3 RESULTS
The N:P ratios of vegetation and soil for the A. ordosica ,S. alopecuroides and S. bungeana communities are presented in Table 1.
From the data distribution, the vegetation N:P ratios of the A. ordosica communities were lower than those of the S. alopecuroides and S. bungeana communities (Table 1, Figure 1). The vegetation N:P ratios generally decreased in the order S. bungeana communities > S. alopecuroides communities > A. ordosica communities. As a leguminous plant,S. alopecuroides has strong nitrogen fixation capacity. The vegetation N:P ratio was higher at sites where S. alopecuroidesdominated. In S. alopecuroides communities, the minimum N:P ratio was 7.56, and the maximum was 23.88.
TABLE 1 N:P ratios of vegetation and soil in communities of different successional stages in a semi-arid area
FIGURE 1 Distribution of vegetation N:P ratios in a successional series in a semi-arid area
In the early successional stage, represented by the A. ordosicacommunities, there was a significant power function regression relationship between plant total density and vegetation N:P ratio (p <0.05). As the vegetation N:P ratio increased, plant total density increased (Figure 2). Furthermore, the aboveground biomass of the A. ordosica communities showed an increasing trend with increasing total plant density, although the regression relationship was not significant (F = 1.274, R 2 = 0.053,p = 0.271).
FIGURE 2 Relationship between total density and vegetation N:P ratio in an Artemisia ordosica community representing the early successional stage in a semi-arid area
In the mid-successional stage represented by the S. alopecuroidescommunity, there was no significant regression relationship between plant total density and the vegetation N:P ratio (Figure 3). However, the aboveground biomass of the community showed a significant power function regression relationship with total plant density (p <0.05). With increasing total plant density, aboveground biomass increased (Figure 4).
FIGURE 3 Relationship between total density and the vegetation N:P ratio in a Sophora alopecuroides community representing the mid-successional stage in a semi-arid area
FIGURE 4 Relationship between aboveground biomass and total density in a S. alopecuroides community representing the mid-successional stage in a semi-arid area
FIGURE 5 Relationship between total density and the vegetation N:P ratio in an A. ordosica community representing the early successional stage and a S. alopecuroides community representing the mid-successional stage in a semi-arid area
Upon pooling the vegetation N:P ratio and total density data of theA. ordosica and S. alopecuroides communities, i.e., the early successional and mid-successional stages, a significant quadratic function relationship was observed between total density and vegetation N:P ratio, and the vertex coordinate was (16.6, 353.3) (Figure 5).
In the late successional stage represented by the S. bungeanacommunity, a significant negative regression relationship between plant total density and the vegetation N:P ratio was observed (p <0.05). As the vegetation N:P ratio increased, plant total density decreased (Figure 6). Regression is based on a correlation between variables and does not imply causation, but for this late successional stage S. bungeana community, a negative correlation between plant total density and vegetation N:P ratio was observed. In addition, the aboveground biomass of the S. bungeana community decreased with increasing total density, although the regression relationship was not significant (F = 0.843,R 2 = 0.035, p = 0.368).
FIGURE 6 Relationship between total density and the vegetation N:P ratio in a Stipa bungeana community representing the late successional stage in a semi-arid area