Long-term effects of forest thinning on soil respiration and its components in a pine plantation

Mounting evidence has indicated that forest thinning would increase soil respiration (SR) in the early stage (c. < 5 years) after thinning. However, the responses of SR and its components to different thinning intensities in the long-term have not been sufficiently studied. In April 2010, four levels of thinning intensities including control (CK, no thinning), light thinning (LT, 20% basal are removed), moderate thinning (MT, 40% basal area removed), and heavy thinning (HT, 60% basal area removed) were conducted in a Chinese pine (Pinus tabulaeformis) plantation in northern China. SR and its components were measured monthly in growing seasons (May to October) from 2016–2018. After 6–8 years of thinning, on average, HT significantly reduced SR by 24.56%, compared to the control. Declines in SR were ascribed to the decreased heterotrophic respiration (HR) and autotrophic respiration (AR) in HT plots, because heavy thinning inhibited soil nutrients, microbial biomass carbon (MBC), and fine root biomass (FRB). Thinning increased temperature sensitivity (Q10) of HR (CK = 2.24, LT = 2.58, MT = 2.82, HT = 2.51), but decreased the Q10 values of AR (CK = 2.76, LT = 1.14, MT = 1.82, HT = 2.23). Soil moisture had a positive relationship with HR but did not correlate with AR (expect for MT). In addition, two-factor models combining soil temperature and soil moisture explained 40–64% and 1–12% of variation in the HR and AR, respectively. The effects of thinning on soil respiration and its components varied substantially over time. Our study highlights that fine root and microbial biomass should be incorporated into biogeochemical models when accurately predicting long-term effects of thinning on soil respiration and its components.