Shifts in soil microflora induced by velvetbean (Mucuna deeringiana) in cropping systems to control root-knot nematodes

This project is part of work underway in our laboratories to test the hypot hesis that the induction of soil suppressiveness to plant parasitic nematod es that occurs following planting of velvetbean (Mucuna deeringiana (Bort) Merr.) is associated with the development of an antagonistic microflora in soils and rhizospheres. The specific objective of this investigation was to examine long-term microbial shifts associated with the use of velvetbean i n rotations to control nematodes. A crop rotation study was conducted in mi croplots, consisting of three crop cycles. Cycle 1 involved planting of eit her velvetbean or cowpea (Vigna unguiculata L.) in the first spring. Cycle 2 during the next fall and winter was fallow or cover-cropped with wheat (T riticum aestivum L.) or crimson clover (Trifolium incarnatum L.). Cycle 3 t he next spring was soybean (Glycine max (L.) Merr.). Populations and specie s diversity of bacteria and fungi in soils or rhizospheres were investigate d at the end of each cropping cycle. Rhizosphere fungal populations were si gnificantly smaller on velvetbean than on cowpea at the end of cycle 1. The use of velvetbean in cycle 1 significantly decreased rhizosphere bacterial populations on crops in cycle 2, compared to treatments which had cowpea i n cycle 1. Velvetbean also influenced bacterial diversity, generally increa sing frequency of bacilli, Arthrobacter spp. and Burkholderia cepacia, whil e reducing fluorescent pseudomonads. Some of these effects persisted throug h cycle 3. Fungal diversity was influenced in cycle 1 by velvetbean; howeve r, effects generally did not persist through cycles 2 and 3. The results in dicate that the use of velvetbean in a cropping system alters the microbial communities of the rhizosphere and soil, and they are consistent with the hypothesis that the resulting control of nematodes results from induction o f soil suppressiveness. (C) 2000 Academic Press.