Incorporation of plant residue in corn monoculture selectively enriches antagonistic phenotypes among lignin-degrading vs non-degrading Streptomyces

Abstract

Competition for limited carbon resources in the soil can drive complex microbe-microbe dynamics including selection for antagonistic resource competitors. Lignin, one of the main components of plant residue, is one source of carbon input into soil. However, the highly recalcitrant nature of lignin requires degrading organisms to invest in extracellular enzymes, whose breakdown products are susceptible to poaching by opportunistic competitors. Here, we tested two hypotheses: 1) lignin-degrading Streptomyces are more inhibitory than non-degrading isolates, and 2) competition for lignin contributes to the frequency and intensity of antagonistic phenotypes. We collected 134 Streptomyces from the soil of 55-year corn monocultures that had been managed in a +/- fertilizer by +/- residue removal factorial design. Using a subset of 24 lignin-utilizing and 24 non-utilizing isolates we measured the inhibition of each isolate against ten Streptomyces standards. Lignin-degrading Streptomyces had larger mean zones of inhibition than non-lignin-degrading populations, but only in plots where residue was retained, suggesting competition for lignin promotes the evolution or maintenance of antagonistic interactions when plant residues provide abundant lignin substrates. Among non-lignin-degrading Streptomyces, there was no difference in the frequency or intensity of antagonistic phenotypes with fertilizer or residue practices. Our results suggest that competition for byproducts of lignin degradation by lignin-degrading Streptomyces imposes selection for antagonistic phenotypes in habitats with regular residue inputs, but not in non-lignin-enriched soils. This work has potential implications for the development of antibiotic-mediated disease suppressive soils, the survival of plant pathogens within residue, and the structure and function of the soil microbial community.