Soil Microbial Populations and Activities under Conventional and Organic Management

Evaluation of microbial populations and activities, and their relationship to N cycling in soils under organic and conventional farm management was conducted in eastern Nebraska in 1981 and 1982, on an experiment initiated in 1975. The experimental treatments consisted of 3 × 4 factorial with three management systems (organic, fertilizer only, and fertilizer plus herbicide) for a 4-yr grain/legume crop rotation plus one treatment of continuous corn (Zea mays L.) receiving fertilizer, herbicide, and insecticide (including one subplot without insecticide). Soil physical, chemical, and microbiological characterizations were made at soil depth intervals of 0 to 7.5, 7.5 to 15, and 15 to 30 cm. Soil chemical properties were significantly influenced by chemical management, primarily the application of beef (Bos taurus) feedlot manure in the organic management system. Total organic C, Kjeldahl N, and potentially mineralizable N in manure-amended surface soils (0–7.5 cm) were 22 to 40% greater than nonmanured soils receiving fertilizer and/or herbicide. Soluble P levels were eightfold greater in manure-amended surface soils, and soil nitrate levels after harvest in 1981 were two- to threefold greater to a depth of 30 cm than nonmanured chemical treatments. Soil microbial biomass, bacterial and fungal counts, dehydrogenase activity, and CO₂ evolution were greater in soils planted to oat/clover (Avena sativa L./Trifolium pratense L. + Melilotus officinalis Lam.) and treatments receiving manure. Increases in microbial populations and their activities paralleled increases in soil organic C content, Kjeldahl N, and water-filled pore space. Differences in N₂ fixation and denitrification between crops and management systems were minimal—possibly resulting from suboptimal water availability at midseason sampling. No significant differences were found in measured soil physical, chemical, or biological properties due to herbicide or insecticide at field application rates. Contribution from the USDA-ARS in cooperation with the Agricultural Res. Div., Univ. of Nebraska, Lincoln. Published as Journal Series no.. 8493.