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.