4.4. Linkages between soil stoichiometry and enzyme stoichiometry
In this study, the average enzyme stoichiometry of AG, WM, and PC was 0.96: 0.76: 1, 0.94: 0.74: 1, and 0.86: 0.73: 1, respectively, which deviated from 1: 1: 1. Moreover, the enzyme stoichiometry and soil stoichiometry showed the significant linear correlation in each soil layer. These results indicated that enzyme stoichiometry of land use patterns in the YRD was not homeostasis, but nutrient dependent (Qiu et al., 2021). According to RDA analysis, the main driving factors of community variation in AG, WM, and PC were TP, TP and WSA (2-1 mm), and NO3--N, respectively, and these factors were positively correlated with the enzyme activities (βG, NAG, and ACP) and enzyme C: P and N: P ratios, but negatively correlated with enzyme C: N ratios. Studies have shown that extracellular enzyme activity is mainly determined by the number of available substrates and the microbial biomass for extracellular enzyme synthesis (Blagodatskaya et al., 2014). The increase of TP provided sufficient substrate for ACP. Meanwhile, the fungal biomass would affect the microbial biomass involved in extracellular enzyme synthesis, and the fungal biomass was easily affected by tillage (Li et al., 2019a). Therefore, the main driving factors of community variation in WM and AG were TP and WSA (2-1 mm), and TP, respectively. According to the viewpoint that microorganisms optimize resource allocation to obtain the most limited resources (Bai et al., 2021; Zhang et al., 2019d). According to RDA analysis, when P was no longer the nutrient limiting factor of microorganisms with the increase of P content, microorganisms would promote the synthesis of βG and NAG enzymes, thus increasing the enzyme C: P and N: P ratios. However, the increase of enzyme C: P and N: P ratios would lead to the decrease of enzyme C: N ratio, which would cause the N limitation of microorganisms. This is consistent with the conclusion of Yang et al. (2020), who found that the nutrient limitation of soil microorganisms changed from P limitation to N limitation with the plant restoration process to about 15 years. It also explained that NO3--N was the main factor driving the variation of PC community without human disturbance for a long time. Therefore, although soil microorganisms in the YRD were generally limited by P, the microbial nutrient limitation would change from P limitation to N limitation over time. Our results should contribute to the development of soil improvement strategies in the YRD.