Modeling ecological determinants of the symbiotic performance of introduced rhizobia in tropical soils

Abstract

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 rhizobia g-1 soil prevented a significant response to inoculation. Inoculation response and competitive success of inoculant rhizobia were inversely related to numbers of indigenous rhizobia. Hyperbolic and log-linear equations, respectively, were most useful in quantifying these relationships. Combining indices of soil N with hyperbolic-response models yielded useful equations for determining the need to inoculate and predicting success of inoculant strains introduced into new environments. Rhizobial interstrain competition studies identified both highly and poorly competitive inoculant strains across diverse environments. Symbiotic crops attained, on average, only 88% of maximum yield as defined by the fertilizer N treatment. Nitrogen source also significantly affected crop development. Crops supplied with urea had higher rates of vegetative growth, but, delayed reproductive maturity compared with crops relying on soil N and nitrogen fixation. Results of 4 soybean trials were compared with output from an existing soybean crop model. Difficulty in accurately simulating field results was encountered, indicating the need to address both source and supply of N when predicting legume yield and inoculation success.