Effects of Close-to-Nature Transformation on Soil Enzyme Activities and Organic Carbon Fractions in Cuninghamia lanceolata and Pinus massoniana Plantations

Soil enzymes are involved in the process of mineralization of soil organic matters. The close-to-nature transformation (CNT) of plantations changes the soil enzyme activities by changing the composition of stand vegetation, which in turn affects the change process of soil organic carbon. We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana and Cunninghamia lanceolate, to explore the effects of CNT on soil enzyme activities and soil organic carbon, and the relationship between them is explored through comparative study. Compared with control stands (CCK and PCK), CNT enhanced soil organic carbon; the content of water-soluble organic carbon in the 0-10 cm soil layer of the transformed C. lanceolata plantations (CCN) is 81.29% higher than those in the control stands (p < 0.05); the contents of particulate organic carbon and water-soluble organic carbon in the 10-30 cm soil layer are 95.42% and 48.68% higher than those in the control stands (p < 0.05), respectively; after the CNT, the protease, urease, and acid phosphatase in C. lanceolata plantations were higher than control stands, while protease and catalase in P. massoniana plantations were higher than control stands. Correlation analysis showed that catalase and protease were more closely related to organic carbon components than other organic enzymes; redundancy analysis (RDA) results show that pH and total nitrogen are key factors that cause changes in carbon fractions after the CNT. In general, CNT enhanced soil organic carbon in coniferous plantations, which was more conducive to soil organic carbon accumulation but had a negative effect on soil organic carbon stability to a certain extent. Therefore, the effect of tree species configuration on soil carbon stability components should be considered in the forest management practice.