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from the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N2 fixation.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.

Global inputs of biological nitrogen fixation in agricultural systems

Wild Crop Relatives: Genomic and Breeding Resources

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).

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

Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30–40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legumegrain may be relatively small.Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability of N in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogenemitted from nodules as a by-product of N2, fixation.

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems.

This review reveals that crop residues of common cultivated crops are an important resource not only as a source of significant quantities of nutrients for crop production but also affecting soil physical, chemical, and biological functions and properties and water and soil quality. When crop residues are returned to the soils, their decomposition can have both positive and negative effects on crop production and the environment. Our aim as agricultural scientists is to increase the positive effects. This can only be achieved with the better understanding of residue, soil, and management factors and their interactions, which affect the decomposition and nutrient release processes. Data on nitrogen benefits and nitrogen recoveries from residues show that a considerable potential exists from residues, especially leguminous residues, not only in meeting the N demands of the succeeding crops, but also in increasing the long-term fertility of the soils. In addition, crop residues and their proper management affects the soil quality either directly or indirectly. Intensive cropping systems are very diverse and complex, so no one residue management system is superior under all situations. Ideally, crop residue management practices should be selected to enhance crop yields with a minimum adverse effect on the environment. It is suggested that in each cropping system, the constraints to production and sustainability should be identified and conceptualized to guide toward the best option. Multidisciplinary and integrated efforts by soil scientists, agronomists, ecologists, environmentalists, and economists are needed to design a system approach for the best choice of crop residue management system to enhance both agricultural productivity and sustainability.

Crop Residues and Management Practices: Effects on Soil Quality, Soil Nitrogen Dynamics, Crop Yield, and Nitrogen Recovery

N2 fixation and its potential contribution to increasing soil total N were estimated in field-grown crops of lupin and pea in 21 trials at 10 locations in New South Wales and Victoria, during 1984 to 1987. Chickpea, faba bean and annual medic were included at some sites. Across experiments there were differences in annual rainfall (267 to 646 mm), soil N (0.02 to 0.20%), soil pH (CaCl2,4.3 to 8.0) and sowing date (24 April to 16 June). Most experiments were conducted on acidic (pH < 4.8) red-earth, the others on grey-cracking clay or sandy soil, both of higher pH The differing sites, seasons, and sowing time contributed to variation in legume biomass (2.02 to 14.33 t/ha) and total N (45 to 297 kg N/ha), and the amount of N harvested with grain (8 to 153 kg N/ha), which were related.Lupin fixed an average of 65% of total crop N, and pea 61%, but there was considerable variation about these averages (20 to 97%). Significant differences in % N2 fixation between legumes within sites were few. The amount of N2 fixed averaged 98.5 kg N/ha by lupin and 80.5 kg N/ha by pea, varying 26 to 288 kg N/ha and 16 to 177 kg N/ha, respectively. Variation in proportional and total N2 fixation was associated with biomass, soil mineral N, and sowing date. N2 fixation increased with more biomass and declined with higher soil mineral N, and later sowing (lupin). Each additional tonne of dry matter increased fixed N by c. 20 kg N/ha. Differences in amounts of fixed N between legumes within sites were due primarily to biomass differences.N2fixed by lupin contributed an average of 38.2 kg N/ha to soil N, and by pea, 17.9 kg N/ha. The contribution was variable, -41 to 135 kg N/ha (lupin) and -32 to 96 kg N/ha (pea), and correlated with proportional and total N2 fixation. Positive increase to soil total N occurred when lupin fixed at least 50% of its crop N, and pea 65%. This occurred in most crops. Legumes frequently used less of the available soil N than cereals.

N2 fixation and its value to soil N increase in lupin, field pea and other legumes in south-eastern Australia

Dry matter, biomass N, N2 fixation (determined by the 15N isotope dilution method), grain yield and grain N, were determined for five pea cultivars grown with three sowing times at six sites in south-eastern Australia. Earlier sowing (late April to early May) increased N2 fixation (by as much as 96 kg N/ha compared to sowing in late June to early July). Early sowing improved the probability of peas contributing to soil total N. The potential increment in soil N from the pea stubbles left after harvesting grain, averaged over the varieties, was as high as 98 kg N/ha with early sowing, and as low as -38 kg N/ha with late sowing. The benefits from earlier sowing were due to larger dry matter production with a higher N concentration and a greater proportion of plant N from N2 fixation. Varietal variation in fixed N and potential for augmenting soil total N, was generally smaller than the variation in these parameters due to different sowing dates.

Sowing Date and Varietal Effects on the N2 Fixation of Field Pea and Implications for Improvement of Soil Nitrogen

Plant-available phosphorus (P) has been found to be limiting crop and pasture production in Australian dryland, broadacre, organic farming systems. The present review examines the mechanisms that act to provide organic sources of P to soil or mobilise P stored within the soil. A range of products is available to exploit one or more of these mechanisms to achieve a claimed improvement in P fertility. These products are described, and where possible, scientific research of their effectiveness is reviewed. The use of microbial inoculants, although successful in laboratory and glasshouse experiments, has returned varied results in field trials. The addition of organic fertilisers, such as composted or elemental sulfur (S) enriched reactive phosphate rock (RPR), tended to produce more reliable results. The variable nature of the composting process creates complexity in the production of composted RPR. The increased dissolution of RPR by the oxidation of added S has been successful in increasing available P content above that of RPR alone. This is especially significant to low-rainfall areas where RPR tend to be ineffective. This paper highlights the need for development and optimisation of the many organic fertilisers and additives available to organic producers. In all cases, products still require rigorous field and economic evaluation so that organic producers can be confident in making decisions that are informed, correct, and profitable with regard to P fertility. The alleviation of P deficiency is vital to the increased adoption and sustainability of boardacre organic farming in Australia.

New fertiliser options for managing phosphorus for organic and low-input farming systems

An indirect, competitive, enzyme-linked immunosorbent assay (ELISA) was developed to indentify strains (SU391, SU303, WSM937, NZP5472) of Rhizobium leguminosarum bv. viciae in the nodules of Pisum sativum and Vicia faba and to study the effect of legume species and indigenous R. l. viciae on the nodulating-competitiveness of these strains, applied as seed inoculants. The study involved field-grown crops at Avondale and Merredin in Western Australia and Wagga Wagga in New South Wales. A multivariate statistical procedure was used to determine an optimal classification rule by which nodule isolates were allocated as an inoculant strain or indigenous strain. The legume significantly affected the proportion of nodules formed by inoculant strains at Merredin. Strains (SU303, WSM937 and NZP5472) with V. faba cv. Fiord were very competitive with indigenous rhizobia, forming 53–92% of the nodules. In comparison, inoculant strains on P. sativum at Merredin were found in 500 g−1 soil, the nodulating-competitiveness of inoculant strains applied to faba bean. No inoculant strain was remarkably better than another at competing with indigenous R. l. viciae. We conclude that seed inoculation often may be more successful with V. faba than P. sativum, which is fortuitous because indigenous R. l. viciae are more likely to be less efficient for N2 fixation with faba bean (cv. Fiord) than with pea. The relatively poor success with inoculation of P. sativum suggests the need to determine its efficiency for N2 fixation with indigenous R. l. viciae, and to ensure novel cultivars are effectively nodulated by these rhizobia. The poor representation of inoculant strains in nodules on faba bean grown in soils with very large numbers of soil-borne R. l. viciae may severely reduce its productivity. Moreover, we propose that seed inoculation may not achieve a solution to this problem.

Nodulation of field-grown Pisum sativum and Vicia faba: Competitiveness of inoculant strains of Rhizobium leguminosarum bv. viciae determined by an indirect, competitive ELISA method

Burning of rice straw after rice harvest in the Rice Wheat System of North west India can be overcome by sowing wheat into rice straw with the help of a machine called the ‘Happy Seeder’. The adoption of zero tillage and retaining rice straw on the soil surface alters the nitrogen demand of the wheat crop due to changes in soil temperature and soil moisture under rice straw mulch, which in turn affects microbial growth. The microbes can either immobilise nitrogen or mineralise it and thus affects the plant growth. A field experiment was conducted to optimise the management of N fertilizer for wheat production under rice straw mulch so as to ensure high grain yield, high N use efficiency. Whilst band placement of nitrogen fertiliser at 180 kg/ha resulted in higher grain yield when rice straw was burnt, banding was not effective in increasing yield when straw was retained. The retention of rice straw as a mulch also resulted in higher mineral N concentrations remaining in the soil after harvest which may be used by subsequent crops in leaching can be minimised. This work provides evidence that retention of rice straw is not detrimental to yield if N management is optimised.

/pdf/nitrogen-management-in-wheat-sown-in-rice-straw-as-mulch-in-4q2vmln8pm.pdf

Nitrogen management in wheat sown in rice straw as mulch in North West India.

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