Indirect effects of earthworms on microbial assimilation of labile carbon

Interactions between earthworms and microorganisms can be important in regulating the rate of soil carbon turnover and maintaining soil fertility in agroecosystems. Despite the significance of earthworms in nutrient cycling in agroecosystems, the indirect influence of earthworms on C assimilation by microorganisms has not been adequately quantified. We assessed microbial assimilation of C-13-labeled acetate in earthworm (Lumbricus terrestris) middens and surrounding soil collected from maize agroecosystems. Incorporation of C-13 into microbial lipids was used as an indicator of microbial growth rates. Earthworm middens had significantly lower concentrations of microbial phospholipid phosphates and lower natural abundance delta(13)C than the surrounding soil. After incubation with C-13-labeled acetate, microbial communities in earthworm middens had greater C-13/C-12 ratios of microbial lipids than microbial communities from surrounding soil. The C-13 enrichment per unit of microbial phospholipid was much greater in middens than in surrounding soil indicating that: (i) microbial lipid synthesis was significantly higher in the earthworm middens; (ii) microbial assimilation efficiency for C-13-labeled acetate was greater in midden soil; or (iii) assimilation of C-13-labeled acetate relative to other C sources was proportionately greater in middens than in the surrounding soil. Our results suggest that there were functional differences between microbial communities in earthworm middens and surrounding soil, probably due to a combination of physical, chemical, and biological changes in the midden microenvironment. The resulting differences in microbial communities or activity increased microbial growth rates and assimilation of readily available C substrates in middens relative to surrounding soil. (C) 2002 Elsevier Science B.V. All rights reserved.