Effects Of Soil Fauna On Leaf Litter Decomposition In A Subalpine Forest

Litter decomposition is a key process of nutrient restoration in forest ecosystems and is closely related to plant growth and productivity.Climatic conditions,substrate quality and decomposer activity are the three most important factors to regulate litter decomposition.Soil fauna communities can affect the decomposition rates and element release process of litters by direct feeding,crushing,mixing and changing the decomposition microenvironment,and indirect controlling the quantity,structure and activity of soil microorganisms by selective predation.However,present studies on the effects of soil fauna on litter decomposition are mostly carried out by single physical barrier method(litterbags with different pore size)or chemical reagent removal method(e.g.naphthalene),and seldom involve the combination of the two methods to study the effects of soil fauna on litter decomposition.Therefore,the methods of biocide application(naphthalene addition or non-naphthalene)and litterbag method with different pore sizes(3mm,1mm and 0.05mm)were adopted to study the effects of soil fauna on litter decomposition of Minjiang fir(Abies faxoniana),birch(Betula albo-sinensis),poplar(Populus simonii)and bamboo(Fargesia spathacea)in a subalpine forest of western Sichuan from April 2016 to April 2018.The contribution of soil fauna to litter mass loss,elements release(C,N and P),and lignin and cellulose degradation were measured during litter decomposition.Moreover,the differences in faunal effects were compared between the biocide application method and litterbag method,this will favor to in-depth understanding of the roles of soil fauna in the biogeochemical cycle of subalpine forest.The main results are as follows:(1)After two years of decomposition,the litter mass remaining of the fir,birch,poplar and bamboo were 48.5-56.6%,44.9-54.12%,36.6-47.2% and 47.4-56.7%,respectively.The participation of soil fauna significantly reduced the litter mass remaining.The contribution of soil fauna to the mass loss of litters decreased with the increase of decomposition time.Under the condition of naphthalene treatment,the contribution rate of soil fauna was the highest after 120 days of decomposition.The contribution rates of soil fauna in the fir,birch,poplar and bamboo litterbags were 26.1%,22.1%,14.5% and18.7%,respectively.Contributions of soil fauna in the different pore sizes litterbag method reached a peak value between 120 days and 240 days of decomposition.Specificly,soil fauna contribution rates for the 3 mm litterbags of the fir,birch,poplar and bamboo were22.2%,18.3%,14.2% and 20.9% respectively.This for the 1 mm litterbags of the fir,birch,poplar and bamboo were 16.2%,22.3%,15.5%,23.3%.Contributions of the small and medium-sized soil fauna(1 mm pore size)in birch,poplar and bamboo were greater than that of the large soil fauna(3 mm pore size),but opposite results were observed for the fir The decomposition coefficient(k)of litters was the highest in the poplar litterbag and the lowest in the fir litterbag,respectively.The participation of soil fauna increased the decomposition coefficient(k)and shortened the decomposition time of litters.(2)During the whole study period,the organic carbon,nitrogen and phosphorus residues of the four litters showed similar dynamicss.The effects of soil fauna on the release of carbon,nitrogen and phosphorus from litter were significantly different compared with the research methods and tree species.Under the naphthalene treatment,soil fauna participated in the significant reduction of organic carbon residues in four tree species,and promoted the litter nitrogen release of of fir(1.1%)and litter phosphorus release of poplar(1.0%),but retarded the litter nitrogen release of birch(-3.2%)and poplar(-15.6%)and the litter phosphorus release of fir(-1.6%)and birch(-10.2%),respectively;for the different pore sizes litterbag treatment,there was no significant effect of soil fauna on the total organic carbon residue of four litters,and soil fauna involvement significantly promoted the nitrogen(3.4%)and phosphorus(5.1%)releases of poplar,but not for the other three litter.Moreover,the phosphorus release of the fir,birch and poplar was inhibited by soil fauna,and the contribution rates of soil fauna were-4.2%,-6.5% and-1.1%,respectively.(3)Soil fauna participation generally promoted the release of cellulose and inhibited the degradation of lignin,but did not reach a significant level under the conditions of the two soil fauna removal methods.The decomposition of cellulose and lignin by soil fauna is significantly different under the two methods.For example,under the first naphthalenetreatment,soil fauna promoted the cellulose decomposition of poplar,and the faunal effects on the other three types of litter were not noticeable,but soil fauna inhibited the lignin degradation of fir,birch,poplar and bamboo.Furthermore,bamboo had an important faunal effect in different pore size litterbag treatment.Soil fauna retared the decomposition of cellulose in four litters,which promoted the lignin degradation of fir,birch and poplar eventually.In conclusion,the mass loss,element release and component degradation of litter showed similar changes under the naphthalene treatment and different pore size litterbag treatment.Moreover,the participation of soil fauna accelerated the decomposition rate of litter and promoted the release of carbon,nitrogen and phosphorus from litter but shortened the decomposition time,which indicating that the two methods can be used to research soil fauna ecology in subalpine forests.Besides,the differences in degradation of cellulose and lignin under the two soil fauna treatment methods indicate that the components(such as lignin and cellulose)have a significant influence by the different research methods and soil fauna groups involved in decomposition.