| Despite selection of inoculant strains for improved nitrogen fixation capacity and competitive ability, rhizobial inoculation frequently fails to improve crop yield. The natural diversity in rhizobial population size, soils, and climates present at five sites on Maui, Hawaii, was used to examine, under field conditions, the role that indigenous rhizobia and other environmental factors play in determining the symbiotic performance of inoculant strains. Eight inoculation trials were conducted using 2-4 legumes from among 9 species which yielded 29 legume/site observations. Uninoculated, inoculated, and fertilizer N treatments evaluated the impact of indigenous rhizobial populations and soil N availability on inoculation response and yield potential. Inoculation increased yield by 62% on average. A significant inoculation response was obtained in 38% of the trials and varied by both legume species and site. Significant responses to N application, significant increases in nodule parameters, and greater than 50% nodule occupancy by inoculant rhizobia did not necessarily coincide with significant inoculation responses. Size of indigenous rhizobial populations and soil N status had the greatest influence on inoculation response. As few as 54 rhizobial g... |
