Effects And Mechanism Of Throughfall Exclusion On Leaf-litter Decomposition In Subtropical Forests

The decomposition of litter is a basic process of the biogeochemical cycle,which has an important regulating effect on the carbon(C)and nutrient mineralization of terrestrial ecosystems.Studying the response of litter decomposition to precipitation reduction is essential for understanding the dynamics of terrestrial C and its feedback to climate change.In order to understand the impact of reduced precipitation on the decomposition of subtropical litter,this study selected three types of litter(Castanopsis carlesii,Cunninghamia lanceolata,and mixture of Castanopsis carlesii and Cunninghamia lanceolata)as the research object.In October 2011,we established three treatments,containing control without throughfall exclusion(CK),30%of throughfall exclusion(PE1),and 60%of throughfall exclusion(PE2)in a natural Castanopsis carlesii forest in Sanming City,Fujian Province.Leaf litters were placed in different treatments for one-year decomposition.This experiment was to explore the response of leaf litter decomposition to throughfall exclusion and its influencing factors.The research results showed as follows:With the increase of decomposition time,the percentage of three types of leaf litter mass remaining showed a steady downward trend,but the decline range of dry mass remaining rate of different litters was different,which was represented by Castanopsis carlesii>mixture of Castanopsis carlesii and Cunninghamia lanceolata>Cunninghamia lanceolata litter.From the perspective of different throughfall exclusion treatments,the mass remaining rates of the three types of litter were the lowest in the CK treatment.In particular,the decrease in throughfall has a significant inhibitory effect on the mass remaining rate of Castanopsis carlesii leaf litter,which indicated the different litter quality.There was the difference in the substrate loss response to different intensity of throughfall exclusion.For CK and PE2 treatments,the contents of litter C,N,extractables,cellulose and lignin and C/N,N/P,lignin/N ratios had significant effects on leaf litter decomposition,while for three treatments,the effects of cellulose and N/P ratio on leaf litter decomposition were significant.The effects of nutrient elements and chemical composition of litter on decomposition were different between throughfall exclusion treatments.Generally speaking,litter quality had the largest effect on leaf litter decomposition in CK and the least effect in the PE2 treatment,which indicated that compared to the low-quality litter(Cunninghamia lanceolata),the decompositons of the high-quality litter(Castanopsis carlesii)and the litter mixture of Castanopsis carlesii and Cunninghamia lanceolata were more sensitive to throughfall exclusion.Due to the significant differences in the substrate quality of three types of leaf litters,and the significant differences in the microbial biomass,community structure,and extracellular enzyme activities of different litters,the mechanisms were different during the leaf decomposition.Redundant analysis showed that the microbial community structure and extracellular enzyme activities mainly regulated the response of leaf litter decomposition to different throughfall exclusion treatments.Among them,the?G and GP/GN ratio of Castanopsis carlesii leaf litter,the AMF and?G of Cunninghamia lanceolata leaf litter,and the MBC,NAG,AMF and AP of litter mixture of Castanopsis carlesii and Cunninghamia lanceolata were the important factors that caused different response of leaf litter decomposition to throughfall exclusion.The main driving factors affecting soil microbial community structure were different under different decomposition stages and throughfall exclusion treatments.The main driving factor in the P₀ stage was soil mineral N,in the P₁₈₀ stage were soil SWC and DOC,and in the P₃₆₀ stage were soil SWC and DON.These indicated that the potential changes in soil microbial community structure after throughfall exclusion were mainly affected by soil substrate availability and nutrient utilization efficiency.The basic soil physical and chemical properties and the structure of microbial community under different throughfall exclusion treatments affected the decomposition of three types of leaf litters.The redundancy analysis found that SOC was the main influencing factor for the decomposition of Castanopsis carlesii leaf litter,soil DOC was the main factor influencing the Cunninghamia lanceolata leaf litter decomposition,while soil p H,soil water content and mineral N were important for the decomposition of litter mixture.In addition,soil microbial biomass and community structure were also the important factors affecting the decomposition of leaf litter under different throughfall exclusion treatments.The decomposition coefficient of Castanopsis carlesii leaf litter was significantly correlated with soil MBC,MBN,MBP,GP,AMF,and T_PLFA.The decomposition coefficient of Cunninghamia lanceolata leaf litter was significantly correlated with soil MBC and MBN and the decomposition of litter mixture had a significant correlation with soil MBC,MBN,MBP,GP and AMF.In summary,by simulating the impact of reduced precipitation on the decomposition of three leaf litters in the subtropical forest and analyzing their mechanisms,it can provide theoretical guidance for assessing forest ecosystem C and nutrient cycling,and predict the future global climate change.