Contributions of rhizobia to soil nitrogen fertility

Abstract

Agricultural systems are dependent on the input of nitrogen (N) to offset losses sustained through produce removal and through processes such as leaching, denitrification and the volatilisation of ammonia (Ladha 1995). A major source of N in agricultural systems is provided through the symbiosis between rhizobia (root nodule bacteria) and legumes. Rhizobia enter legume root systems through root hair infection or through cracks in the epidermis and induce cortical cells to divide and form nodules (Kijne 1992). Within the root nodules, rhizobia transform N between its abundant form as atmospheric gas (N2) which is metabolically unavailable to plants and metabolically available combined N (NH3). This process is known as biological N fixation. After export from the nodule, the fixed N can be used directly for growth by the crop or pasture legume. It has been estimated that N fixation by the legume-rhizobia symbiosis contributes at least 70 million metric tons of fixed N per year into terrestrial ecosystems (Brockwell et al. 1995). This accounts for up to 40% of the total N fixed on earth (Paul and Clark 1996). In areas of arable agriculture, N fixation by crop and pasture legumes is believed to be the dominant N input (Peoples and Craswell 1992) and estimates exceeding 50% of N are frequently reported in the literature (Burns and Hardy 1975, Peoples and Craswell 1992, Unkovich et al. 1995, Grey 1999, Peoples and Baldock 2001). In Australia, about 1 × 106 t N are fixed annually by crop and pasture legumes, which is equivalent to $1 billion in fertiliser N application (Herridge et al. 2001). Fixed N may also be used indirectly by other plant species through legume root exudates and the breakdown of legume residues. The mineralisation of legume residues contributes varying quantities of N to agricultural systems and is dependent on grazing practices, efficiency of carbon use by decomposers, N demand, plant C:N, lignin:N and polyphenol:N ratios and a range of soil factors (Fillery 2001). In pasture based systems, or where the legume is grown as a green manure, these inputs can be substantial (Brockwell et al. 1995). However, even when a large proportion of fixed N is removed as a grain crop, N fixation still improves the N economy of soils (Peoples et al. 1995). Legumes commonly increase the yield of subsequent crops by the equivalent of 30-80 kg fertiliser N ha-1 and provide the additional advantage of a disease break between crops (Peoples et al. 1995). For the full N benefits to be realised, legumes must be nodulated by rhizobial strains that are effective in their N fixing capacity. In countries with mechanised agricultural production, this is usually achieved through inoculation of legume seed with selected rhizobial strains in the form of commercially available preparations such as gamma-irradiated peat (Date 2001). For example, in Australia, commercial rhizobial strains are selected to be: highly effective in their N fixing capacity in association with target host legumes capable of persisting in soil over the range of environments in which they are likely to be introduced either effective on, or incapable of nodulation with, non-target legumes grown in the same agricultural systems Stringent quality control of Australian commercially produced inoculants by an independent agency (the Australian Legume Inoculant Research Unit, New South Wales Agriculture, Gosford NSW, Australia) ensures high quality peat inoculants that contain at least 1 × 109 viable rhizobia g-1. As a result, farmers have the potential to achieve high rates of N fixation through crop and pasture rotations in agricultural systems. Unfortunately, legumes do not always become nodulated by the applied inoculant strain. A key limitation that prevents nodule occupancy and N fixation by rhizobial inoculant strains is the presence of background populations of variably effective rhizobia. In the following chapter, we review factors that contribute to the size, strain composition, strain dominance and N fixing effectiveness of background rhizobial populations. We also investigate potential management strategies for enhancing N inputs from fixation through manipulation of background populations to favour nodulation by applied inoculant strains. We conclude by speculating on the impacts that conservation farming practices may have on competition between background populations and introduced inoculant strains. © 2007 Springer.