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 community litter chemicals to N enrichment has been seldom quantitatively addressed.

Dr. HOU Shuangli and her colleagues from the research group of Ecological Stoichiometry in Institute of Applied Ecology carried out a long-term field N addition experiment in a semi-arid grassland with a wide range of N addition rates.

They hypothesized that species turnover and intraspecific trait variation effects should reinforce each other and the relative impacts of plant turnover versus intraspecific trait variation on changes in total ecosystem litter quality inputs would shift from low N deposition to high N deposition treatments.

They investigated the response of species composition to the increasing N addition rates and examined the concentrations of nitrogen, phosphorus, lignin, cellulose, hemicellulose and lignin:N in litters of each species from each plot. After that, they calculated the community means of chemical traits with either changed or unchanged in community composition. Furthermore, they quantified the relative contribution (RC) of changes in community composition to the total changes of community litter chemical characteristics in response to seven-year N addition.

They found that increasing N addition altered community composition by facilitating the dominance of one species with higher litter quality (higher N and lower lignin, cellulose and hemicellulose concentrations), and being more responsive to N addition at intraspecific level. Community litter N and C chemicals were more sensitive to N enrichment when changes of community composition were considered. Overall, changes in species composition, intraspecific changes and their interaction all led to higher litter quality under N deposition treatments. The relative contribution of species turnover 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.

This study demonstrated the positive impacts of increasing N deposition on litter quality through changing intraspecific N and C chemistry and species turnover, which has potential consequences for litter decomposition and nutrient cycling in ecosystems. Their results shed new light on the dual role of species turnover, both directly and in interaction with intraspecific responses, in mediating the ecosystem biogeochemical responses to N deposition.

This work has been published in Biogeochemistry as an article entitled ‘Quantifying the indirect effects of nitrogen deposition on grassland litter chemical traits’.