UC Riverside

In the hundred-plus years since the invention of the Haber-Bosch process, industrial production of nitrogen (N) fertilizers has increased agronomic production and, consequently, the human population. Synthetic N fertilizers also promote the emission of reactive nitrogen forms, which can significantly damage the environment and human health. One alternative to N fertilizer is the use of beneficial nitrogen fixing soil bacteria (rhizobia) that form tumor-like structures called nodules on the roots of legumes. The most effective rhizobia for nitrogen fixation have been used to develop bioinoculants for field application, which have the potential to decrease synthetic N fertilizer use. But importantly, inoculant strains are often outcompeted by indigenous strains that fix little nitrogen. Because of this, bioinoculants are often only marginally effective, and plant growth suffers. To resolve this rhizobial competition problem and improve the N fixing benefits to plants, we must gain a better understanding of i) competitive interactions among rhizobia, ii) the epidemiological patterns of strains that dominate natural populations and spread among them, and iii) gain insight of N fixing efficiency and developmental variation of bacteroids inside the nodule. This dissertation research uses native Californian legumes in the genus Acmispon, which naturally host Bradyrhizobium strains. The three parallel aims are to: i) characterize rhizobial competition and model plant performance based on nodule occupancy of competing strains, ii) investigate the epidemiology of rhizobial genotypes across multiple hosts in a large transect (>800 km) of California, and iii) examine the nodule-bacteroid morphology and nitrogen fixation efficiency of rhizobia that vary in N fixation. The main findings from this dissertation reveals a competitive-dominance hierarchy of rhizobial strains which leads to reduced host-benefit, the epidemic spread of certain dominant Bradyrhizobium strains in natural populations, and ultrastructural variation in nodule bacteroid morphology during different stages of nodule development. This dissertation research holds significant importance in improving crop yields, reducing dependency on synthetic nitrogen fertilizers, and directly benefiting agricultural research aimed at boosting bioproduct yields from legumes without the use of synthetic nitrogen fertilizers.