Predicting the abundance of indigenous and the persistence of introduced rhizobia in tropical soils

Abstract

Our ability to effectively manage the Rhizobium-legume symbiosis is often limited by the saprophytic performance of rhizobia prior to infection of the host and subsequent nitrogen fixation. A series of investigations on rhizobia in soils was conducted on the Island of Maui. Plant infection procedures were improved through the development of MPNES, a computer program that allows population estimates to be assigned to data not included in most-probable-number tables. The relationship between environment and the abundance of indigenous rhizobia was described by enumerating rhizobia at 14 diverse sites. Populations varied from 1.1 to 4.8 log10 cells/g soil. Total rhizobial populations were correlated with mean annual rainfall, legume cover, soil temperature and soil pH. In a later study, the population dynamics of Trifolium repens and associated rhizobia in Pennisetum clandestinum pastures demonstrated the importance of host legumes in maintaining rhizobia in soils. Population sizes ranged from 1.78 to 5.49 log10 cells/g soil and were positively correlated with host presence. A two year study was conducted to measure the persistence of 18 strains representing 6 species of rhizobia introduced into 14 field sites. Peat based inoculant was applied at the rate of 1.5 x 105 cells/strain/g soil. The sites were kept fallow. The rapid decline in numbers followed by either non-recoverability or stable population sizes in soils was best described mathematically by the Mitscherlich equation. The rates of decline and persistence were not related, suggesting difficulty in extrapolating the results obtained in short-term studies. Significant interactions were observed between species and sites. These results allow for the targeting of saprophytically competent rhizobia into specific environmental stress regimes. Slope, aspect, rainfall and the size of the source population contributed to the rate of dispersal away from the release areas. Predictions based on these studies can be used by agricultural planners as a means to determine which agro-ecosystems may benefit from the application of rhizobia.