| Forest litter plays an important role in forest ecosystem. Litter decomposition is a main way of material cycling and energy flow in the forest ecosystem. It is not only very important in protecting and permanent utilizing forest resources, but also significance in maintaining forest land productivity. Eucalyptus grandis is one of the tree species in the engineering project of conversion of cropland to forest. It has been planted in large area in the south of China. From January 2008, the experiment choose the E. grandis plantation planted in 2003 in Ya'an Sichuan, for the study on annual litter production, litter decomposition, nutrient release and soil respiration in different density plantation. The results showed that:(1)The stands with density of 2222(1.5m×3m),1333(1.5m×5m),833(1.5m×8m) give the annual litter production of 7655.58 kg·hm-2,6247.78 kg·hm-2,4079.93 kg·hm-2 respectively. Leaf litter is main component, and account for 87.89%,94.85%, 87.18% respetively. The amount of litter components of different density stands was leaf>twig>bark, annual litter in every stand with different density exhibited a clear seasonal pattern, with the highest litter in spring and summer, the peak in May,2008.(2) The leaf and twig litter had different decomposition rates, and the higher one was leaf litter. After decomposing for 12 months, the 2222,1333,833 remained litter mass rate of leaf were 42.86%,39.56%,11.57% respectively, and the remained litter mass rate of twig were 64.28%,56.35%,47.07% respectively, the differences between leaf and twig were significant (p<0.01).Leaf litter had a faster decomposition speed during the 5th month to the 8th month, while the decomposing rate of twig was faster from the 4th month to the 10th month. It would take 3.322 a,2.989 a,1356 a for 95% of leaf litter to decompose in density of 2222,1333,833 and 6.147 a,4.783 a,3.604 a for twig litter, respectively. The coefficient of litter decomposition for different density was K 833>K1333>K2222.The initial concentration of lignin in leaf and twig litter were 211.07 g·kg-1,1242.56 g·kg-1,and the cellulose were 152.32 g·kg-1,315.70 g·kg-1.After decomposing for 12 months, the cellulose was lower than initial concentration cellulose, and the lignin was higher than initial concentration lignin in different density.(3) After decomposing for 12 months, the six nutrient elements could be arranged with respect to nutrient release rate in this sequence:2222(leaf) N>C>Mg> P>K>Ca,1333(leaf) N>Mg>C K>Ca,833(leaf) N>C>Mg>P>K>Ca,2222(twig) C>Mg>N>K>P>Ca,1333(twig)Mg>C>N>K>P>Ca,833(twig)C>Mg>N>P>K>Ca, All nutrient release rates in leaf litter were higher than those in twig litter in the stands with different densitiy. During the decomposing process of leaf litter, C, K, Mg featured a pattern of release, the dynamics of Ca featured decrease-increase-decrease,N, P showed different releasing models in different densities.As for the decomposing process of twig litter, C, Mg featured direct release, K consistent accumulated,N, P was as the same as the leaf,,and Ca featured decrease-increase.(4) The initial C/N ratios in leaf and twig litters were 43.51 and 285.78.In the decomposing process of litter and tiwg, the C/N had an decreasing trend in different densities. After decomposing for 12 months, C/N of leaf and twig litters could be arranged in this sequence 833<1333<222 and 1333<833<2222 respectively.(5) Soil respiration in every stand with different density exhibited a clear seasonal pattern, with the highest rate in July and lowest in January, and the stand with density 883 had higher soil respiration rate(Rs), while the density 2222 had lower rate; The average respiration rate in 24 hours of per day in April,July, October in 2008 indicated the order of Rs(833)>Rs(1333)>Rs(2222), and Rs(July)> Rs(April)>Rs(October).(6) 833 and 1333,833 and 2222 existed significant different in microbial biomass carbon and nitrogen (p<0.05). Soil organic matter content and root biomass in 10cm soil layer showed the same trend, the smaller the density, the more soil organic matter content and the more root biomass;(7)Temperature was the leading factor of soil respiration variation. Two-factor model (Rs= aebtWc) of soil temperature and moisture was better than single-factor models (Rs=aebt,Rs= aW+b), and they could explain 78.3%-91.5% monthly variation of soil respiration; As the Q10 value of stands with different density, the sequence was 3.65(833)>2.60(1333)>2.55(2222), implying the trend that the Q10 values decreased with enhancement of stand density.
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