Rhizobial persistence and its role in the development of sustainable agricultural systems in Mediterranean environments

The objective of this review is to emphasise the continuing role of rhizobial persistence as a selection criterion for inoculant strains. Traditional ley-farming systems rotate cereals with pasture legumes, the latter regenerating to modulate with soil-borne rhizobia. This has required the selection of rhizobial strains for clovers and medics capable of persisting as soil saprophytes in the absence of their hosts. In the last decade, economic factors have dictated the diversification of on-farm enterprises. One means to achieve this has been through the development of alternative legumes, principally grain legumes. In Western Australia grain legume usage increased from 40,000 ha in 1978 to 950,000 ha in 1992. Lupins and peas have been used in rotation with cereals to take advantage of the high price received for high protein grains. Whilst rhizobial inoculation of grain legumes at sowing is possible, the reliance on seed-borne fungicides to control diseases, and the incompatibility of these fungicides with rhizobia, has again required the selection of rhizobial inoculants capable of persisting in soil in the absence of their respective hosts. ergence of herbicide resistance in grasses, and increasing disease pressures in the legume phase, have cast doubt upon the sustainability, and certainly the profitability, of continuous cropping. To control grasses without selective herbicides implies the use of a pasture phase, whilst longer breaks between legume crops is a means to reduce disease. Thus, complex rotations are being investigated in Western Australia, including LWSSW* in regions where herbicide resistance is an issue, and LWFW* in regions where pathogens of lupins are an issue. For very low input systems, WWVV* rotations are being explored. Clearly, once again rhizobial persistence over several seasons in the absence of a host is required for those strains used as inoculants in these more complex rotations. The selection of rhizobial inoculant strains capable of persisting over seasons in the absence of their hosts, and in sufficient number to promptly nodulate their hosts when required, is a continuing research priority to underpin the development of sustainable agricultural systems.