Effect And Mechanism Of Fine Roots Of Litter Layer On Litter Decomposition In Subtropical Plantations

Litter decomposition plays an important role in forest ecosystem.Fine root growth in litter layer is also a common phenomenon,but there are few reports on the effect of fine root on litter decomposition.The sample is located in Sanming Castanopsis kawadamii Nature Reserve(26°10?N,117°27?E).Phoebe zhennan and Castanopsis kawadamii plantations were selected in this study.Phoebe zhennan is an arbuscular mycorrhizal fungus(AMF)specie and Castanopsis kawadamii is an ectomycorrhizal fungus(EMF)specie.In this experiment,four treatments(control treatment,mycorrhizal fungi treatment,rootless treatment and mycorrhizal + dead root treatment)were set up in the two plantations,taking the litter leaves in litter bags at different decomposition time as the research object.Contribution of fine roots to litter decomposition can be judged by comparison of control treatment and rootless treatment.And the contribution of mycorrhizal fungi(AMF,EMF)in litter decomposition can be inferred by comparing mycorrhizal fungi treatment and rootless treatments.The effects of nutrient uptake by fine roots and root exudates on litter decomposition can be evaluated by comparing the control treatment and mycorrhizal + dead root treatment.In addition,the physicochemical properties of fine roots in litter layer and 0-10 cm soil layer were compared,the dry weight and nutrients remaining rate of litter leaves at different decomposition stages were calculated,and the microbial enzyme activities of litter leaves at different decomposition stages was measured.Accordingly,the effects of fine roots on litter decomposition through nutrient uptake,mycorrhizal fungi and root exudates,and the changes of microbial enzyme activities induced by fine roots were evaluated.The main conclusions are as follows.(1)The branching density,branching ratio,N and P contents of fine roots in litter layer of two tree species were significantly higher than those in 0-10 cm soil layer.while the diameter,C/N and C/P of fine roots were significantly lower than those in 0-10 cm soil layer.It indicated that the morphological structure of fine roots was also changed with sufficient nutrients in litter layer,which made fine roots easier to obtain nutrients and performed nutrient absorption function as well as accelerated nutrient releasing.(2)The mass remaining rate and litter nutrients remaining rate in the control treatment of Phoebe zhennan and Castanopsis kawakamii plantations were significantly lower than those in rootless treatment.The microbial enzymes activities of litter leaves in the control treatment of two species were significantly higher than those in rootless treatment.These results suggest that fine roots in litter layer may enhance microbial enzyme activities and promote litter decomposition and nutrient releasing through mechanisms of nutrient uptake,mycorrhizal fungi and root exudates.(3)After three months of decomposition of Phoebe zhennan litter leaves,P remaining rate was significantly reduced by 16.83% under mycorrhizal treatment compared with that under rootless treatment and the activities of AP and NAG were significantly higher than that without roots.After six months of decomposition,the N remaining rate under mycorrhizal treatment was significantly reduced by 21.46% compared with that under rootless treatment and the activities of PPO and PER were significantly higher than that under rootless treatment.The results of the study showed that arbuscular mycorrhizal fungi(AMF)could increase microbial enzyme activities and promote litter decomposition and nutrient releasing.However,during one year decomposition period of Castanopsis kawakamii litter leaves,the mass remaining rate and N,P remaining rate under mycorrhizal treatment had no significant difference compared with the remaining under rootless treatment,which indicated that ectomycorrhizal fungi(EMF)had no significant effect on the litter mass remaining and nutrient releasing.(4)The mass remaining rate and C,N,P remaining rate of litter leaves in Phoebe zhennan and Castanopsis kawakamii under control treatment were significantly lower than that in mycorrhizal + dead root treatments.The results showed that nutrient uptake by fine roots and root exudates could promote dry weight loss and nutrient release of litter leaves,and root exudates played a major role in both.Nutrient competition between fine roots and microorganisms limits nutrients in litter layer and stimulates the microorganisms to secrete extracellular enzymes and mineralized organic matter(N mining).Moreover,root exudates can directly provide energy to microorganisms and stimulate the microorganism activity.