Effects of thinning on soil nutrients in a chronosequence of Chinese fir in subtropical Chinaforests

Thinning is a common silviculture technology in forestry, but there is considerable uncertainty about the changes of soil nutrients due to thinning practice in different aged forests. The influence of light thinning (reduction of 20% of stand density) on soil organic carbon (SOC) and other mineral elements (N, P, K, Cu, Fe, Zn, Mn and S), as well as soil pH and soil bulk density were investigated in 6-, 13-, and 23-year-old growth stands of Chinese fir forests in subtropical China. Six 10m ×10m plots (three thinning and three non-thinning plots) were set up in each aged forests. Soil samples were taken from different soil layers from these plots for nutrient analysis. Results showed that SOC increased 59.4, 48.9, and 62.0% in topsoil layer (0-30 cm) in 6, 13, and 23 year-old growth stands, respectively when compared to the control, and the content of N increased by 20.5, 44.6, and 54.1% corresponding after two year post-thinning. In the thinned forests, soil P slightly increased at 6- and 23 year-old growth forests dude to microclimates were improved, and K decreased because of leaching. The response of micronutrients Cu, Fe, Zn and Mn to thinning varied in all examined Chinese fir forests. Thinning decreased soil bulk density and soil pH in 6 and 23 year-old growth stands. Our study provided scientific references for sustainable management of soil nutrients under forest operations in Chinese fir forest ecosystem in subtropical China.