Forest thinning and organic matter manipulation drives changes in soil respiration in a Larix principis-rupprechtii plantation in China

Plantation management practices may influence carbon dioxide (CO₂) uptake dynamics; however, the responses of carbon losses, such as soil respiration, to management (e.g. thinning) have not been sufficiently studied. We evaluated the effects of forest thinning (no thinning (NT), low thinning (LT), medium thinning (MT), and high thinning (HT) and organic matter manipulation (intact soil (untreated control, UC), exclusion of aboveground litter (EL), and exclusion of both aboveground litter and roots (ELR)) on soil respiration in a Larix principis-rupprechtii plantation. During the period from 2015–2018, soil respiration was continuously monitored from May to October (growing season). Meanwhile, soil temperature and soil moisture at a depth of 5 cm were also measured. In addition, three soil samples were collected to measure biochemical properties of soil. We found that mean soil respiration (Rₛ) was significantly greater in MT than that in NT (by 21.5 %). RS in HT was significantly greater than NT in 2015 (by 25.0 %) and then finally lower than NT (14.8 %) in 2018. However, RS in LT compared with NT showed no response to thinning during our observation period. The organic matter manipulation also affected soil respiration. Mean reduction of soil respiration in EL (REL) and in ELR (RELR) compared to Rₛ ranged from 25.9%–39.2% and 40.8%–53.6% across all thinning regimes, respectively. The temperature sensitivity (Q₁₀) values of RS, REL, RELR ranged from 2.23 to 2.72, 2.21–2.60, and 2.31–2.90, respectively. The model with the best fitting temperature and moisture factors explained 63.8 % - 75.4 % changes in RS, 65.9 % - 74.7 % changes in REL and 70.4 % - 74.1 % changes in RELR. Stepwise regression analyses showed that soil temperature, ratio of soil carbon and nitrogen in the topsoil explained the pattern of RS across the thinning scenarios. REL showed a high sensitivity to changing in some soil chemical properties and hydrolases activities, whereas RELR showed a high sensitivity to changing in microbial activity combined with temperature and pH of soil. Overall, our data show that the response of organic matter inputs to thinning has an important impact on soil CO₂ fluxes of plantation, and emphasize the difference of the response between aboveground and underground organic matter inputs to soil CO₂ fluxes.