Responses of decomposition rate, nutrient return, and composition of leaf litter to thinning intensities in a Pinus tabulaeformis plantation

It is important to study the effect of tree density on the substrate quality and decomposition rate of leaf litter in plantations. In 2002, an experiment of the effects of thinning intensities at four different levels (i.e., 0 (I), 35.7% (II), 49.2% (III), and 64.2% (IV)) on undergrowth were carried out in an 18-year-old Pinus tabulaeformis plantation at an initial density of 3130 trees/hm² in the middle of the hills of Yingpan, Yanqing County, Beijing. Three years later, the rates of decomposition, the amount of nutrients returned, and the characteristics of leaf litter were compared by a litter bag method. The results show that the annual loss of dry matter of leaf litter in plots I, II, III, and IV was 25.81%, 26.25%, 27.68%, and 25.96%, respectively. The turnover of leaf litter was 10.04, 9.84, 9.24, and 9.97 years, respectively. Therefore, it is feasible and convenient to evaluate the effect of thinning on the rate of decomposition. In the first two months, the N, P, K, and Mg nutrients were released quickly. During the entire observation period of 14 months, the return of both N and Mg nutrients in the four plots exhibited a release-accumulation type of cycle. The return of P and K indicated a trend of release-accumulation and a relative balance. However, the return of Ca was far different from that of the other nutrients in the four plots. The total nutrient return of the four elements N, P, K, and Mg in the plots was 10.806, 31.016, 31.798, and 39.365 g/kg, respectively. Specifically, the quality of leaf litter in plot I was the worst in that N and Ca accumulated only 2.567 and 0.767 g/kg, respectively. Thinning did decrease the ratios of lignin to N and C to N and accelerated the rate of decomposition of leaf litter. The content of crude ash in leaf litter was enhanced by thinning, which prevented acid material, such as tannins and resins, from returning to the soil. The effect of thinning intensity was evaluated by the ratio of lignin to N, the ratio of C to N, and the ash content in leaf litter.