Fertilizer and Residue Incorporation Effects on Pathogen Inhibitory Potential of Soil Streptomyces.

Antibiotic-producing bacteria can suppress soil-borne crop diseases. For example, increases in the density and frequency of pathogen-inhibitory Streptomyces in soil have been associated with reduced pathogen survival and disease severity in several crops. Ecological and evolutionary models predict that selection for antibiotic production will be strongest at intermediate to high resource availability. Crop management, including fertilization and crop residue incorporation, may alter the amount of resources available to soil microbes. We studied how long-term fertilization and crop residue management affect the frequency of antibiotic production and resistance in Streptomyces, a ubiquitous genus of antibiotic–producing bacteria. Streptomyces isolates (n = 148) were collected randomly from soil in a long-term corn (Zea mays L.) monoculture experiment in southeastern MN. Fertilizer (103 kg ha^-1 nitrogen versus no fertilizer) and above-ground residue (removed versus incorporated) treatments have been maintained in these plots for 57 years. Streptomyces isolates were assayed to determine their ability to inhibit target organisms, resist antibiotics, and grow on 95 carbon sources. Soil total carbon and nitrogen and Streptomyces density were also measured. Fertilizer increased soil carbon and nitrogen (p < 0.05), while crop residue incorporation had no effect. Fertilizer increased Streptomyces density (p = 0.05), but reduced the frequency of Streptomyces isolates that inhibited at least one target from 42% to 25% (p = 0.01). Both fertilizer and residue incorporation reduced inhibitor density (p < 0.05). The frequency of resistance to antibiotics was greatest where plots received both fertilizer and crop residue (p = 0.07). Neither fertilizer nor residue incorporation affected carbon use phenotypes. Contrary to our expectations, high resource availability in fertilized plots reduced the Streptomyces community’s inhibitory potential. Further work to unravel the mechanism behind this effect is essential before specific methods of managing the soil microbial community for disease suppression can be recommended.