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).