3 | Results

3.1 | Soil environmental conditions

In our common garden experiment, soil temperature and moisture (at the depth of 10 cm) was significantly influenced by N addition and plant diversity loss (Fig. 1). Soil temperature (ST) significant increased but soil moisture (SM) had no change after N addition. Soil temperature and moisture changed markedly along the plant diversity gradient, but the changing trend was contrary under different N addition level (Fig. 1c and d). ST decreased but SM increased with decreasing species richness at the control treatments, while ST increased and SM decreased with plant diversity loss at the N addition treatments (Fig. 1c and d).

3.2 | The effects of plant diversity loss and N addition on phenology

The flowering phenology of M. Sativa was significantly affected by N addition and plant diversity loss, but the effects was varied with phenological stages (Fig. 2). N addition delayed the first flowering day (FFD) of M. Sativa by 1.9 days, but plant diversity loss had no effects on it (Fig. 2a). The last flowering day (LFD) was earlier by 1.2 days after N addition, but it was delayed under plant diversity loss (Fig. 2b). LFD was delayed by on average of 0.51 and 0.77 days per species lost at the control and N addition treatment, respectively (Fig. 2b). Obviously, the delay of FFD and the advance of LFD inevitably shorten of the flowering duration (FD). N addition shortened FD ofM. Sativa by 3.2 days, while FD was extended under plant diversity loss (Fig. 2c). FD was extended by 1.08 and 0.82 days per species lost at the control and N addition treatment, respectively (Fig. 2c). Moreover, our study also showed that flower numbers (FN) significantly decreased under N addition and plant diversity loss (Fig. 2d).

3.3 | The effects of N addition and plant diversity loss on functional traits

Six different leaf traits of M. Sativa was measured in our study. N addition significantly increased leaf mass and area, but had no effects on specific leaf area, leaf carbon and nitrogen content, and leaf C/N ratio (Fig. 3). However, with losing of plant diversity, specific leaf area and leaf nitrogen content significantly increased, leaf C/N ratio decreased, but leaf mass, leaf area, and leaf carbon content had no changes (Fig. 3).
The relative biomass, abundance, and height of M. Sativa were used to represent the plant traits in our study. We found that N addition had no effects on plant traits, but plant traits was significant changed under plant diversity loss (Fig. 4). The biomass (Fig. 4a), relative biomass (Fig. 4b), and relative abundance (Fig. 4c) of M. Sativa were significantly increased with plant diversity loss, while relative height significantly decreased (Fig. 4d).

3.4 | Ecological factors influencing flowering phenology

Three statistical methods, including partial correlation, variation partitioning, and structural equation modelling analysis, were used to discern the influence of ecological factors on flowering phenology. Partial correlation analysis showed that with controlling N addition and plant diversity levels, FFD was closely correlated with leaf carbon content, C/N ratio, and available soil N; LFD was closely correlated with leaf nitrogen, C/N ratio, and ST; FD was correlated with relative abundance and ST; FN was closely correlated with leaf mass, C/N ratio, and plant height.
Variation partitioning analysis indicated that light and nutrient acquisition traits explained a much greater portion of the variance in FFD (21% and 40%), LFD (41% and 62%), FD (57% and 73%), and FN (52% and 64%), respectively (Figure 6). Structural equation modelling analysis showed that the changes in FFD was positive correlated with available soil N and negative correlated with leaf carbon content (Fig. 7a). Leaf nitrogen content and ST jointly explained 44% of the variance in LFD, and the negative effects of plant diversity on FFD was mainly through its negative effects on leaf nitrogen content (Fig. 7b). ST and relative abundance jointly explained 38% of the variance in FD, and the negative effects of plant diversity on FD was mainly through its negative effects on relative abundance (Fig. 7c). Leaf mass and leaf C/N ratio jointly explained 30% of the variance in FN, and the positive effects of plant diversity on FN was mainly through its negative effects on leaf C/N ratio (Fig. 7d).