Effects Of Soil Fauna On Leaf Litter Decomposition And Soil Respiration In Poplar Plantations

Soil fauna is an important component of the forest ecosystem and a key link to the soil food web.They would affect the litter decomposition process and soil organic carbon mineralization through burrowing,feeding,and crushing on soil and litter,which play an essential role in soil nutrient cycling and the energy transformation process.However,there is still limited research on the role and mechanism of soil fauna’s contribution to the litter decomposition and soil organic carbon mineralization process in the forest ecosystem.Poplar plantation is an important part of Chinese plantation,which area ranks second in our domestic plantation and is widely distributed in China.In this study,we first explored the effects of soil fauna on forest litter decomposition and its regulating factors through a global meta-analysis,and then combined with field control experiments（poplar plantation in northern Jiangsu as the research object）to analyze it at a local scale.We set up a two-year litter decomposition experiment（employed both different mesh size litterbag and naphthalene application to repel soil fauna）,and monitored the effects of soil fauna on soil carbon dioxide（CO₂）fluxes,as well as analyzing the effects of naphthalene application on soil fauna and fungal community structure to find the pros and cons of naphthalene application,for the aim of exploring the effects of soil fauna on litter decomposition,nutrient release,and soil respiration process in poplar plantations,and ultimately providing scientific basis for further reveal the mechanism of soil fauna effect on soil carbon cycling.The main results are as follows:（1）The global meta-analysis shows soil fauna contributes 30.9±6.8%（mean±standard error）on average to the forest litter decomposition rate.The influences of soil fauna on litter decomposition rate vary in the different forest ecosystems,with the strongest soil fauna effect occurring in tropical wet forest and the weakest occurring in cold or dry systems.The positive effects of soil fauna on litter decomposition increased with the mean annual temperature,mean annual precipititation,and cellulose concentration of the initial litter.Moreover,as the experiment duration increased（>360days）,the role of soil fauna might be weakened.（2）Naphthalene application greatly suppressed soil fauna abundance and taxonomic richness,and had no significant effect on soil microbial biomass carbon and nitrogen.The abundance of Thelephorales in naphthalene treated soil was significantly lower than in the control soil,and the abundance of Capnodiales was the opposite.The most abundant fungal phylum under TR are still Ascomycota and Basidiomycota.Thus,naphthalene application can result in both the suppressed soil arthropod abundance and the changed specific soil fungal taxa.（3）Soil fauna accelerated the decomposition of poplar leaf litter.After two years of field decomposition experiment,soil fauna contributed 19.5±4.9%to poplar leaf litter mass loss under naphthalene treatment method,and contributed 18.2±6.1%to poplar leaf litter mass loss under mesh-size litterbag method.Both naphthalene application and mesh-size litterbag are effective ways to repel soil fauna,and the variation trend of poplar leaf litter mass remaining denoted similar under these two methods.Combining naphthalene application and the smallest mesh size litterbag together would achieve a better effect on repelling soil fauna,and represent the effect of soil fauna to a greater extent.The litter lignin content increased first and then decreased during poplar litter decomposition.Structural equation model showed that soil fauna abundance not only directly promoted litter decay rate but also indirectly through influencing carbon and cellulose release.（4）During the experimental period,soil CO₂ flux under naphthalene treatment(0.18～4.94μmol·m^-2·s^-1)was higher than that in the control group(0.07～3.51μmol·m^-2·s^-1).This may be because repelling soil fauna reduced the inhibition of soil fauna to soil microbial respiration and root respiration.On the other hand,the application of naphthalene may stimulate the growth of some specific microorganisms and thus increase soil respiration.The temperature has a significant positive effect on soil CO₂flux,and it showed significant and rapid growth between 15～30℃under naphthalene treatment.Soil water content was negatively correlated with soil CO₂ flux,while the influence of precipitation on soil CO₂ flux was not significant.In conclusion,soil fauna would promote the global forest litter decomposition process,which is mainly moderated by mean annual temperature,mean annual precipitation,and the initial cellulose content of litter.Field control experiment shows that soil fauna not only directly promotes the litter decomposition rate but also indirectly by influencing the release of carbon and cellulose in the decomposition process.Lignin content first increased,and then decreased during poplar leaf litter decomposition.Both naphthalene application and mesh-size litterbag method showed similar effect on litter decomposition rate,and the soil fauna contribution assessed by naphthalene application is slightly higher than by mesh-size litterbag.Combining the smallest mesh size litterbag with naphthalene application would repel soil fauna better.Note that although naphthalene application could effectively suppress soil fauna abundance and taxonomic richness,it would also affect the relative abundance of specific soil fungal communities（Thelephorales and Capnodiales）,thus causing variations in the estimation of soil fauna respiration.Therefore,this study provides a new scientific basis for exploring the contribution of soil fauna to forest litter decomposition on a global scale.Meanwhile,our field experiments also demonstrated the contribution of soil fauna to the poplar leaf litter decomposition,nutrient release,and soil respiration process,and pointed out the reliability of naphthalene application,and its potential effects on soil fungi,to further investigate the influence of soil fauna on the mechanisms of the litter decomposition process and the soil carbon cycle in future studies.