Plant community temporal stability in response to nitrogen addition among different degraded grasslands

Atmospheric nitrogen (N) deposition is generally believed to decrease plant community temporal stability. However, it remains unclear whether the responses of community temporal stability to N deposition vary with disturbance regimes, such as among different grasslands with degrees of degradation. We established a 4-year field experiment with six N addition levels on four grassland sites in northern China with varying degradation statuses (non-degraded, moderately, severely and extremely degraded grasslands). We examined the response of community temporal stability (quantified as the ratio of the mean of community biomass to its standard deviation) to N addition and important regulating factors. Asynchrony was calculated as the difference between one and synchrony (quantified as the ratio of the variance of community biomass to the square of the sum of the standard deviation of species biomass). The most interesting result we found was that community temporal stability of the moderately and severely degraded grasslands was relatively higher among the four grasslands without N addition. This was attributed to shifts in dominant species composition rather than species diversity. Community temporal stability of nearly all sites were not significantly affected by N addition except the moderately degraded grassland. Community temporal stability of the moderately degraded grassland responded to N addition non-linearly, being promoted by low N addition levels (10 and 20 g N m^-2 yr^-1) and decreased by high N addition levels (30-50 g N m^-2 yr^-1). Furthermore, community temporal stability of the non-degraded and moderately degraded grasslands was mainly driven by species asynchrony. Whereas, in the severely and extremely degraded grasslands, community temporal stability was mainly regulated by species richness and dominant species stability, respectively. These findings highlight the importance of grassland degradation in the response of community temporal stability to N deposition and provide scientific support for the management and restoration of degraded grasslands.