The Effects Of Soil Fauna On Litter Decomposition And Its Driving Mechanism In Zijin Mountain

Forest litter is an important storage of organic matters and nutrients.Litter decomposition paly a necessary role in maintaining forest ecosystem productivity,soil organic matter formation and community succession.With the population growth,fuel burning,atmospheric nitrogen(N)deposition has been greatly increased.Excessive atmospheric N deposition has led to a series of serious ecological problems such as soil acidification,imbalance of soil nutrient storage,and also having an important effect on litter decomposition process.Though studies about effects of three primary factors on litter decomposition process have been carried out widely,the interaction of the these factors in litter decomposition process and its regulatory mechanism on the functions of forest ecosystem are known less,especially the responses of soil decomposers to environment change and the plant and litter quality.Therefore,the effects of atmospheric N deposition,soil microenvironment and plant secondary compounds on litter decomposition were studied from the aspects of soil detritus food chain,and the interactive effects of climate change,litter quality and soil decomposers on the regulation of forest carbon cycling were discussed from the aspects of community composition and functional diversity of soil decomposers.The study of interaction of three factors has important theoretical and practical significance.In this study,three types of subtropical forests(broad-leaved forest,coniferous forest and mixed coniferous broad-leaved forest,respectively)in Zijin Mountain,China were chosen as the experiment sites.First,we collected the dominant species of Ouercus variabilis and Pinus massoniana litter in the broad-leaved forest and coniferous forest respectively,as experimental materials.Two kinds of litter were placed in the litter layer and soil layer of the two forests for a ten-month(2013.2-2013.12)field experiment.We compared the community composition of soil mesofauna,microbial biomass and soil enzyme activity involved in Q.variabilis and P.massoniana litter decomposition in litter layer and soil layer of the two forests.At the same time,the roles of soil fauna and microorganisms on litter decomposition were distinguished using fine mesh and coarse mesh litterbags.On the basis of this,to further explore the comprehensive regulation mechanism of soil decomposers,environmental factors and litter quality on litter decomposition,we performed a reciprocal litter transplant experiment(see Appendix 1)using coarse mesh and fine mesh litterbags under N addition treatment in the broad-leaved forest dominated by Q.variabilis and coniferous forest dominated by P.massoniana.The added N concentration was 47 kg ha-1 yr-1.The field transplant experiment lasted 10 months(2014.2-2014.12).Finally,at the decomposers community level,we carried out a laboratory incubation experiment to explore the effects of plant secondary compounds on the role of soil decomposers playing in litter decomposition.We obtained the aqueous extract and main secondary compounds,tannins extract from P.massoniana litter,respectively in the coniferous and broad-leaved mixed forest.The aqueous extract solution and tannins solutions were set as high and low concentration,respectively,and were evenly sprayed in the incubation boxes with Quercus soil or Pinus soil in which Q.variabilis litter decomposed.At the same time,all incubation boxes with Quercus soils or Pinus soils were divided into two groups:with isopods and without isopods.Distilled water was sprayed as control treatment(see Appendix 2).The experiment period is:2015.3-2015.9.During the above three experiments,we investigated litter decomposition rates,community composition of soil fauna and microbial biomass,soil physical and chemical properties(such as pH,moisture content,etc.)and related enzyme activities.The main results of this study are as follows:(1)No matter in broad-leaved forest or coniferous forest,the decomposition rate P.massoniana in coarse mesh litterbags was higher in soil layer than in litter layer(p<0.05).However,vertical position of soil didn't influence Q.variabilis decomposition of both fine mesh litterbags and coarse mesh litterbags in two forests.The effect of position on litter decomposition was significantly related to litter type(F=13.624,p=0.001),but not to forest type(F=0.757,p=0.391).No matter in broad-leaved forest or in coniferous forest,both the abundance and species richness of soil fauna in soil layer were greater than in litter layer(p<0.01).Besides,due to the specificity of the detritivorous mites to recalcitrant P.massoniana litter,which result in the decomposition rate of P.masoniana was greater than Q.variabilis in soil layer.However,the soil enzymes activities did not have a significant effect in two litter types decomposition in fine mesh litterbags(p=0.552).(2)There no significant difference in decomposition between the two dominant litters in the fine-mesh litterbags at home and away habitats under control and N addition plots.P.massoniana litter in the coarse-mesh litterbags decomposed twice as fast at home than in away habitats under N addition(decomposition constant k at home was 0.36 vs.0.18 at away habitat,p<0.05).The result suggests a positive HFA effect in the coniferous forest under N addition(HFAI = 26.892,p =0.044),but no significant HFA effect was observed in the control plots.N addition did not enhance O.variabilis litter decomposition in the home habitat.The positive HFA effect of P.massoniana litter in the coarse-mesh litterbags in N addition plots was associated with more abundant soil fauna than in the control plots.However,N addition had no significant effect on the activity of most soil enzyme during litter decomposition.Moreover,soil microbial biomass showed no relationship with the HFA of litter decomposition.Our findings suggest that N addition likely enhances the feeding activity of soil fauna by increasing fauna abundance.(3)In Pinus soils,litter decomposition was enhanced significantly after high concentration aqueous extract addition with isopods,decomposition rate was 1.16 times that of the control(p<0.05),suggesting that the higher decomposition rate with aqueous extracts addition was driven by micro-arthropods instead of microorganisms.Tannins addition significantly inhibited O.variabilis litter decomposition(50.6%lower than the control,p<0.05)and soil microbe biomass in Quercus soils,especially high concentration tannins.However,there was no significant reduction in the litter decomposition after tannin addition in the Pinus soils.This partly due to soil microorganisms and isopods that were well adapted to local resources,particularly Pinus soils rich in secondary compounds.The study tested the inhibition of tannins on soil processes by comparing it with water-soluble secondary compounds,suggesting that secondary compounds could reduce litter decomposition through tannins.In summary,we illustrated:(1)the effect of soil vertical positions on litter decomposition mainly due to the changes in soil fauna abundance and community composition largely caused by changes in the microenvironment and substrate specificity of soil fauna;(2)N addition could reinforce the habitat specificity of soil mesofauna in coniferous forests,resulting in a positive HFA of P.massoniana litter decomposition;(3)the potential role of secondary compounds in influencing ecosystem processes relating to decomposition and as a driver of soil mesofauna under a certain condition.Our study provides a reliable way to realize the effective regulation of litter,understand the matter cycling of terrestrial ecosystem and study the structure and function of ecosystems.