Atmospheric nitrogen (N) deposition is one of the major drivers of global diversity loss, yet the mechanisms behind this decline remain controversial. N-enhanced litter accumulation has been suggested to be one of the causes of N-induced diversity loss. We tested mechanisms of N-addition-induced diversity change in a field experiment for 7 growing seasons with N and litter treatments. Using structural equation modeling (SEM) and nonmetric multidimensional scaling (NMDS), we examined the roles of competition, interspecific interactions, and litter accumulation in structuring the plant community.
Results/Conclusions
Nitrogen effects changed over time. Nitrogen addition changed species composition, reduced Shannon-Wiener diversity H' and evenness from the second to the fourth growing season but increased plant H' and evenness in the sixth and seventh growing seasons, while litter addition increased plant diversity indirectly in the third, fourth and seventh growing season and directly in the sixth growing season. The N-induced diversity loss resulted from increased litter accumulation and the interspecific competition between the dominant grass and other species. The recovery of plant diversity was attributed to the increased intraspecific competition of dominant species. Litter addition increased plant diversity through decreasing the abundance of dominant grass, because increased litter accumulation enhanced light interception, which reduced the number of buds and the density of the dominant grass. The negative feedback loop between the dominant grass and litter accumulation driven by N could maintain plant diversity in the long term. Thus, grassland management that affects litter accumulation such as grazing and mowing can change the pattern of eutrophication-driven diversity change and have important implications for grassland response to N deposition.