| Increasing human activities are driving a decrease in area of natural habitats and simultaneously isolating the remaining natural habitats into discrete fragments, and have led to expanding problems that affect the function and composition of forest ecosystems. In fragmentized habitats, the diversity of plants, animals and soil microbial organisms are restricted and depauperate due to the influence of biogeographic processes and interference, which has attracted widely attention. Islands as a typical and independent fragmentation habitat are generally considered an "ideal model" for ecological research. At present, the research of habitat, plant and vertebrate diversity are still very hot for island ecosystem response to global change around the world. However, there are few studies on the differences of the decomposition function in soil system of island habitats and its response to the change of environments. Thus, there be an advantage to island ecosystem that explores soil microbial function differentiations for better understand the potential ecological process. Exploring and identifying the response of different forests to climate change on litter decomposition system is an important task.Two dominant litter types were chosen from Thousand Island Lake region in East China:Castanopsis sclerophylla and Pinus massoniana leaves from 29 island forests. Litter decomposition is mainly determined by substrate quality and soil decomposer, and it is easily vulnerable to environmental factors. The biogeochemical features of islands are complex because of the uncontrollable environmental factors. Thus, firstly a field investigation experiment was performed experimental factors of island attribute, soil environment properties, and soil invertebrate fauna which combined with indoor test and lab analysis. Soil arthropod fauna improve soil nutrient status through metabolic activity ultimately effect litter decomposition. And soil microorganisms are the main participants during litter decomposition. Thus, in our study, we conducted a simulation experiment under standard conditions from microbial perspective to avoid the influence of various environmental. In our experiments, the both litter samples were incubated microcosms with each original forest soils. And inorganic N was used to fertilize the soils to simulated N deposition during decomposition for six months, and samples were harvest monthly. During the above three experiments, chemical composition of soil and litters, and decomposition parameters were determined. The results of this study are as follows:(1) The invertebrate communities contribute to the litter decomposition in soils are mainly distributed in the litter layer, the humus layer and the top soil layer. Collembola and Acarina were the dominated groups of soil invertebrate. Anoplura and Isopoda with saprophagy fauna were mainly dominant types in the humus layer. Chilopoda, Oligochaeta and Isopoda were mainly dominant types in the soil layer. The diversity groups of invertebrate were greater in the humus layer than that of the litter and top soil layers in the same habitat. The density of fauna was not significantly different in the vertical level, and it was not enhanced with the increase of island area. However, most invertebrate in mainland soil was higher than that in all island habitats. In addition, the results also show that the island size is not the main factor affecting the distribution of soil faunas, the island attributes, soil nutrients and their interaction affect the distribution of soil invertebrates and metabolism of soil microbial.(2) In terms of litter quality, the decomposition rate of low-quality P. massoniana litter was significantly less than that of high-quality C. sclerophylla. There were significant differences in litter decomposition rate among island soils. Along island gradient, we concluded that soil N deficiency existed in some large island soils. Substrate-induced respiration was decreased along island area. After litter decomposition, litter N content was negatively correlated with island area. Phenol oxidase, peroxidase activities were positively correlated with decomposition rate of P. massoniana litter, implying that special soil enzymes dominated and improved P. massoniana decomposition. Result showed that P. massoniana was sensitive to home-field advantage in these potential island ecosystems, while C. sclerophylla litter did not significantly differ among the soils. In summary, soil N is the major limiting factor, the responses of microbes to island ecosystems were dependent on soil nutrient balance and interaction between litter property and soil microbial communities.(3) Inorganic N subsidy accelerated litter decomposition of both litters rapidly during decomposition period, especially C. sclerophylla litter in large island soils, that is to say C. sclerophylla litter is sensitive to soil N subsidy especially in large islands, this implying that soil N addition had alleviated the limitation of nutrient. The decomposition coefficient k of of C. sclerophylla and P. massoniana were 20.48% and 19.86%higher than that of control, respectively. Based on islands gradient, N subsidy accelerated substrate induced respiration and most enzyme activities. Moreover, N subsidy enhanced the activities of microbial decomposers towards to C, N and P metabolism while decreased the soil pH and N metabolism related enzyme activities.In summary, during litter decomposition, soil nutrients, especially nitrogen, is an important regulating factor. The structure and distribution of soil arthropod in the decomposition layer may via alter the soil nutrient state eventually effect litter decomposition process. Human activities and climate change, resulting isolation of forests and soil faunas with high heterogeneity; atmospheric N deposition effect of habitats caused by litter decomposition eventually presumably alter the balance of forest nutrient cycling and humus formation. This study can be used as a reference for the future establishment of the island forest reserve and the balance of the ecosystem. |
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