4.2. Soil enzyme activity of different type vegetations
Our study shows that different vegetation types affect soil enzyme activity differently (Fig. 3). Urease, a key enzyme that regulates soil nitrogen transformation, comes mainly from plants and microbes and plays a key role in nutrient cycling (Zhao, Li, & Wang, 2012). Soil urease activity in XS vegetation is higher than in the others (Fig. 2C). The high urease activities in XS vegetation may be due to both microbial growth and stimulation of microbial activity by enhanced resource availability (Li et al., 2014). At the same time, higher soil nutrients (Table 2) and SOC contents (Fig. 2D) provide microorganisms with a rich source of nitrogen and carbon, which significantly adds to the nutrients accumulated by transformation (Cui et al., 2019). Improving the physical properties of soil creates an environment that benefits microorganisms (Iovieno, Morra, Leone, Pagano, & Alfani, 2009) and increases urease activity.
Catalase can decompose hydrogen peroxide into molecular oxygen and water to prevent cells from being damaged by reactive oxygen species (Bartkowiak & Lemanowicz, 2017). In this study, we found no significant difference in soil catalase activity under different vegetation types. This may be due to less rainfall in this area, and small differences in soil microbial activity (MBC, Fig. 2) and soil properties (bulk density and porosity, Table 2), leading to there was no significant difference in soil catalase activity. Furthermore, microbial communities, litter decomposition, and soil pH are also important factors affecting soil catalase activity (BrzeziƄska et al., 2005; Gu et al., 2009; Kannan and Wei, 2008).
Soil amylase and cellulose enzymes are responsible for the rate and course of plant material decomposition and plant debris degradation (Piotrowska, 2014). Significant differences in soil amylase and sucrase activities were observed under the four vegetation types (P < 0.05). The activities of amylase (Fig. 3A) and sucrase (Fig. 3D) in GL vegetation in the 0-20 cm layer were significantly higher than in the other three vegetation types. In the 20-40 cm layer, the soil amylase activity in the HR vegetation was the highest, while there was no significant difference in the other three vegetation types. The soil sucrase in the XS vegetation was significantly higher than that in the other three vegetation types. Because there are more types of vegetation and litter on the surface of GL, the content of SOC fractions is higher (Fig. 2), and soil organic matter has a higher input capacity, which affects the community structure and growth of rhizosphere soil microorganisms (Prescott, 2010). GL vegetation is also dominated by low, herbaceous vegetation (Table 1). The shade effect of this vegetation is small, and soil temperature is higher than in the other three vegetation types, resulting in higher soil amylase and invertase activities in GL vegetation. The higher MBC, POC contents (Fig. 2A, C), and total porosity (Table 2) in the 20-40 cm layer of HR vegetation provide a source of oxygen for microbial activity, while the root system of GL vegetation is mainly concentrated in the 0-20 cm layer (An, Huang, & Zheng, 2009), meaning that amylase in HR vegetation is more active in the 20-40 cm layer.
In all four vegetation types, the soil amylase, urease, and sucrase activities were greater in the upper layer than in the lower layer, while the soil catalase activity did not change significantly. Due to the high SOC content (Fig. 2), there are sufficient nutrient sources to facilitate the growth of microorganisms. In addition, higher surface temperatures and better ventilation enable soil microorganisms to quickly grow and metabolize (Chen, Shang, Cai, & Zhu, 2019). The underground biomass in the 20-40 cm soil layer was reduced, which reduces the source of soil nutrients, while this reduction of SOC content and plant roots often leads to a decrease in enzyme activity (Xiao, Huang, & Lu, 2015). These results suggests that the effects of vegetation on soil enzyme activities are different under different soil types and environmental conditions.