Impacts of thinning on soil carbon and nutrients and related extracellular enzymes in a larch plantation

Thinning, an important management practice for reducing plant competition and improving wood production in forests, profoundly influences the soil carbon (C) and nutrient cycling by changing the microclimate, quantity and quality of litter inputs of the systems. This study aimed to evaluate responses of the C, nitrogen (N) and phosphorus (P) concentrations of the topsoil (0-10 cm) to long-term thinning treatments and explore their underlying mechanisms in a 60-year-old Korean larch (Larix olgensis) plantation in northeastern China. The treatments included quartic thinning with low-intensity (LT4), triple thinning with medium-intensity (MT3), twice thinning with high-intensity (HT2), and control. We found that thinning increased the soil total C concentration by 48.7-50.3%. Both the labile and recalcitrant C fractions had positive responses to thinning, but the response of the latter was significantly greater than that of the former. The treatments increased the recalcitrant C concentration by 69-75%, suggesting that the increase of the recalcitrant C fraction was mainly responsible for the thinning-induced increase in the total C concentration, which supports that root-derived C is retained more efficiently in soils than the leaf-derived C. However, the oxidative and hydrolytic C-degrading enzymes were inhibited (peroxidases) or not significantly affected (phenol oxidase, alpha-1,4-glucosidase, beta-1,4-glucosidase) by thinning, which may be conducive to the soil C sequestration especially for the recalcitrant C. Additionally, thinning significantly increased the soil N and P availability and the activities of N-acetyl-beta-glucosaminidase and acid phosphatase, suggesting that the thinning-induced increments of soil N and P availability may be related to the increases in the activities of N and P-releasing enzymes. Overall, our findings illustrate that the long-term thinning not only increased soil C sequestration and stability, but also improved soil nutrient availability in the larch plantations, which may be mediated by shifts in the activities of extracellular enzymes.