Moderate thinning increases soil organic carbon in Larix principis-rupprechtii (Pinaceae) plantations

Forest thinning management practices such as thinning play a major role in the process of soil organic carbon (SOC) sequestration, however, the mechanism for SOC variations is still unclear. The objectives of this study were to estimate SOC stock and its active chemical components following a short term density adjustment of stand woods. Three treatments, low intensity thinning (removal of 15% of the trees, three 25 * 25 m repeated plots), moderate thinning (30% removal), and heavy intensive thinning (50% removal) were compared to control plots with no thinning three years before sampling of soil carbon composition. A number of carbon components (i.e. SOC; soil total nitrogen, TN; permanganate oxidizable C, POXC; dissolved organic carbon, DOC; microbial biomass carbon, MBC) were measured in five soil layers within the Larix principis- rupprechtii plantation throughout the growing seasons of 2015 and 2016. Results indicated both SOC content and its active component, POXC content, were maximized under the moderate density adjustment (p < 0.01). Density adjustment significantly affected POXC/SOC ratio (p < 0.001), with maximum POXC content per unit SOC content also under moderate density adjustment. However, density adjustment hardly had any impact on DOC or MBC, both of which showed more obvious seasonal dynamics than SOC, TN and POXC. Summer affected SOC storage by altering the POXC, TN and MBC. Our results suggest that effects of density adjustment on POXC drive variation in SOC.