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 occu rs, the specificity of the process and its interaction with soil manag ement factors remain poorly described. In a series of field trials, we measured changes in the size of indigenous populations of Bradyrhizob ium in response to cropping of host and non-host legumes under two N f ertilizer regimes. Uninoculated cowpea (Vigna unguiculata) and soybean (Glycine max) were grown with or without applied urea (900 kg N ha(-1 )) at three field sites on the island of Maul, HI, U.S.A., not previou sly cropped with legumes. Using the most-probable-number plant infecti on method and Siratro (Macroptilium atropurpureum) as the host, the po pulation density of Bradyrhizobium in the bulk soil at each site was m easured at planting and at grain maturity and compared to the populati on density in adjacent fallow soil. When the size of the initial indig enous population was low (18 and 580 cells g(-1) soil), significant in creases in the population density compared to fallow soil were observe d only in soils cropped with cowpea receiving no applied urea. When th e size of the initial indigenous bradyrhizobial population was high (5 .8 x 10(4) cells g(-1) soil), no significant increase in the populatio n density was observed. These results suggested that enrichment of soi l bradyrhizobial populations was host-specific, that symbiotic legumes can enrich their soil environment with microsymbionts up to a thresho ld level and that such enrichment can be curtailed by soil management practices that suppress nodulation.