Effects of Thinning on Soil Organic Carbon Fractions and Soil Properties in Cunninghamia lanceolata Stands in Eastern China

Soil organic carbon (SOC) fractions, along with soil properties, are greatly affected by forest management. In this study, three thinning treatments: control (conventional management mode), moderate thinning intensity and heavy thinning intensity, were applied in Chinese fir (Cunninghamia lanceolata) plantations in eastern China. The dissolved organic carbon (DOC), soil light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC), total SOC, DOC/SOC and LFOC/HFOC were not affected by thinning treatments. In the heavy thinning treatment, soil bulk density decreased, and soil water holding capacity and porosity increased in the topsoil layers (0–10 cm and 10–20 cm). Total nitrogen, hydrolysable nitrogen, and zinc concentrations increased in the topsoil layers (0–20 cm) in the heavy thinning treatment compared to the control treatment, while the available potassium concentration reduced. The moderate thinning treatment had little effect on the soil physical and chemical properties. Moreover, the variation of SOC fractions was strongly correlated to soil physical and chemical properties. These results suggest that thinning has little effect on the total SOC and its fractions in one rotation of Chinese fir tree in eastern China. In contrast, however, results also suggest that thinning has a positive effect on soil quality, to a certain extent.