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).