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).