Long-term nitrogen addition in maize monocultures reduces in vitro inhibition of actinomycete standards by soil-borne actinomycetes

Management of soil microbial communities for enhanced crop disease suppression is an attractive approach to biocontrol, but the effects of agricultural practices on the disease-suppressive potential of the soil microbial community remain unknown. We investigated the effects of long-term nitrogen addition (103 kg ha−1 nitrogen as urea vs. no fertilizer) and crop residue incorporation vs. removal on in vitro antibiotic inhibitory capacities of actinomycetes from 57-year maize (Zea mays L.) monocultures in southeastern Minnesota. We hypothesized that both nitrogen and crop residue addition would increase inhibitor frequencies by increasing microbial population densities and thus increasing the importance of competitive interactions among microbes to their fitness. We found that although soil carbon and nitrogen and microbial densities (actinomycete and total colony-forming units) tended to be greater with nitrogen fertilizer, the frequency of in vitro inhibitory phenotypes among culturable actinomycetes in fertilized plots was approximately half that in non-fertilized plots. Residue incorporation had little to no effect on soil chemistry, microbial density and inhibitor frequency. These results suggest that density-mediated processes alone cannot explain the effects of amendments on inhibitor frequencies. Fitness costs and benefits of inhibitory phenotypes may vary over time and may depend on the type of resource amendment.