Enhancing crop legume N2 fixation through selection and breeding

Legume N₂ fixation is variable, but nonetheless is a valuable process in world agriculture. There is great potential to increase the contribution by the crop legumes to the world's supply of soil.N. This will be achieved by (i) increasing the area of legumes sown by farmers; (ii) improved management of the crops in order that the major determinants of productivity, e.g. land area, water availability, are converted to harvested product with maximum efficiency; and (iii) genetic modification of the commonly-grown species to ensure high dependence of the legume crop on N₂ fixation at all levels of productivity. Currently-used methods for measuring N₂ fixation and for assessing heritability and repeatability of N₂ fixation in breeding and selection programs are reviewed. Results from research programs to define genetic variation in N₂ fixation and to enhance N₂ fixation through selection and breeding are presented with particular emphasis on common bean (Phaseolus vulgaris) and soybean (Glycine max).     

Quantifying below-ground nitrogen of legumes

Quantifying below-ground nitrogen (N) of legumes is fundamental to understanding their effects on soil mineral N fertility and on the N economies of following or companion crops in legume-based rotations. Methodologies based on ¹⁵N shoot-labelling with subsequent measurement of ¹⁵N in recovered plant parts (shoots and roots) and in the root-zone soil have proved promising. We report four glasshouse experiments with objectives to develop appropriate protocols for in situ ¹⁵N labelling of the four legumes, fababean (Vicia faba), chickpea (Cicer arietinum), mungbean (Vigna radiata) and pigeonpea (Cajanus cajan). Treatments included ¹⁵N-urea concentration (0.1–2.0% w/w), feeding technique (leaf-flap and petiole), leaflet/petiole position (top and bottom of shoot) and frequency of feeding (one and two occasions). ¹⁵N-labelling via the leaf-flap was best for fababean, mungbean and pigeonpea, whilst petiole feeding was best for chickpea, in all cases at the lower-stem nodes 3 or 4 using 0.2 mL volumes of 0.5% urea (98 atom% ¹⁵N excess). Fed leaflets and petioles were removed within 2 weeks of labelling. Uneven ¹⁵N enrichment of the nodulated roots because of effects of the less-enriched nodules meant that root derived N in soil would be overestimated if recovered roots were more heavily nodulated than unrecovered roots. One possible solution would be to assume crown nodulation of the plants. Thus, recovered roots would be nodulated; root-derived N remaining in soil may be without nodules. The ratios of nodulated root to unnodulated root enrichments could then be used as an adjustment in the calculations, i.e. in the case of fababean and chickpea, by dividing calculated root-derived N in soil by 1.12 (fababean) and 1.56 (chickpea).     

Morphological divergence in giant fossil dormice

Insular gigantism—evolutionary increases in body size from small-bodied mainland ancestors—is a conceptually significant, but poorly studied, evolutionary phenomenon. Gigantism is widespread on Mediterranean islands, particularly among fossil and extant dormice. These include an extant giant population of Eliomys quercinus on Formentera, the giant Balearic genus †Hypnomys and the exceptionally large †Leithia melitensis of Pleistocene Sicily. We quantified patterns of cranial and mandibular shape and their relationships to head size (allometry) among mainland and insular dormouse populations, asking to what extent the morphology of island giants is explained by allometry. We find that gigantism in dormice is not simply an extrapolation of the allometric trajectory of their mainland relatives. Instead, a large portion of their distinctive cranial and mandibular morphology resulted from the population- or species-specific evolutionary shape changes. Our findings suggest that body size increases in insular giant dormice were accompanied by the evolutionary divergence of feeding adaptations. This complements other evidence of ecological divergence in these taxa, which span predominantly faunivorous to herbivorous diets. Our findings suggest that insular gigantism involves context-dependent phenotypic modifications, underscoring the highly distinctive nature of island faunas.     

Global inputs of biological nitrogen fixation in agricultural systems

Biological dinitrogen (N₂) fixation is a natural process of significant importance in world agriculture. The demand for accurate determinations of global inputs of biologically-fixed nitrogen (N) is strong and will continue to be fuelled by the need to understand and effectively manage the global N cycle. In this paper we review and update long-standing and more recent estimates of biological N₂ fixation for the different agricultural systems, including the extensive, uncultivated tropical savannas used for grazing. Our methodology was to combine data on the areas and yields of legumes and cereals from the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N₂ fixation. As the FAO lists grain legumes only, and not forage, fodder and green manure legumes, other literature was accessed to obtain approximate estimates in these cases. Below-ground plant N was factored into the estimations. The most important N₂-fixing agents in agricultural systems are the symbiotic associations between crop and forage/fodder legumes and rhizobia. Annual inputs of fixed N are calculated to be 2.95 Tg for the pulses and 18.5 Tg for the oilseed legumes. Soybean (Glycine max) is the dominant crop legume, representing 50% of the global crop legume area and 68% of global production. We calculate soybean to fix 16.4 Tg N annually, representing 77% of the N fixed by the crop legumes. Annual N₂ fixation by soybean in the U.S., Brazil and Argentina is calculated at 5.7, 4.6 and 3.4 Tg, respectively. Accurately estimating global N₂ fixation for the symbioses of the forage and fodder legumes is challenging because statistics on the areas and productivity of these legumes are almost impossible to obtain. The uncertainty increases as we move to the other agricultural-production systems—rice (Oryza sativa), sugar cane (Saccharum spp.), cereal and oilseed (non-legume) crop lands and extensive, grazed savannas. Nonetheless, the estimates of annual N₂ fixation inputs are 12–25 Tg (pasture and fodder legumes), 5 Tg (rice), 0.5 Tg (sugar cane), <4 Tg (non-legume crop lands) and <14 Tg (extensive savannas). Aggregating these individual estimates provides an overall estimate of 50–70 Tg N fixed biologically in agricultural systems. The uncertainty of this range would be reduced with the publication of more accurate statistics on areas and productivity of forage and fodder legumes and the publication of many more estimates of N₂ fixation, particularly in the cereal, oilseed and non-legume crop lands and extensive tropical savannas used for grazing.     

Use of the ureide technique to describe the nitrogen economy of field-grown soybeans

Nitrogen fixation was estimated in `Bragg,' `Forrest,' and `Bethel' soybean (Glycine max [L.] Merrill) from seven locations northwest of New South Wales, Australia, by relating ureide and nitrate contents of plant parts sampled at regular intervals during growth to standard curves derived under controlled nitrate regimes. Estimates were combined with data on crop growth and mineral N contents of soils to (a) determine the total requirements for N by the crops, (b) determine the contributions of N₂ fixation to crop growth, and (c) relate symbiotic dependence ([N₂ fixed/total plant N] × 100) of the crops to levels of mineral N in the soil at sowing. At two locations, Myall Vale and Glenara, levels of ureides in the shoot axes and roots of unnodulated seedlings were surprisingly high at the first time of sampling, perhaps reflecting effects of uptake of ammonium-N by the soybeans or breakdown and remobilization of cotyledonary protein. Ureides in plant parts declined significantly by the second (V5 to V7 growth stage) sampling. Subsequently, ureide contents increased whereas levels of nitrate in plant parts decreased. The relative abundance of ureides ([ureide-N/ureide-N + nitrate-N] × 100) in the shoot axes and nodulated roots of both crops increased linearly from almost zero during mid-vegetative growth (V5 to V7) to virtually 100% during late reproductive growth (R4 to R5, Myall Vale and R6, Glenara). The data suggest a steady transition in soybeans at both locations from dependence upon mineral N for early growth to complete reliance upon fixed N during late reproductive growth. Estimates of seasonal N₂ fixation for soybeans at the seven locations ranged from 73 to 288 kilograms per hectare N (shoot axes ureides) and from 147 to 337 kilograms per hectare N ha (nodulated roots ureides).     

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N₂ via biological N₂ fixation (BNF).Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N₂ fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N.