| Litter decomposition is the vital link between material circulation and energy flow in forest ecosystem,and litter decomposition plays an important role in maintaining soil fertility,sequestering carbon（C）and improving soil biodiversity.Litter decomposition is affected by global change factors,such as increased nitrogen deposition and altered precipitation regimes.Under the background of global change,it is of great significance to understand the decomposition characteristics of leaf litter in natural evergreen broad-leaf forest for a comprehensive understanding of the material cycle of forest ecosystem and their response to climate change.To understand the effects of N deposition and reduced precipitation on litter decomposition,a 2-year field litter decomposition experiment was conducted using the litterbag method in a natural evergreen broad-leaved forest in western China.Two levels of N deposition and three levels of throughfall reduction were established:control(CK;0 kg·N·ha-1·year-1),N deposition(N;150·kg·N·ha-1·year-1),T1（10%throughfall reduction）,T2（20%throughfall reduction）,NT1(150 kg·N·ha-1·year-1+10%throughfall reduction)and NT2(150 kg·N·ha-1·year-1+20%throughfall reduction).The simulated N deposition was around 160%of the ambient rate of wet N deposition.Linear mixed effects models employing the restricted maximum likelihood estimation method were applied to inspect effects of N deposition,throughfall reduction,sampling time and their interactions on the decomponstion rate,elements release,components degradation,as well as the activities of soil animals,microorganisms and enzymes during decomposition.Partial least square regressions were used to assess the relative importance of quality for leaf litter cumulative mass loss.Structural equation modeling were used to quantify the relative contributions of factors（soil animals,microorganisms and enzymes during decomposition as well as leaf litter water content and temperature）influencing litter decomposition,elements release and components degradation under the combined effects of N deposition and reduced precipitation were comprehensively analyzed.The results suggested that:（1）Throughfall reduction significantly decreased leaf litter water content,by contrast,the temperature in litterbags did not differ significantly between treatments during the study period.（2）During the whole two-year decomposition period,29,154 individuals of soil meso-and microfauna were captured,belonging to 52 families,13 orders and 4 classes of Arthropoda.Community structure and distribution of soil fauna were different in different treatments.N deposition and throughfall reduction increased the relative proportion of dominant groups of soil fauna during the decomposition.N deposition significantly reduced the individual density of soil fauna during decomposition of leaf litter;10%reduction of throughfall significantly increased the individual density of soil fauna,and the interaction between N deposition and throughfall reduction on individual density were not significant.N deposition,throughfall reduction and their interaction had no significant effects on the number of soil animal groups during the decomposition.N deposition significantly increased the Pielou evenness index of soil fauna during the decomposition,whereas had no significant influence on shannon-wiener diversity index,Simpson dominance index and Margalef richness index.10%reduction of throughfall significantly reduced shannon-wiener diversity index and Pielou evenness index.Furthermore,throughfall reduction significantly increased the Simpson dominance index,but had no significant effect on the Margalef richness index.The interaction between N deposition and throughfall reduction significantly affected shannon-wiener diversity index,Pielou evenness index and Simpson dominance index,but had no significant influence on Margalef richness index of soil fauna.（3）N deposition significantly decreased microbial biomass carbon,nitrogen and phosphorus（MBC,MBN and MBP）during the decomposition period,20%reduction of throughfall significantly decreased MBC,whereas 10%reduction of throughfall significantly increased MBN,throughfall reduction significantly decreased MBP.N deposition had no significant effect on microbial biomass C-to-N ratio and C-to-P ratio during the decomposition,whereas 10%reduction of throughfall significantly increased the C-to-N ratio,and throughfall reduction significantly increased the C-to-P ratio.The interaction between N deposition and throughfall reduction on MBC,MBN and MBP as well as microbial biomass C-to-N ratio and C-to-P ratio were not significant.（4）N deposition significantly reduced the activities of sucrase and catalase in the leaf litter during decomposition period,however,significantly increased the activity of peroxidase and had no significant effect on urease and acidic phosphatase.Throughfall reduction significantly reduced the activities of sucrase and urease,but had no significant effect on acidic phosphatase,peroxidase and polyphenol oxidase.20%reduction of throughfall significantly reduced the activity of catalase.The interaction between N deposition and throughfall reduction significantly affected the activities of sucrase,urease and peroxidase,but not acid phosphatase,catalase and polyphenol oxidase.（5）N deposition and throughfall reduction significantly inhibited the leaf litter decomposition in the evergreen broad-leaved forest,whereas their interaction had no significant effect on the cumulative mass loss,decomposition constant and limit value,indicating that N deposition and throughfall reduction had no significant interaction effect on leaf litter decomposition.Leaf litter quality during decomposition explained the most variations of cumulative mass loss.We found that the concentrations of manganese（Mn）,zinc（Zn）,copper（Cu）and total phenols（Phen）explained most of the variation in cumulative mass loss among all treatments.The quality control factor of litter decomposition in different treatments was Mn concentration during the decomposition period,regardless of N deposition and throughfall reduction.The PLS variable of importance of projection values of Ctr,N,T1,T2,NT1 and NT2 were 1.93,2.06,2.11,1.95,1.65 and 1.69,separately.Under the combined effects of N deposition and throughfall reduction,the biggest influencing factor account for litter decomposition was microbial biomass,and the standardized total effect value was-0.62,indicating that microorganisms play a regulatory role in litter decomposition under the background of increasing N deposition and decreasing precipitation in the future.（6）The macronutrients of C,N,potassium（K）and magnesium（Mg）generally showed release pattern during the two-year decomposition period,while P and calcium（Ca）showed immobilization pattern,there were no net release of P and Ca from litter until short before the end of the experiment.N deposition and throughfall reduction did not change the release or immobilization pattern of the macronutrients.N deposition significantly reduced the releases of C,N,K and Mg,and throughfall reduction significantly reduced C releases;20%reduction of throughfall significantly reduced the releases of N and K as well as Ca immobilization,whereas increased P immobilization.The interaction between N deposition and throughfall reduction on macronutrients release or immobilization were not significant.Under the combined effects of N deposition and throughfall reduction,the biggest influencing factor account for the dynamics of C,N,K and Mg was microbial biomass,and the standardized total effect value were-0.57,-0.52,-0.59 and-0.40,separately;and the factor account for the P and Ca dynamics was oxidoreductase activity,and the standardized total effect value were 0.80 and 0.79,separately.Those results indicated that microorganisms play a regulatory role in C,N,K and Mg release and oxidoreductase activity in P and Ca immobilization under the background of increasing N deposition and decreasing precipitation in the future.（7）The micronutrients of iron（Fe）and Zn generally showed immobilization patterns during the two-year decomposition period,while Mn showed release pattern and Cu showed immobilization-release pattern,N deposition and throughfall reduction did not change the release or immobilization pattern of the micronutrients.N deposition and 20%reduction of throughfall significantly increased the Fe immobilization,whereas throughfall reduction significantly reduced Mn release.The interaction between N deposition and throughfall reduction on micronutrients release or immobilization were not significant.Under the combined effects of N deposition and throughfall reduction,the biggest influencing factor account for the dynamics of Fe,Zn and Cu was oxidoreductase activity,and the standardized total effect value were 0.79,-0.45 and 0.65,separately;and the factor account for the Mn dynamic was microbial biomass,and the standardized total effect value was-0.66.Those results indicated that microorganisms play a regulatory role in Mn release and oxidoreductase activity in Fe and Zn immobilization as well as Cu immobilization-release under the background of increasing N deposition and decreasing precipitation in the future.（8）N deposition significantly suppressed the degradations of lignin,cellulose and total phenols,throughfall reduction had no significant effects on the degradation of lignin,cellulose,total phenols and condensed tannins,whereas the interaction between N deposition and throughfall reduction were significant.Under the combined effects of N deposition and throughfall reduction,the biggest influencing factor account for the lignin and total phenols dynamic were oxidoreductase activity,and the standardized total effect value were 0.55 and0.42,separately;the factor account for cellulose and condensed tannins dynamic were microbial biomass,and the standardized total effect value were-0.48 and-0.55,separately.Those results indicated that oxidoreductase activity play a regulatory role in lignin and total phenols degradations and microorganisms in cellulose and condensed tannins degradations under the background of increasing N deposition and decreasing precipitation in the future.In conclusion,N deposition and throughfall reduction significantly inhibited the decomposition of leaf litter in the evergreen broad-leaved forest,whereas their interaction was not significant.Leaf litter quality during decomposition explained the most variations of cumulative mass loss,the quality control factor of litter decomposition was Mn concentration during the decomposition period,regardless of N deposition and throughfall reduction.With increasing N deposition and decreasing precipitation in the future,microorganisms play a regulatory role in litter decomposition,C,N,K,Mg and Mn release as well as cellulose and condensed tannins degradation;and oxidoreductase activity in P,Ca,Fe,and Zn immobilization,Cu immobilization-release and lignin and total phenols degradation.The results of this study will help to understand the forest decomposition process and provide a theoretical basis for the regulation and management of the natural evergreen broad-leaved forest ecosystem. |
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