4.2. Soil enzyme activity of different type vegetations
Our study shows that different vegetation types affect soil enzyme activity differently (Figure 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 (Figure 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 (Figure 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.
We found that the different vegetation types do not significantly affect soil catalase activity, which could be related to the metabolic activity of aerobic organisms(Brzezińska, Włodarczyk, Stępniewski, & Przywara, 2005). It can decompose hydrogen peroxide into molecular oxygen and water to prevent cells from damage by reactive oxygen species (Bartkowiak & Lemanowicz, 2017). As such, we observed no significant change in catalase activity in adverse environments.
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 (Figure 3A) and sucrase (Figure 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 (Figure 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 (Figure 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, 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 (Figure 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). The effects of vegetation on soil enzyme activities (i.e., amylase, catalase, urease and sucrase) are different under different soil types and environmental conditions.