Contributions Of Soil Fauna To Litter Decomposition In Ropical Evergreen Broad-leaved Forests In Sichuan Basin

Litter decomposition is one of the essential processes in terrestrial forest ecosystems, which could contribute to material turnover and then plant productivity. Although primary decomposition of litter is ultimately the result of the catabolic activities of soil microbes, soil faunas have been well known that they can accelerate the rates of litter decomposition and nutrient cycling in forest ecosystems. Soil faunas have body sizes large enough to disrupt physical structure of litter directly by fragmenting and comminuting of litter, and promote litter decomposition indirectly by altering litter surface and regulating microbial activities. However, both litter quality and microclimate play important roles in the contributions of soil fauna to litter decomposition. The annual critical phenological stages in plant growth do not only represent the change of climate environment, but also indicate the nutrients needs in plant growing. The key stages in plant phenology do not only represent the dynamics of climate, but also indicate the nutrients needs in plant growing. Therefore, in order to understand the relationships between litter decomposition and plant growth, the contributions of soil faunas to litter decomposition were studied in the key stages of Plant phenology periods (leave falling stage, leave budding stage, leaf expanding stage leave maturing stage, leave graving period and leave senescing stage) according to local plant phenology from November 2011 to August 2013. Four kinds of foliar litter were sampled as Pinus massoniana and Cryptomeria fortune from plantation, and Cinnamomum camphora and Quercus acutissima from secondary ecosystem in subtropical regions of Sichuan basin in southwestern China, respectively. Litterbags with different mesh sizes (0.04 mm and 3.00 mm) were used to exclude soil faunas during litter decomposition.Soil fauna had significant contributions to litter decomposition in each key stage during the decomposition regardless of species. We captured a total of 10386 soil fauna individuals in all litterbags, which belonged to 22 orders and 73 families. Soil fauna individuals showed:P. massoniana (2863)> Q. acutissima (2713)> C. camphora (2704)> C. fortunei (2106) Soil fauna community structure was significantly varied with different litter substrate at different key stages as litter decomposition proceeding, indicating liter quality could have close relationships to soil fauna during litter decomposition. The leave falling stage, leave budding stage and leaf expanding stage, predatory soil fauna dominated soil fauna community during P. massoniana litter decomposition, and fungivore dominated the other stages. Fungivore also dominated soil fauna community at eave falling stage and leave senescing stage during C. camphora decomposition, but predatory dominated the other stages. In contrast, soil fauna community during C. fortunei litter decomposition was dominated by fungivore at all investigated stages. Although fungivore dominated soil fauna community at early three stages during Q. acutissima litter decomposition, phytophagy dominated the later stages.Compared with other stages, leave falling stage displayed the lowest contribution rates. The highest contribution rate of soil fauna to P. massoniana litter decomposition was observed in the leave senescing stage, but the highest contribution rates for the other three species were detected in leave graving stage. The contribution rates of soil fauna were higher during broadleaf litter decomposition than coniferous litter decomposition. The contribution rate of soil fauna in the first decomposition year related to initial cellulose content and the ratio of C/N, but which in non-growing period related to initial N content, the ratios of C/N and lignin/cellulose, which in growing period showed significant relationships with the ratio of lignin/N. The effects of soil fauna on nitrogen and phosphorus concentrations, release rates and release velocities during litter decomposition at different phenological stages were investigated in this study. In the first year, soil fauna obviously promoted nitrogen release in the C. fortune and Q. acutissima litter, and promoted phosphorus release in the P. massoniana during litter decomposition. But soil fauna inhibition of nitrogen release in the P. massoniana and C. camphora during litter decomposition, with C. camphora, C. fortune and Q. acutissima in the process of phosphorus release also show the negative effects. Relative to other phenological period, soil fauna suppresses the nitrogen and phosphorus releases of the P. massoniana and C. camphora in the leave falling stage, leave budding stage during litter decomposition, to facilitate the nitrogen and phosphorus releases of the P. massoniana, C. camphora and Q. acutissima in the leaf expanding stage, leave maturing stage and leave graving period.Research showed that the individual density of soil fauna was significantly related to Nitrogen and Phosphorus content in leave falling stage and leave budding stage, but which closely related to lignin content at leave senescing stage. Moreover, the contribution of soil fauna to litter decomposition showed a significantly relationships with the individual density of soil fauna at leave falling stage, which significantly related to the group density and the soil faunal functional group at other critical stages during litter decomposition. The results here provide efficient evidences in understanding litter decomposition and its relationship with plant growth in subtropical evergreen broad-leaved forests.