Figure
3 (a) Trends of difference in species-richness distribution for plot
group of the total (blue), mowing (green), N-addition (orange), and
frequency of N fertilization (red) are distributed, respectively. The
difference between years was measured by Lloyd’s ratio from the variance
and mean on species richness. Under mowing treatments, the network
complexity changed with logarithm of N-addition rate (b). The solid dot
in red and blue represent coordinate of network complexity and
degeneracy under the same nitrogen (\(\log_{10}(N)\)) with mowing (red)
and no-mowing (blue). The solid lines represent linear least square
regression lines. With the increase of N addition rates, complexity
decreased significantly, but the results treated with mowing are always
above that with no-mowing. Note: **<0.01, * * * <
0.001.
As
shown in Fig. 3a, with over years, the differences in distribution of
species richness gradually increased. Before the experiment, the
distribution of species richness in 2008 was relatively uniform,
differences of which were small. Water is the most critical resource of
the grassland ecosystem,so the drought in 2009 and the beginning
N-addition in the year resulted in little difference in the
distributions within each group and between the groups (total, mowing,
N-addition, frequency, in Fig. 3a). In other words, N fertilizer
intensified the response of the grassland vegetations to the drought,
resulting in the most uniform distributions. Nitrogen enrichment was the
key factor that caused the difference in species richness distribution,
while fertilization frequency and mowing had relatively small effects on
it (\(R=0.166,\ \ P=0.001\), in Fig. S1). With the increase of
nitrogen enrichment, the complexity gradually decreased, but the
negative effect of nitrogen could be significantly offset by mowing (in
Fig. 3b). Nitrogen enrichment led to the expansion of difference in
competition levels and the superior species gradually played the
dominant role in the networks, resulting in the network degenerated into
a structure with more transitive chain (in Fig. S2).