Drivers of Soybean Rhizobia Diversity in Uninoculated Soils of Smallholder Farms in Malawi.

Improving growth and production of nitrogen-fixing-legume crops in smallholder farming systems will lead to enhanced soil nutrient status and contribute increased agricultural yields and improved food security in these systems. Doing this will require an improved understanding of soil rhizobia ecology. This study is investigating biotic and abiotic that factors drive genetic diversity of soil rhizobia capable of nodulating promiscuous soybean on smallholder farms in Malawi. Soil was collected from 39 sites in the Ekwendeni region of Malawi known to vary in land use and cropping history. Soils were tested for texture, extractable P and organic matter as well as total N content. Soils were then used as inoculants on promiscuous soybean var. TGx1740-2F, locally named “Tikolore”, grown in sterilized sand with an N-free nutrient solution in order to extract rhizobia. Total nodules, plant mass and total N fixed were determined. 20 nodules from each soil were surface randomly selected and rhizobia cultured. Stains were fingerprinted using repetitive element polymerase chain reaction (rep-PCR) using the BOX A1R primer. Molecular results were analyzed using GelCompar software to create phylogenetic dendrograms, and to generate diversity indexes for each site sampled. Soils were found to range widely in particle size, Mehlich-3 extractable P and total organic matter(OM) content. Rhizobia strains orginated from diverse genera and represented both fast and slow-growing species.  Diversity was found to correlate highly with field history.