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).