Effects Of Soil Fauna On Forest Litter Decomposition In The Upper Reaches Of The Yangtze River

Litter decomposition is one of the essential components of material cycles in terrestrial ecosystems, determining global carbon budget and nutrient turnover. The decomposition process is mainly regulated by climate, litter quality and soil organisms. Soil faunal community has been shown to accelerate the rates of litter decomposition and nutrient cycling in forest ecosystems. Their body sizes are large enough to disrupt physical structure of soil and litter, and then affect organic matter decomposition directly by fragmenting and comminuting of litter, further indirectly by altering soil moisture, soil structure and litter surface and regulating microbial activities during litter decomposition. Moreover, the influences of soil faunal community on litter decomposition could depend strongly on the conditions of microclimate, litter quality and soil faunal structure. The forests in the upper reaches of the Yangze River serve as fundamental ecological barriers to the Yangze River. The climate condition, plant composition and soil faunal structure in these forests significantly vary with the change in altitudes in the Yangze River valley. Hence, in order to assess the effects of soil faunal community on litter decomposition processes, we conducted an experiment using litterbag method in the representative forest ecosystems of the upper reaches of the Yangze River. Our objectives were to explore:(1) whether the contribution of soil fauna to litter decomposition could exhibit the influences of climate and litter quality on litter decomposition,(2) whether the contribution of soil fauna to litter decomposition would decrease with the increase in latitudes and altitudes,(3) what are the differences in the contributions of soil fauna to litter decomposition between subalpine and alpine forests and subtropical evergreen broad-leaved forests.(1) The air temperature and preciptation significantly (P<0.05) decreased with increasing altitude. The maximum and minimum of air temperature and preciptation were observed at the453m and3582m, respectively. The differences in temperature and preciptation between the453m and3582m were12.89℃and479mm, respectively. Moreover, the mean temperature in litterbags also significantly (P<0.05) decreased with increasing altitude. The temperature in litterbags at the945m and453m remained above0℃throughout the decomposition process, but those at the3582m and3023m experienced obvious freeze-thaw cycles with the air temperature fluctuates above and below0℃in the winter. Additionally, the moisture in litterbags significantly changed with the dynamics of preciptation during the whole decomposition process. These results suggest that the striking differences in temperature and moisture in litterbags can profoundly affect the contribution of soil fauna to litter decomposition in different forest ecosystems.(2) There were significantly differences in the composition of dominant groups and ordinary groups, density of individual and group as well as the rate of Acarina to Collembola of the soil faunal communities in the litterbags at different altitdues. Meanwhile, the composition of dominant groups and functional groups, density of individual and group, diversity index and similarity index of the soil faunal communities significantly changed as the decomposition processed, and showed obviously correlated with litter temperature and moisture, litter chemical properties and litter quality. Additionally, the altitude, litter quality and sampling date significantly influenced the soil faunal structure in the litterbags. These results suggest that the soil faunal structure changes during the litter decomposition process and shows sensitive response to the changes in altitude, litter quality and season, which can profoundly affect the litter mass loss and nutrients mineraliztion in different forest ecosystems.(3) As compared with soil fauna removal, soil fauna significantly (P<0.05) increased the litter mass loss and decay rat, and reduced the litter decomposition times. However, the contributions of soil fauna to litter mass loss did not decrease with the increase in altitudes. During the study period, the contributions of soil fauna to litter mass loss and decay rate were15.44%-42.56%and11.24%-36.35%, respectively. The contributions of soil fauna to litter mass loss and decay rate for the broadleaf species significantly (P<0.05) decreased with the increase in altitudes, but which for the coniferous species insignificantly (P>0.05) decreased with the increase in altitudes. The time of contribution effects delayed with increasing altitudes, and the contribution rate increased with increasing decomposition times. The contribution rates were higher (P<0.05) for the coniferous species than those for the broadleaf species at the3582m and3023m, but which were higher (P<0.05) for the broadleaf species than those for the coniferous species at the945m and453m. Additionally, the contribution rate showed obviously correlated with litter temperature and moisture, litter chemical properties and soil faunal structure and were significantly influenced by the altitude, litter quality and sampling date. These results suggest that the soil faunal community profoundly accelerates litter mass loss, and the differences in the contribution of soil fauna to litter can display the influences of climate and litter quality on litter decomposition.(4) The contribution of soil fauna to litter elements release varied with the change of element types. In general, as compared with soil fauna removal, soil fauna significantly (P<0.05) increased the content of N and P in litters by promoting the retention of N and P by microbes, but it showed little (P>0.05) effect on the C content. Moreover, soil fauna also significantly increased the content of lignin and reduced the content of cellulose. Additionally, litter elements release showed obviously correlated with litter temperature and moisture and soil faunal structure and were significantly influenced by the altitude, litter quality and sampling date. These results suggest that the soil faunal community profoundly influences litter elements release and nutrients turnover, and the contribution of soil fauna to litter elements release shows sensitive response to the changes in altitude, litter quality and season.(5) Soil fauna significantly (P>0.05) changed the litter quality throughout the decomposition process compared with soil fauna removal. In general, soil fauna significantly reduced the rates of C/N, C/P, and lignin/N during the early stage of litter decompotion, in particular for the coniferous species. However, soil fauna significantly increased the rate of N/P during the early stage of litter decompotion and the rate of lignin/cellulose during the whole decomposition process. Additionally, litter qualities showed obviously correlated with litter temperature and moisture and soil faunal structure and were significantly influenced by the altitude, litter quality and sampling date. These results suggest that the soil faunal community increases litter mass loss and nutrients turnover and has important contribution to litter decomposition process. Taken together, our results indicate that the forest ecosystems in the upper reaches of the Yangze River keep high diversity of soil faunal community during litter decomposition. Soil faunal community generally has positive effects on litter mass loss and nutrient release as decomposition processed by directly and indirectly improving litter decomposability. Furthermore, the positive effects show sensitive response to changes in climate and litter quality.