| There are serious ecological problems such as single forestation species, conifer problem and soil degradation of artificial forest in our country. Tree-grass complex ecosystem is a sustainable land use pattern which has enormous potentiality in controlling soil erosion, improving ecological environment, soil improvement, increasing biological productivity, making full use of natural resources and increasing economic benefit. Therefore, choosing appropriate tree-grass complex ecosystem has important significance in understanding soil degradation mechanism, adopting corresponding maintenance measures and sustainable forest management.Alnus formosana, also known as Taiwan, which belongs to Betulaceae alnus, is a nonleguminous fast-growing species of nitrogen fixation. Alnus formosana originated from Taiwan is an important local light demanding pioneer species, which has effect on soil improvement. Alnus formosana is a fast growing tree species with straight form and good quality in timber, and it has some other good characters such as strong adaptability, barren resistance and nitrogen fixation. Also it is secondary commercial timber of Taiwan. At present, the studies on Alnus formosana are very few in our country. And a systemic research on quantity, composition, decomposition and nutrient release of its litter is absent.The aim of this paper is to study the bio-ecological character of Alnus formosana Hemarthria compressa, and its differences with Alnus formosana forest. From July,2008, two patterns were established (A:Alnus formosana-Hemarthria compressa; B:Alnus formosana) according to the local natural environmental conditions in Danling, Meishan city, Sichuan province. A systemic research on quantity, composition, seasonal dynamics and nutrient release during decomposition of litter was made for one year to evaluate the patterns' ability of sustainable forest management. Results were as follows:(1) The annual litter production of the two modes were 5671.88 kg-hm"2 and 5010.61 kg·hm-2 respectively, A>B; litter production of leaf was A>B; litter production of branch was B≈A; litter production of root was A>B. View from the seasons, the change trends of litter production in different season were as follows:leaf:winter>spring>autumn> summer; branch:summer>autumn>spring>winter; root:summer>autumn>winter> spring.(2) In A and B modes, the annual decomposition rate were in sequence of leaves> roots>branches. According to exponential model y=ae-kt, exponential expression equation of different litter weight residual rate with time were set up, and it had high fitting degree (P<0.01). The weight loss rate of all components showed an exponential relationship with time.(3) Correlative analysis of litter initial quality showed that there was a significant correlation between lignin and litter decomposition rate of each component (P<0.01). Therefore, lignin content was an important index to predict litter decomposition rate. Compared with other area's litter decomposition, the annual loss rate of branches, leaves and roots of the two patterns were 54.7%,66.6% and 45.1% respectively after one year, which were all in the range of subtropical litter's annual loss rate (40%-70%).(4) In the early stage of decomposition process, the concentration of N and Ca showed an obviously rising trend, while there was a significant downtrend in the concentration of C and K, and the concentration of P and Mg changed slightly with irregular. The concentration of C of all components decreased smoothly, and then tended to be stable. All the nutrient elements except Ca had an obvious downtrend during decomposition process. The variation trends of N, P and K were similar, while the others showed slightly different.(5) In the litter decomposition process, the variation trends of the residual rate of C, K and Mg were similar with those dry weight residual rates. And the residual rate of N, P and Ca decreased relatively slow, while C was always net release in all patterns. In the root decomposition process, the release rates of C, N, P, K and Ca in tree-grass mode were faster than those in pure forest. There were no significant differences of leaf release rates of N, P, K, Ca, Mg and C between the two patterns. The differences of N, P and Mg between the two patterns were obvious in branches, but K, Ca and C were not. There were significant differences of release rates in all nutrient elements except Ca in roots.(6) In this one year, the total release amount of N, P and K were 26.16 kg·hm-2,3.34 kg·hm-2 and 23.79 kg·hm-2 respectively in the litter (including leaves, branches and roots) of Alnus formosana-Hemarthria compressa mode, while the ones were 22.87 kg·hm-2, 2.87 kg·hm-2 and 20.16 kg·hm-2 respectively in mode B. There were great differences (P<0.05) of nutrient release amount of branches, leaves and roots between two patterns, which indicated that the litter quality was the key factor in litter decomposition and nutrient release. The total release amount of N, P and K of mode A were larger than those of mode B, which showed that the return amount of litter and nutrient release amount of Alnus formosana-Hemarthria compressa mode were larger than those of pure Alnus formosana forest, and the former was more beneficial to nutrient recycling and played an important role in keeping soil fertility.(7) Except soil residual available P, the contents of soil organic matter, total N, total P, hydrolytic N and available P were increase in different degree after one year. The content of soil organic matter and nutrients in 0-10 cm soil layer of forest-grass mode and natural mode were more than those in 10-20 cm soil layer in most time of this one year. The content of soil organic matter and nutrients in 0-10 cm and 10-20 cm soil layer of forest-grass mode were more than those in pure forest mode in most time of this one year. The analysis of litter nutrient release amount and soil nutrients content indicated that the nutrient release during litter decomposition process had obvious effect on soil nutrients content, which increase the soil nutrient content in different degree. But the correlative analysis showed that the nutrients except total K and available K were uncorrelated with litter release. The correlative analysis of soil pH/T/H and litter decomposition indicated that except branches, soil pH/T/H had significant positive correlation with litter decomposition in different patterns (P<0.05, P<0.01). The importances to litter decomposition in sequence are:soil moisture, soil temperature and soil pH.
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