Effects of management history and legume green manure on soil microorganisms under ‘organic’ vegetable production

Abstract

Pasture and conventionally cultivated vegetable cropping land were converted to ‘organic’ vegetable cropping, where synthetic fertilisers and pesticides were not used. Compost was applied and 0, 1, or 2 legume green manure crops grown in rotations. Soil bacterial numbers and fungal hyphal length (microscopic observation), microbial biomass carbon (C) and nitrogen (N) (fumigation–incubation): and water content were monitored over 18 months. All variables were initially greater in ex-pasture than in ex-vegetable soils. Bacterial count declined during the first 6 months under both ex-pasture and ex-vegetable, and after 2 months there was no difference between histories. Microbial biomass C and N declined during the first 6 months in ex-pasture soils, but remained larger than in ex-vegetable soils, in which there was little change in microbial biomass C and N with time. Fungal hyphal length showed no decline with time and remained greater under ex-pasture. Soil water content was greater under ex-pasture for the duration of tile experiment. Crop rotation had no effect in ex-pasture soils. In ex-vegetable soils, water content, fungal hyphal length, and microbial C and N were increased by the inclusion of 2 (and sometimes 1) legume phases in the relation, but only after the legume residues had been substantially decomposed. The history and rotation effects were attributed to greater organic matter and water contents.