Influence of earthworms on microbial dynamics and nutrient cycling

Abstract

Functioning of the soil system is dependent on the composition and structure of the biotic community. Earthworms have a strong influence on nutrient cycling through associated changes in soil microbial biomass and activity and decomposition of returnable resources. The impact of earthworms on nutrient turnover varies spatially and with composition of the community. It has been established that the interaction between earthworms and soil microbes drive the patterns of increased nutrient turnover observed. Investigations of native and exotic assemblages of earthworms in Florida, and Puerto Rico were undertaken to understand resource use patterns, linkage with microbial dynamics and influences on nutrient cycling. Two laboratory incubation studies involved methods to 1. determine changes in microbial activity and N mineralization with activity of native Diplocardia mississippiensis from Florida, and 2. determine resource use by native Estherella sp. and exotic Pontoscolex corethrurus assemblages found in Puerto Rico and impacts on C and N mineralization. Using substrate induced respiration and microbial inhibitors it was found that D. mississippiensis stimulated both bacterial and fungal respiration and decreased microbial biomass C in proportion to total soil C. Greater soil respiration and greater N mineralization were attributed to the interaction of D. mississippiensis and microbial biomass C. Resource use by native and exotic assemblages was investigated with the use of materials enriched with 13C and 15N. Each species was found to have assimilated slightly different resources with variation in stable isotopic signatures varying with interaction between the two species. Mineralization C and N were influenced in the presence of both species. Individual assemblages of the exotic (P. corethrurus) stimulated more mineralization than individual assemblages of the native (Estherella sp.). The greater mineralization found with earthworms was attributed to soil turnover in earthworm cast production. Soil ingestion, gut passage, and cast production were established as the sites of earthworm-microbial interaction and the direct mechanism for impacts on nutrient dynamics. The data collected from the two laboratory incubation experiments was used to construct, calibrate and validate a nutrient cycling model. The model, developed with STELLA Software, was used to understand earthworm resource use patterns and differences due to interaction with microbial processes.