Soil Microbial Populations and Activities in Reduced Chemical Input Agroecosystems

This study, conducted in the Piedmont of North Carolina, was initiated to determine how reductions in N fertilization and green-manuring with crimson clover (Trifolium incarnatum L. cv. Tibbee) would affect populations and activities of soil microorganisms. Four continuous corn (Zea mays L.) treatments were used: no-till (receiving herbicides and soil insecticides) with 0 or 140 kg N ha⁻¹ as NH₄NO₃; conventionally tilled, receiving 140 kg N ha⁻¹, but no pesticides; and conventionally tilled with a crimson clover green manure, but no fertilizer or pesticides. Populations were determined using selective media for culturable bacteria, gram-negative bacteria, fungi, actinomycetes, Bacillus spp., and Pseudomonas spp. Microbial activities were estimated by enzyme assays for acid and alkaline phosphatase, arylsulfatase, and β-glucosidase. Microbial biomass C was determined by a chloroform fumigation-extraction procedure and levels of available N were measured after anaerobic incubation. Surface soil (0–7.5 cm) from the no-till treatment receiving 140 kg N ha⁻¹ contained significantly more fungi than did soil from the unfertilized, no-till treatment. Microbial biomass C and available N were not affected by N addition, but levels of acid phosphatase and β-glucosidase were significantly higher in the fertilized soil than in the unfertilized soil. Surface soil from a crimson clover-corn rotation contained significantly larger populations of Bacillus spp. (260% more), actinomycetes (310% more), and culturable bacteria (120% more) than did soil from the well-fertilized conventionally tilled, no-pesticide treatment. Also, microbial biomass, available N, and levels of alkaline phosphatase, arylsulfatase, and β-glucosidase were significantly higher in surface soil from the crimson clover treatment than the nonmanured soil. Although the soil biological properties changed significantly during the year, seasonal variations were similar across treatments. Microbial numbers and activities were high in the spring and fall and low during the late summer. This work was supported in part by a grant from the USDA-LISA Program and North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products used nor criticism of similar ones not mentioned.