The impact of background rhizobial populations on inoculation response.

Agronomic programs that target the introduction of legumes into different agro-ecosystems focus initially on the selection of legumes that have the ability to tolerate edaphic constraints that include disease resistance, water stress and tolerance to salinity, acidity and sodicity. Within such selection programs the interaction of the legume with soil Rhizobium has largely been ignored. In order to maximise agricultural production, specific programs must link strain selection with plant development together with an understanding of the impact of background populations. PASTURE and crop legumes have been used extensively in agriculture over the past century in Australia, mainly for maintaining soil fertility. These agricultural soils are often constrained in their ability to sustain productive farming systems due to factors associated with low fertility, sodicity, salinity and extremes of acidity and alkalinity. These same attributes can also have a negative impact on the legume-Rhizobium symbiotic relationship reducing the ability of rhizobia to form nodules with optimal N2-fixing capacity, thus impeding the continued success of legumes in Australian agricultural systems. Considerable variation in the soil populations of Rhizobium spp has been found in soils throughout Australia, with the range varying from less than 10 to in excess of 106 Rhizobium bacteria g/m soil (Gibson et al. 1975; Slattery et al. 1999), generally with an average soil population above 1 × 104 Rhizobium bacteria g/m soil. The size of the soil population is dependent on field history, location of sampling, soil characteristics and the presence of a host plant. Where there are low ( 103 Rhizobium bacteria g/m soil) introduction of new strains can be difficult and often unsuccessful (Thies et al. 1991; Brockwell et al. 1995). The Australian continent provides a large landmass that constitutes a wide spectrum of climatic conditions and soil environments, in which to maintain a viable agricultural industry. Soil types in the pulse growing regions of southeastern Australia vary from alkaline (NW Victoria and much of South Australia), neutral acidic soils (Wimmera, central and southern Victoria) to highly acidic soils (NE Victoria and southern NSW). This wide variation in soil type has serious implications for Rhizobium survival, Rhizobium effectiveness and the need for re-inoculation of following pulse crops. Techniques used for the development of elite inoculant strains have been discussed in an accompanying workshop paper (Slattery and Pearce 2001). The final criteria adopted for the selection of Rhizobium strains are the assessment of germplasm for edaphic adaptation and field performance (Howieson et al. 2000; Slattery and Pearce 2001). The capacity of inoculant strains to colonise soils in sufficient quantity to provide effective nodulation is very much dependant on the soil type. Thus, a thorough understanding of the presence of background populations will enable recommendations to be made regarding the need for re-inoculation of pulse legumes. In this paper, we study the effect of the soil environment on the ability of rhizobia to form nodules and the need for rhizobial inoculation when introducing pulse legumes into different soil environments. A comprehensive soil survey across the pulse legume growing regions of Victoria provide a critical 1 Rutherglen Research Institute, Department of Natural Resources and Environment, Rutherglen, Victoria 3685, Australia