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M.B. Peoples1, K.E. Giller2, D.F. Herridge3, J.K. Vessey4

'CSIRO Plant Industry, GPO Box 1600 Canberra, ACT 2601, Australia 2Dept of Soil Science & Agric. Engineering, University of Zimbabwe, MP 167 Harare, Zimbabwe

3NSW Agriculture, RMB 944 Tamworth, NSW 2340, Australia

4Dept of Plant Science, University of Manitoba, Winnipeg, MB, R3T 2N2 Canada

1. Summary

Biological systems that can fix atmospheric N2 include a range of free-living microorganisms and associative or symbiotic relationships between microbes and plants. But although there are many potential sources of fixed N in terrestrial ecosystems, symbiotic N2 fixation by legume-rhizobia associations provide the largest inputs of N for agriculture. Environmental and management limitations to legume growth are the major factors regulating N2 fixation, although practices that either limit the presence of effective rhizobia in the soil, or enhance soil nitrate concentrations can also be critical. There is the potential for large increases in N2 fixation and enhanced benefits for farmers in the short-term through the wider availability of high quality rhizobial inoculants, basic improvements in crop agronomy, the introduction of legumes to new areas, and changes in residue management. However, since much of these technologies are already known, the prospects for such increases must be considered within the context of the present constraints to the adoption by farmers of existing knowledge. Further increases in the inputs of fixed N into agro-ecosystems might come from rhizobial strain selection and plant breeding, or the use of modern molecular techniques to transfer the capacity to fix N to non-legume crops. But compared to the large potential gains that could be made via improved management, the impact of genetic changes to either the microsymbiont or host is likely to be relatively marginal.

2. Introduction

The fixation of atmospheric N2 can occur in the free-living state with some diazotrophs or via associative relationships on the roots or within the tissues of plants, while other organisms require a symbiosis with specific host plants. Inputs of fixed N by these diverse systems provide a renewable source of N for many terrestrial ecosystems. Biological N2 fixation (BNF) contributes directly to agricultural production where the fixed N is harvested in grain or other food for human or animal consumption, and indirectly by adding N to the soil for the benefit of companion plant species or following crops.

3. Comparative Inputs of Fixed N by Different Organisms

Experimental estimates of N2 fixation by various organisms are presented in Table 1. Free-living N2-fixers probably contribute only small amounts of N to farming systems (Table 1). The data tend to be inconclusive concerning the role of diazotrophs associated with non-legumes in temperate agriculture although studies have demonstrated potential for significant inputs of fixed N by some tropical grasses and crops such as sugarcane (Table 1). Symbiotic associations between Anabaena and the aquatic fern Azolla, or Frankia and actinorrhizal trees such as Casuarina and Alnus may also fix useful amounts of N in agro-ecosystems. However, it is the symbiosis between legumes and rhizobia (Rhizobium, Bradyrhizobium, Allorhizobium, Azorhizobium, Mesorhizobium and Sinorhizobium spp.) that is generally responsible for the largest amounts of fixed N (Table 1).

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