treatments. The relative contribution of species composition on the responses of litter chemical traits to N deposition also increased with N addition rate, ranging from 5 to 40% for litter N, and from 2 to - 30% for the three structural carbon components. Our results demonstrate the positive impacts of increasing N deposition on litter quality through changing intraspecific C and N chemistry and species turnover, which has potential consequences for litter decomposition and nutrient cycling in ecosystems. Further, we highlight the important contribution of shifts in species abundance to the plant-mediated biogeochemical responses to N deposition