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

LI Guo-lei~1;LIU Yong~1;LI Rui-sheng~2;XU Yang~1;GUO Bei~1.1 Key Laboratory for Silviculture and Conservation of Ministry of Education,Beijing Forestry University,100083,P.R.China;2 Beijing Municipal Bureau of Landscape and Forestry,100029,P.R.China.  
It is vital to study the effects of density on the substrate quality and decomposition rate of the leaf litter.In 2002,the experiment of thinning intensities with four different treatments of 0(Ⅰ),35.7%(Ⅱ),49.2%(Ⅲ) and 64.2%(Ⅳ) on the undergrowth were carried out in the 18-year-old Pinus tabulaeformis stands with initial density of 3 130 trees/hm~2 in middle hilly lands of Yingpan of Yanqing Country,Bejing.Three years later,the decomposition rate,amount of nutrient return and characteristics of leaf litter were compared by litter bag method.The results show that the annual dry matter loss of leaf litter in the plots of Ⅰ,Ⅱ,Ⅲ and Ⅳ was 25.81%,26.25%,27.68% and 25.96%,respectively.The turnover via leaf-litter fall was 10.04,9.84,9.24 and 9.97 years,respectively.Therefore,it is feasible and convenient to evaluate the thinning effect on decomposition rate.In the first two months,the nutrient of N,P,K and Mg was released sharply.In the whole observation of 14 months,the return of both nutrient N and Mg in the four plots exhibited a cycle with release-accumulation.The return of P and K indicated the trend of release-accumulation-relative balance.However,the return of Ca was different from each other in the four plots. The total nutrient return of the four elements(N,P,K,Mg) in the plots was 10.806,31.016,31.798 and 39.365 g/kg,respectively.Specifically,the quality of leaf litter in the plot Ⅰ was the worst that the N and Ca were accumulated only 2.567 and 0.767 g/kg,respectively.Thinning could decrease the ratios of lignin to N and C to N and accelerate the decomposition rate of leaf litter.The content of the crude ash in the leaf litter was enhanced by thinning.It prevents the acidic material such as tannin and resin from returning to the soil.The effect of thinning intensity is evaluated by the ratio of lignin to N,the ratio of C to N and the ash content in the leaf litter.
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