3.2 ANPP and the aboveground biomass of plant species
Precipitation and N and P addition interactively altered ANPP over the
two experimental years (Table 1). We found the significant positive
interaction between ANPP and N and P combined only in 2019 (the normal
year); P alone enhanced ANPP by 63.95%, but N + P addition together
increased it by 199.40% (Fig. 1a). However, in the dry year (2020), P
input alone increased ANPP by 139.08% (Fig. 1b). Mycorrhizal
suppression had only a minimal effect on ANPP in both 2019 and 2020
(Fig. 1a and b, p > 0.05).
Meanwhile, the shoot biomass of plant species exhibited a much higher
degree of inter-annual fluctuation than ANPP (Table 1). In the normal
year (2019), mycorrhizal suppression reduced the aboveground biomass ofS. breviflora by 63.11% wherear enhanced it by 42.19% and
179.63%, respectively, in N + P addition plots (Table 1; Fig. 2a, F× N
× P: F 1, 28 = 8.79, p < 0.01);
however, there were no significant interactions between these factors
during the dry year (Fig. 2b). The aboveground biomass of C.
squarrosa and C. aristatum was higher in the normal year (2019)
than that in the dry year (2020), but the opposite was true for A.
mongolicum (Fig. 2c–h). Importantly, in the dry year (2020), the
aboveground biomass of C. aristatum was drastically decreased
(Fig. 2h). Across all years, P input alone significantly enhanced the
aboveground biomass of A. mongolicum , C. aristatum andC. squarrosa (Fig. 2 c–h, Table 1, P: F 1,
28 = 27.83, p < 0.01; F 1, 28 =
8.27, p < 0.01; F 1, 28 = 54.11,p < 0.01, respectively).
The best SEM model explained 89% of the variation in ANPP (Fig. 3;
χ2 = 15.167, p = 0.232). Precipitation and N
and P input explained 47% of the deviation in the aboveground biomass
of C. aristatum ; AM fungi had a limited impact on this parameter
(Fig. 3). The aboveground biomass of C. aristatum (standardised
effect size: 0.42), A. mongolicum (standardised effect size:
0.51), S. breviflora (standardised effect size: 0.30) andC. squarrosa (standardised effect size: 0.11) contributed
positively to ANPP (Fig. 3).