Environmental drivers of soybean-nodulating-rhizobia diversity in un-inoculated smallholder farms in Malawi

Background/Question/Methods:

Legumes associate with soil-dwelling nitrogen fixing rhizobia bacteria, and through this relationship can maintain and increase soil fertility while improving human nutrition, an important function in low-resourced nations such as Malawi.  Increasing legume production requires the presence of appropriate rhizobia partners.  In recent years soybean (Glycine max) in Malawi has increased as an economically and nutritionally important crop, however since Malawi has a lack of soybean production history, appropriate rhizobia populations are unlikely to exist in most soils.   The purpose of this project is to reveal the environmental drivers of soybean-nodulating-rhizobia population size and diversity in un-inoculated soils, and use this information to improve nodulation and nitrogen-fixation of soybean grown on resource-limited farms.Soils were collected from 39 small-holder farms in the Ekwendeni region of northern Malawi from fields with a history of soybean production (11), non-soybean legume production (8), non-legume crop production (12) and non-cultivated areas (8).  Soils were analyzed for Mehlich 3 phosphorus (P), particle size distribution and total organic matter (OM) content.  Extracted soybean-nodulating rhizobia were characterized using rep-PCR and the BOX A1R primer and diversity indexes calculated from resulting fingerprints. 

Results/Conclusions:

Soils varied in extractable P from 2 – 112 mg kg^-1 soil, OM from 0 – 7.5% and clay content from 4% to 44%, however, these variables were independent of past cropping history. Soils from fields with a history of soybean production and non-cultivated fields resulted in a greater mean number of nodules per plant (16 and 15, respectively)  than those cultivated fields without a history of soybean (8 and 10, respectively), with total nodulation varying from 0 to 78 nodules per plant.  Rhizobia morphology revealed diverse species in terms of colony morphology as well as growth rate.  Both slow-growing (Bradyrhizobia sp.) and fast-growing (Rhizobia sp. and Sinorhizobia sp.) were present in nodules.   Preliminary analysis of 690 isolate BOX fingerprints revealed high diversity both within and between sites.  Further research will allow identification of factors causing this varied population as well as allow for selection of high-N-fixing strains for use as inoculants in the region.