Ecological interactions of rhizobia and their effects on the nitrogen nutrition of field pea

Abstract

Rhizobial nitrogen (N) fixation is an important source of nitrogen for most legume crops. The amounts of nitrogen added to soils around the world via rhizobial fixation exceeds approximately 20 million tonnes annually. Moreover, rhizobial N fixation has been estimated to replace millions of tonnes of inorganic fertilizer and stimulate other microbial processes like decomposition by adding organic nitrogen into soils. However, rhizobial N fixation is limited in many systems due to several abiotic and biotic factors. For example, having ineffective resident rhizobial populations in soils is a major biotic factor that limits N fixation. Development of effective N fixing rhizobial strains as commercial inoculants is a well-practiced technique in legume industry to enhance plant N nutrition. The success of these commercial inocula under field conditions is still challenging due to the unfavourable competition for nodulation imposed by the ineffective resident populations in field soils. There is a gap between the accurate knowledge on overcoming this inter-strain rhizobial competition and exploiting maximum benefits from rhizobial N fixation. One of the major objectives for this study was to examine the extent of the symbiotic interactions of multiple Rhizobium leguminosarum strains in a single field pea root system. Then to explore the interactions between competitor rhizobial strains and a commercial inoculant and also to examine whether these symbiotic interactions could significantly affect the overall N nutrition of field pea. I also considered these rhizobial interactions in the context of abiotic stress conditions which could constrain symbiotic N fixation. Among the major abiotic environmental stresses, drought is one of the key abiotic constraints in Australian agricultural systems due to lack of rainfall for prolonged periods of time. Water stress disrupts plant growth and function and reduces populations of effective N fixing rhizobia in soils. Therefore, for this thesis, I investigated whether drought could alter the interactions between commercial (WSM1455) and competitor strains of Rhizobium leguminosarum and impact overall N nutrition of field pea hosts (Pisum sativum L).

Date of Award	2020
Original language	English