Compost extract added to microcosms may simulate community-level controls on soil microorganisms involved in element cycling

Interactions among populations of soil microorganisms might alter soil microenvironments sufficiently to allow populations collectively to inhabit sites which individually they could not inhabit. We tested the hypothesis that soluble microbial products in soil microenvironments mediate commensalistic interactions among populations involved in N2-fixation, denitrification, sulfate reduction and P solubilization. We measured the growth of bacteria in microcosms amended with sterile compost extract. Of the 7 Azospirillum isolates tested, 3 fixed more N2 when amended with 50 μg compost extract-C ml−1 medium. Fixation of N2, by one isolate amended with NH4Cl or compost extract decreased with increasing concentrations (0.15–15 μg N ml−1 medium) of NH4Cl, but not with increasing concentrations of compost extract. Optical density of cultures of Bacillus sp. increased 6-fold with addition of 11 μg compost extract-C ml−1 medium under denitrifying conditions. Adding 6 μg compost extract-C ml−1 medium stimulated the growth of all 10 sulfate-reducing enrichment cultures, and three did not grow without compost extract. Addition of 10 μg compost extract-C ml−1 medium, however, increased microbial-P in only one of the 10 cultures in P-limiting medium. This evidence is consistent with the hypothesis that exchange of growth factors among populations in microenvironments contributes to control of microorganisms involved in element cycling.