4.3. Relationship between soil carbon fraction and enzyme activity
Enzymes participate in the transformation process of soil nutrients. Enzyme activity plays a vital role in soil microbial activity and soil quality (Ebhin Masto, Chhonkar, Singh, & Patra, 2006). Under stable organic nutrient conditions, soil enzyme activity is typically higher, and increased mineralization of the soil’s nutrients creates a more favorable environment for nutrient cycling (Roldán, Salinas-García, Alguacil, & Caravaca, 2005). The results of this study demonstrate that catalase activity was significantly related to MBC content, and can reflect the changing process of MBC. Both urease activity and invertase activity displayed significant positive correlations with organic carbon and total N content. Urease and invertase activity can reflect the decomposition of organic matter and nitrogen in soil and can be used as important indicators of soil fertility. In sum, enzyme activity and carbon fraction influence each other’s conversion and circulation of nutrients (Qi et al., 2016; S. Zhao et al., 2016).
5. Conclusions
This study analyzed the responses of soil organic carbon fractions and related enzyme activities to different vegetation types in the northern Loess Plateau. Our results demonstrated that the content of soil SOC, EOC and POC were greater in the upper layer than in the lower layer, except for MBC. This variation was similar for different vegetation types. Vegetation types effected on soil organic carbon fractions differently. The maximum MBC content in the upper soil was observed in HR vegetation, the maximum EOC content was observed in GL vegetation, and the soil POC and SOC contents of SX vegetation were significantly higher than in the other three vegetation types. Moreover, the type of vegetation significantly influenced soil enzyme activities, except for catalase. For all four vegetation types, the soil amylase, urease, and sucrase were all significantly higher in the upper than in the lower layer. In contrast, the soil catalase displayed no significant difference between soil layers. In addition, we also found that the MBC significantly impacted catalase activities, POC significantly impacted urease and sucrase activities, and SOC significantly impacted urease and sucrase activities. Considering that the related effects of vegetation environment on litter decomposition and soil microbial community structure will jointly affect the soil carbon dynamics under human disturbance, more long-term research is needed to better understand the dynamic mechanism of soil organic carbon fractions after returning farmland to forest.