Effects of Leaf Litter Functional Traits on the Annual Turnover Rate of Litter Layer in Subtropical Plantations

Litter turnover is a key ecological process to maintain the material circulation and nutrient reutilization in forest ecosystems. Based on the Nutrient Addition and Biodiversity Ecosystem Function (NaBEF) experimental platform in Shanghang County of Fujian Province, seventeen common subtropical afforestation tree species were selected to determine the leaf litter traits, soil physical and chemical properties and microbial community structure characteristics. Then, the microbial mechanism affecting the annual turnover rate of litter layer was explored. The results showed that: the annual turnover rate of litter layer varied markedly among the 17 tree species, ranging from 0.80 in Lithocarpus glaber to 2.38 in Euscaphis japonica. The stepwise regression analysis of traits showed that the maximum water-holding rate of leaf litters had a significant positive effect on the annual turnover rate of litter layer, while the lignin content and nitrogen-phosphorus ratio of leaf litter had a significant negative effect on the annual turnover rate of litter layer. The stepwise regression analysis showed that the fungal to bacterial ratio, the Gram-positive to Gram-negative bacteria ratio and the total amount of phospholipid fatty acids had significant positive effect on the annual turnover rate of litter layer. The structural equation modeling further demonstrated that: the tree species with higher litter water-holding capacity could indirectly promote the turnover of litter layer by increasing the total amount of microorganisms in the soil, whereas the tree species with higher leaf litter nitrogen-to-phosphorus ratio inhibited the litter turnover by reducing the ratio of soil eubacteria. Although the increase of lignin content did not directly significantly affect the microbial community structure, it indirectly regulated the composition of microorganisms by reducing the soil pH. The studies have shown that the differences in leaf litter traits among tree species could regulate the annual turnover rate of litter layer through direct effects and indirect pathways mediated by the microorganisms or environmental factors, which provided theoretical support for the selection of tree species and the improvement of forest soil nutrients in subtropical plantations.