ENRICHMENT OF BRADYRHIZOBIUM spp POPULATIONS IN SOIL DUE TO CROPPING OF THE HOMOLOGOUS HOST LEGUME

Host legumes can enrich their immediate soil environment with rhizobia through rhizosphere effects. The extent to which this enrichment occurs, the specificity of the process and its interaction with soil management factors remain poorly described. In a series of field trials, we measured changes in the size of indigenous populations of Bradyrhizobium in response to cropping of host and non-host legumes under two N fertilizer regimes. Uninoculated cowpea (Vigna unguiculata) and soybean (Glycine max) were grown with or without applied urea (900 kg N ha-') at three field sites on the island of Maui, HI, U.S.A., not previously cropped with legumes. Using the most-probable-number plant infection method and Siratro (Macroptilium atropurpureum) as the host, the population density of Bradyrhizobium in the bulk soil at each site was measured at planting and at grain maturity and compared to the population density in adjacent fallow soil. When the size of the initial indigenous population was low (18 and 580 cells g-' soil), significant increases in the population density compared to fallow soil were observed only in soils cropped with cowpea receiving no applied urea. When the size of the initial indigenous bradyrhizobial population was high (5.8 x 10° cells g-' soil), no significant increase in the population density was observed. These results suggested that enrichment of soil bradyrhizobial populations was host-specific, that symbiotic legumes can enrich their soil environment with microsymbionts up to a threshold level and that such enrichment can be curtailed by soil management