Effects of the frequency and the rate of N enrichment on community structure in a temperate grassland

Aims Nitrogen (N) enrichment caused by human activities threatens biodiversity and alters plant community composition and structure. It has been found that heavy and infrequent N inputs may overestimate species extinction, but it remains unclear whether plant community structure will equally respond to frequent reactive N enriched conditions. Methods We independently manipulated the rates and the frequencies of N addition in a temperate steppe, northern China, between 2008 and 2013. Important Findings We found that plant community structure changes, measured by 'Euclidean distance' involving species richness, composition and productivity, were significantly positively related to increasing N enrichment rates rather than frequencies. Changes in aboveground net primary productivity (ANPP), plant species richness and shifts in dominant species were observed. Community ANPP increased with N enrichment, whereas species richness reduced. The frequency of N enrichment increased species richness but had no impacts on community ANPP and the relative ANPP of the two dominant species, C-3 perennial bunchgrass Stipa grandis and C-3 perennial rhizome grass Leymus chinensis. The ANPP and relative ANPP of the two dominant species were significantly negatively correlated with each other. Moreover, changes in the relative ANPP of S. grandis was negatively associated with the changes in community structure. After 5 years' treatment, direct influence of the frequency of N enrichment on plant community structure was not observed, but the effects of the rate of N enrichment were apparent. Our results suggested that further study in various ecosystems and with long-term and well-controlled comparisons the frequency vs. the rate of N enrichment may still be needed