Plant community survey and soil nutrient measurement
In August 2013, a permanent quadrat of 1 m × 1 m approximately 1 m away from the edge of the plot was established in each plot. In early August from 2014 to 2018, when vegetation reached its peak biomass, plant species within each permanent quadrat were recorded. Species richness was defined as the total number of species recorded within the permanent quadrat each year. In mid-August from 2014 to 2018, all living plants within three randomly selected 0.4 m × 0.4 m quadrats in each of the 24 plots were clipped at the ground level and sorted by species, and oven-dried at 65°C for 48 h to determine aboveground biomass (AGB). Since nitrogen amendment-induced soil acidification (Tian et al., 2016) and increase in soil inorganic nitrogen (Xu et al., 2015b) may affect species diversity and asynchrony, we measured soil pH and inorganic nitrogen concentration in each plot. In mid-August each year, soil cores (10 cm in depth) were taken from five randomly selected locations in each plot and mixed as a composite sample. Soil pH was measured in a 1:2.5 (soil: water) suspension; soil inorganic nitrogen concentration was determined using a flow-injection autoanalyser (FIAstar 5000 Analyzer, Foss Tecator, Denmark), following extraction with solutions of 2 M KCl (Kaye & Hart, 1998).