4.3. Effects of different land use patterns on soil enzyme activity and stoichiometry
Land use patterns have different effects on soil enzyme activities due to different planting systems and management measures. Some studies showed that the SOC content and βG activity increased gradually with the decrease of soil disturbance and the settlement of plants, especially leguminous crops (Yu et al., 2017). Similar conclusions exist in this study, the βG enzyme activity of AG was higher than that of WM and PC. Meanwhile, the βG enzyme activity of WM was higher than that of PC, which was due to the C source provided by straw returning in wheat maize rotation (Chen et al., 2021). The sequence of NAG activity was similar to that of BG, i.e. , AG > WM > PC. The increase of NAG activity may be due to the increase of soil N availability. According to the theory of resource allocation, soil extracellular enzyme activities are usually significantly positively correlated with nutrient availability before reaching the response boundary of microbial disturbance or fluctuation to nutrient availability (Sinsabaugh et al., 2008; Zhang et al., 2019c). In this study, the LAP activity of WM was significantly higher than that of AG and PC, which may be due to the hydrolysis of cellulose in wheat and corn straw to produce glucose. Some studies have shown that the increase of glucose content would drive the change of soil microbial community (Zwetsloot et al., 2020), and increase the activities of LAP and ACP (Zhang et al., 2019d). This also explained the significant increase of ACP activities of AG in 0-20cm soil layer and WM in 40-60cm soil layer, in which AG was due to the higher βG activity led to the increase of glucose content at the end of hydrolysis, while WM increased the glucose content due to a large amount of carbon source provided by straw returning.
Soil enzyme stoichiometry is used to indicate the nutritional needs of microorganisms and converges to 1:1:1 globally (Sinsabaugh et al., 2008). According to the viewpoint that microorganisms optimize the allocation of resources to obtain the most limited resources, the greater the investment in enzyme activity of C, N, or P, the greater the demand for this nutrient element (Bai et al., 2021). In this study, the enzyme C: N ratios were greater than 1. The enzyme N: P and C: P ratios were less than 1. According to the vector analysis of enzyme stoichiometry, the VL value of PC in 0-40 cm soil layer were significantly lower than that of other land use types, and the VA value of each land use type in each soil layer was greater than 45 °. These results showed that PC was less limited by C, and the microorganisms in each land use pattern were limited by P than N. This may be because PC has no human disturbance for a long time. According to the homeostasis theory of enzyme stoichiometry, soil microorganisms can promote the equilibrium and homeostasis of enzyme stoichiometry (Xiao et al., 2020), thus reducing the C limitation of PC microorganisms. Soil P mainly comes from weathering of primary minerals and cannot be acquired largely from soil (Zhang et al., 2019b). Moreover, in saline-alkali soil, the adsorption of calcium and magnesium ions and the consumption of plants will lead to the decline of P bioavailability (Zhang et al., 2019c). This also explains why soil microorganisms in the YRD were generally limited by P.