Earthworm Effects On The Transformation Of Plant Litter Into Soil Carbon And Nitrogen Fractions

Plant litter is one of the main source of soil carbon and nitrogen.Available carbon and nitrogen released in litter decomposition is of importance in energy flow fueling plant and soil food webs.In the background of rising green-house gas emissions,understanding the controlling factors of the formation and mineralization of soil carbon and nitrogen could help model predictions of future changes in soil and atmospheric carbon and nitrogen pools.The functional role of soil organisms in the transformation of plant litter into soil carbon and nitrogen fractions is still unclear,in particular for soil fauna.This research firstly reviewed previous studies on the efficiency of litter mass loss transformed into soil carbon and nitrogen with isotopic techniques.Second,using 13C/15N labeled litter we explored how earthworms transformed plant litter on soil surface into soil carbon and nitrogen fractions to understand the role of soil fauna in regulating soil carbon and nitrogen pools.By mixing five different plant litters with soil,we compared different fractions of soil carbon and nitrogen pools with the presence or absence of earthworms to explore how earthworms affect soil carbon and nitrogen mineralization by mediating soil microbial communities.Finally,we studied the effects of earthworms on soil available carbon and nitrogen and soil microbial communities in the rhizosphere.The main results are as follows:1.The data re-analysis showed that in the litter decomposition process,about 25%of the litter lost carbon was recovered in the soil,and the recovery efficiency of the litter lost nitrogen in the soil is affected by the experiment type.For field studies,the litter nitrogen recovery rate is 80%,and 60%for the laboratory studies.We found the efficiency of litter nitrogen transferred into soil also decline with decomposition stage.However,unlike litter decomposition rate decreases with decomposition stage,we did not found correlation between decomposition stage and the efficiency of litter carbon recovered in the soil.Effects of plant litter application methods on litter carbon and nitrogen recovery efficiency are different.Litter carbon recovery efficiency is the highest when litter was applied on soil surface,while litter nitrogen recovery efficiency is the highest when litter was mixed with soil.Besides,compared to high-quality litters,low-quality litters are more efficient in transforming to soil carbon and nitrogen fractions.2.Earthworms can accelerate litter decomposition rate and enhance the transformation of plant litter into soil carbon and nitrogen.After six months incubation,the dry weight of remaining litter in the no soil fauna treatment was 0.9 g,the treatment with only millipedes or only earthworms has 0.6 g remaining litter,the treatment with both millipedes and earthworms has only 0.3 g remaining litter.Soil arthropods-millipedes,feeding on plant litter on soil surface,can accelerate the fragmentation of litter and produce a large amount of fecal pellets,therefore enhance the transformation of litter into carbon and nitrogen in surface soil(0-2 cm).The vertical movement of earthworms can accelerate the transfer of plant litter on soil surface to deeper soil layer.Compared to no soil fauna treatment,the presence of millipedes or earthworms increased the total amount of 13C in the soil by 30%,and adding both millipedes and earthworms increased the total amount of 13C in the soil by 50%.The presence of soil arthropods-millipedes accelerate the fragmentation of litter,which further promoted the transfer of organic matter from the surface layer to the deeper soil layer by earthworms.3.The effects of earthworms on soil carbon mineralization is affected by litter quality,but earthworms showed no significant effects on soil nitrogen pools.In general,earthworms promoted the mineralization of organic carbon under high-quality litter(low carbon-to-nitrogen ratio),but did not affect the mineralization of soil organic carbon under low-quality litter(high carbon-to-nitrogen ratio).Under high-quality litter with higher available nitrogen resource,earthworms increased carbon-degrading enzyme activities and increased the proportion of C-degrading enzyme relative to N-degrading enzyme hence leading to soil carbon loss;while under low-quality litter with lower available nitrogen resource,earthworms did not increase carbon-degrading enzyme activities and decreased the proportion of C-degrading enzyme relative to N-degrading enzyme hence having no effects on soil organic carbon.This demonstrated the role of resource quality in regulating the function of earthworms in controlling soil carbon and nitrogen losses.4.The presence of earthworms increased the release of available nutrient from soil organic matter,and enhanced plant growth and nutrient uptake.Moreover,earthworm effects on plant growth and nutrient uptake were depend on plant species.Earthworms only increased the biomass of Lolium perenne and Zea mays by 20%,but have no effects on other plant species.Earthworm activity overall increased the content of microbial biomass carbon as a short-term carbon pool and promoted enzyme activities and soil carbon and nitrogen mineralization.To conclude,earthworms accelerated both the decomposition of plant liter and the transformation of plant litter into soil carbon and nitrogen pools.Furthermore,earthworms could increase the mineralization of plant litter-derived soil carbon and nitrogen.The effects of earthworms on the mineralization process is dependent on litter quality.Earthworms affect the mineralization of soil organic carbon by changing microbial activity,regulating the production of C-and N-degrading enzymes,and changing the structure of microbial community.This study will help to understand how earthworms influence the different processes of soil organic matter transformation,and the functional role of soil fauna in global carbon and nitrogen modeling and predictions for climate change.