Nitrogen fixation in annual Trifolium species in alkaline soils as assessed by the 15N natural abundance method

Annual clover species such as Trifolium purpureum Loisel., T. resupinatum L., and T. alexandrinum L. are adapted to alkaline soil conditions and provide certain agronomic advantages over annual medics (Medicago spp.). Annual clovers have not been widely grown in alkaline soils in Australia, and quantifying their dinitrogen (N-2) fixation in alkaline soils is important in understanding their potential role in mixed farming systems of southern Australia. Using the N-15 natural abundance technique, it was estimated that annual clovers fixed 101-137 kg N/ha at Roseworthy and 59-62 kg N/ha at Mallala, on Calcarosols with soil pH of 8.0 and 8.5, respectively. Species differed in the percentages of fixed N2 estimated in shoot dry matter, which was highest in T. alexandrinum (77-85%), moderate in T. resupinatum (76%), and lowest in T. purpureum (65-74%). Naturally occurring soil rhizobia (Rhizobium leguminosarum bv. trifolii) provided adequate nodulation, as inoculation with different strains of rhizobia had little influence on nodulation or N-2 fixation. These results indicate that clovers can provide a significant contribution of fixed N-2 to mixed farming systems. Examination of nodules indicated variable nodule occupancy by the inoculant rhizobia and that 69% of shoot N was fixed when clovers were nodulated by the soil populations of rhizobia. A simple model is defined to identify the potential interactions between inoculated legumes and soil rhizobia, and the options for enhancing symbiotic effectiveness are discussed.