Rhizosphere microorganisms affected by soil solarization and cover cropping in Capsicum annuum and Phaseolus lunatus agroecosystems

Abstract

Field experiments were conducted to evaluate the effects of soil solarization or cover crop on bell pepper (Capsicum annuum) and lima bean (Phaseolus lunatus) rhizosphere microorganisms. In Experiment I, flat surface solarization (FSS), raised bed solarization (RBS), cowpea (Vigna unguiculata) cover crop alone (CP), and the combination of cowpea cover crop followed by raised bed soil solarization (CP+RBS) were compared with methyl bromide fumigation (MB) and an untreated control (UTC) for effects on rhizosphere bacteria and fungi on bell pepper. In Experiment II, effects of cover crop on rhizosphere microorganisms were examined on lima bean in a 3×3 split-plot experiment with three summer cover crop treatments: sunn hemp (SH) (Crotalaria juncea), cowpea (CP), and fallow (F) as main plots, and three organic amendments: sunn hemp hay (SHH), cowpea hay (CPH), and no hay (NH) as subplots. Rhizosphere bacteria including siderophore producers, gram-positive spore-formers, and fluorescent pseudomonads were assessed, as well as two fungi, Fusarium oxysporum and Rhizoctonia solani. In year 1 of Experiment I, CP+RBS reduced F. oxysporum and R. solani in the rhizosphere, resulting in levels similar to those in MB fumigation. This was not observed in year 2. Heavy rainfall in year 2 resulted in a severe outbreak of Pythium on bell pepper, which was most destructive in MB fumigated plots. Effects of solarization and cover crop on rhizosphere bacteria also differed between years. Fluorescent pseudomonads were abundant across treatments in year 1, but were greatly reduced in year 2, when the Pythium outbreak occurred. In Experiment II, populations of rhizobacteria on lima bean were not affected by cover crop in either year. At the end of year 1, isolation of R. solani from lima bean roots increased in SHH plots. SH cover crop reduced gram-positive spore forming bacteria and fluorescent pseudomonads. In year 2, CP followed by CPH had higher levels of fungi and bacteria than CP followed by either NH or SHH. Overall, cover crop amendments did not result in consistent enhancement of rhizobacteria, and some treatments resulted in increased abundance of pathogenic fungi. The most consistent treatment for reducing rhizosphere fungal populations was MB.