| Larix principis-rupprechtii M ayr is the main afforestation species in northern China. For the reason of its faster growth and quality material, it has a wide range of applications. Due to increase of the forest age and delay of tending measures, the return of nutrient elements in Larix principis-rupprechtii litter is decreased, hence soil fertility tend to degradation in Larix principis-rupprechtii plantation. The research subject of this paper is to study the effect of forest density on decompositon rate of leaf litter and finally on soil fertility. We selected two different site classes 3,4 of Larch Forests in middle mountain, and each site class carried out 4 densites respectively. To investigate Larch plantation forest litter decomposition, the compositon of litter, the characteristic of litter decomposition and the effects of density to litter decomposition. The results show that:1. Larix principis-rupprechtii plantation litter weight loss showed the bimodal curve, the seasons of most quickly loss weight was in spring and fall. After one year decomposition, the total weight loss rate as below:site class 3 was 23.6% and 4 was 21.00%.2. Along with the increasing of forest density, the turnaround coefficient had a significant change, and all of them in the density III had the shortest cycle rate and the highest decomposition coefficient. The result was that site class 3 average turnover was 11.09 years and site class 4 was 11.24 years. The trend was that the former than the latter reducing to 1.35%. Therefore, the lower quality of site condition the lower litter decomposition. Decomposition coefficient also had the same trends with average turnarround coefficient.3. The main reason for significant differeces in litter weight loss rate was that the high content of difficult decomposition material such as lignin, crude fiber and the high ratio of Lignin/N and C/N. From the correlation analysis results we might see that there was significant negative correlation between litter weight loss rate and the ratio of C/N, total C were-0.735 and-0.569. While the litter decomposition had the positive correlation with ash content.4. Along with the decreasing of forest density, the ratio of lignin/N and C/N had first decrease and then increase. Site class 3 litter ash content with the decresing forest density was first increase and then decrease, the maximum appeared in the site class 3-â…¢was 8.98%. crude protein, the initial N concentrations also had the same trend with litter ash content, their value were 8.01% and 1.28%. In site class 4, the litter ash content with the decreasing forest density at first increase and then decrease, the maximum and the minmum ash content were 9.43% and 7.62%. The minmum value of initial N concentration at site class 3-â…¢was 18.87%, the ratio of C/N at site class 3-â…£was 31.84%. The minmum of the ratio of lignin/N and C/N in site class 4 were all in site class 4-â…¢.5. Litter decomposition of the nutrient elements changed significantly with the seasons. The process of N, P trends consistently in all these cases, first accumulation and then release. The enrichment of K elements is slow, then a slow release process. Ca in the whole process was had been released, and was the highest initial concentration of the elements. Mg was a volatile element enrichment-style changes, and the whole change process was not significant.6. Soil enzyme activity changed significant with seasons. In summer, soil enzymes had the most activity, while in spring and fall had the lower activity respectively. With the change in forest density was not apparently. The different activity between topsoil (0-10cm) and 10-20cm soil, the former had the higher activity of catalase, invertase, urease and alkaline phosphatase than the latter.7. Along with the decreasing of forest density, litter decomposition factor composite score gradually increased. The highest score was 0.785 in the site class 3 densityâ…£and other density factor composite score were:densityâ…¢0.128, densityâ…¡-0.166, densityâ… -0.747. In site class 4 densityâ…£had the highest score of 0.916, others were-0.156,-0.404 and-0.356 for densityâ…¢,â…¡, andâ… , respectively.
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