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the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N2 fixation.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

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

Humans are currently confronted by many global challenges. These include achieving food security for a rapidly expanding population, lowering the risk of climate change by reducing the net release of greenhouse gases into the atmosphere due to human activity, and meeting the increasing demand for energy in the face of dwindling reserves of fossil energy and uncertainties about future reliability of supply. Legumes deliver several important services to societies. They provide important sources of oil, fiber, and protein-rich food and feed while supplying nitrogen (N) to agro-ecosystems via their unique ability to fix atmospheric N2 in symbiosis with the soil bacteria rhizobia, increasing soil carbon content, and stimulating the productivity of the crops that follow. However, the role of legumes has rarely been considered in the context of their potential to contribute to the mitigation of climate change by reducing fossil fuel use or by providing feedstock for the emerging biobased economies where fossil sources of energy and industrial raw materials are replaced in part by sustainable and renewable biomass resources. The aim of this review was to collate the current knowledge regarding the capacity of legumes to (1) lower the emissions of the key greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) compared to N-fertilized systems, (2) reduce the fossil energy used in the production of food and forage, (3) contribute to the sequestration of carbon (C) in soils, and (4) provide a viable source of biomass for the generation of biofuels and other materials in future biorefinery concepts. We estimated that globally between 350 and 500 Tg CO2 could be emitted as a result of the 33 to 46 Tg N that is biologically fixed by agricultural legumes each year. This compares to around 300 Tg CO2 released annually from the manufacture of 100 Tg fertilizer N. The main difference is that the CO2 respired from the nodulated roots of N2-fixing legumes originated from photosynthesis and will not represent a net contribution to atmospheric concentrations of CO2, whereas the CO2 generated during the synthesis of N fertilizer was derived from fossil fuels. Experimental measures of total N2O fluxes from legumes and N-fertilized systems were found to vary enormously (0.03–7.09 and 0.09–18.16 kg N2O–N ha−1, respectively). This reflected the data being collated from a diverse range of studies using different rates of N inputs, as well as the large number of climatic, soil, and management variables known to influence denitrification and the portion of the total N lost as N2O. Averages across 71 site-years of data, soils under legumes emitted a total of 1.29 kg N2O–N ha−1 during a growing season. This compared to a mean of 3.22 kg N2O–N ha−1 from 67 site-years of N-fertilized crops and pastures, and 1.20 kg N2O–N ha−1 from 33 site-years of data collected from unplanted soils or unfertilized non-legumes. It was concluded that there was little evidence that biological N2 fixation substantially contributed to total N2O emissions, and that losses of N2O from legume soil were generally lower than N-fertilized systems, especially when commercial rates of N fertilizer were applied. Elevated rates of N2O losses can occur following the termination of legume-based pastures, or where legumes had been green- or brown-manured and there was a rapid build-up of high concentrations of nitrate in soil. Legume crops and legume-based pastures use 35% to 60% less fossil energy than N-fertilized cereals or grasslands, and the inclusion of legumes in cropping sequences reduced the average annual energy usage over a rotation by 12% to 34%. The reduced energy use was primarily due to the removal of the need to apply N fertilizer and the subsequently lower N fertilizer requirements for crops grown following legumes. Life cycle energy balances of legume-based rotations were also assisted by a lower use of agrichemicals for crop protection as diversification of cropping sequences reduce the incidence of cereal pathogens and pests and assisted weed control, although it was noted that differences in fossil energy use between legumes and N-fertilized systems were greatly diminished if energy use was expressed per unit of biomass or grain produced. For a change in land use to result in a net increase C sequestration in soil, the inputs of C remaining in plant residues need to exceed the CO2 respired by soil microbes during the decomposition of plant residues or soil organic C, and the C lost through wind or water erosion. The net N-balance of the system was a key driver of changes in soil C stocks in many environments, and data collected from pasture, cropping, and agroforestry systems all indicated that legumes played a pivotal role in providing the additional organic N required to encourage the accumulation of soil C at rates greater than can be achieved by cereals or grasses even when they were supplied with N fertilizer. Legumes contain a range of compounds, which could be refined to produce raw industrial materials currently manufactured from petroleum-based sources, pharmaceuticals, surfactants, or food additives as valuable by-products if legume biomass was to be used to generate biodiesel, bioethanol, biojet A1 fuel, or biogas. The attraction of using leguminous material feedstock is that they do not need the inputs of N fertilizer that would otherwise be necessary to support the production of high grain yields or large amounts of plant biomass since it is the high fossil energy use in the synthesis, transport, and application of N fertilizers that often negates much of the net C benefits of many other bioenergy sources. The use of legume biomass for biorefineries needs careful thought as there will be significant trade-offs with the current role of legumes in contributing to the organic fertility of soils. Agricultural systems will require novel management and plant breeding solutions to provide the range of options that will be required to mitigate climate change. Given their array of ecosystem services and their ability to reduce greenhouse gas emissions, lower the use of fossil energy, accelerate rates of C sequestration in soil, and provide a valuable source of feedstock for biorefineries, legumes should be considered as important components in the development of future agroecosystems.

/pdf/legumes-for-mitigation-of-climate-change-and-the-provision-jfs23ve9qk.pdf

Legumes for mitigation of climate change and the provision of feedstock for biofuels and biorefineries. A review

Rotational effects of chickpea, an important N2-fixing pulse legume of the northern grains region, on subsequent wheat require quantification of the contribution of the legume to soil N and the N status of the wheat, and of suppression of soil and stubble-borne pathogens, such as crown rot (Fusarium graminearum Schwabe Group 1). Results from selected treatments of 10 experiments in northern New South Wales in which chickpea and wheat in one season were followed by wheat in following seasons indicated generally higher dry matter (DM) and grain yields of wheat after chickpea than after wheat. Responses to chickpea were -0·8 to 3·3 t/ha (shoot DM) and -3 to 39 kg N/ha (shoot N). Responses in wheat grain yields were -0·1 to 1·7 t/ha (mean 0·85 t/ha); grain N responses were -2 to 33 kg/ha (mean 19 kg/ha). Grain protein responses were small (0·6%) and variable. Although these productivity responses could be explained largely in terms of additional nitrate-N following chickpea, we measured reduced incidences of crown rot in wheat after chickpea (range 1-36%, mean of 12%), compared with wheat after wheat (range 5-52%, mean 30%). Modelling the incidence of crown rot indicated highly significant interactions between prior crop and total water (pre-plant soil water plus in-crop rainfall). When wheat followed chickpea, incidence of the disease declined sharply with increasing water. When wheat followed wheat, there was a marginal decline in disease incidence with increasing water. Our results support the strategy of using legumes in rotation with wheat in the northern grains region for enhanced soil-N supply and disease-break effects.

Chickpea in wheat-based cropping systems of northern New South Wales. II. Influence on biomass, grain yield, and crown rot in the following wheat crop

Functions quantifying relationships between N2 flxation by legumes and other factors would be useful to farmers in the management of legumes and nitrogen in their production systems. The 2 most critical factors regulating N2 flxation are legume yield and soil nitrate and both should be included as independent variables in the functions. Data from 9 experiments in northern New South Wales on soil nitrate at sowing, yields of shoot and grain dry matter (DM) and N of chickpea, and - 15 N of shoots of chickpea and non N2-flxing reference crops (wheat, barley, and uninoculated chickpea) were used to determine the percentage of chickpea N derived from N2 flxation (Pflx), total N2 flxed, and N balance (flxed N2 minus grain N). Data were then subjected to simple and multivariate regression analyses with Pflx and total N2 flxed as the dependent variables and soil nitrate, shoot N, and grain yield as the independent variables. Simple regression coe-cients (r 2 ) for Pflx were 0¢26 with shoot N as the independent variable, 0¢59 with soil nitrate, and 0¢62 with grain yield. Coe-cient values were increased in the 2-factor (multiple) regressions to 0¢74 (P<0¢001) for soil nitrate plus shoot N, and 0¢82 ( P< 0 ¢001) for soil nitrate plus grain yield. For total N2 flxed, the regression coe-cient was lower at 0¢68 ( P< 0 ¢001), using soil nitrate plus grain yield. We tested the functions against an independent data set with best prediction of Pflx involving the nitrate and grain yield equation (r 2 =0 ¢83; P< 0 ¢001). Total N2 flxed was not well predicted. We concluded that further development of such functions is warranted to reflne both accuracy and precision for chickpea and to extend the approach to other species. Additional keywords: N2 flxation regressions, natural 15 N abundance.

Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N2 fixation and N balance using soil nitrate at sowing and chickpea yield

Four experiments were commenced after a 1980 wheat crop, and a fifth after the 1981 crop, at different sites representing the major soil types of northern New South Wales in the 550-700 mm rainfall zone, to examine the influence of 3 fallow management practices [no tillage (NT); stubble retention after harvest, cultivation (SM); stubble burning after harvest, cultivation (SB)] on wheat production. Data considered in this paper cover the continuous wheat subtreatments of the 5 experiments (1981-90). Nitrogen applied at 50 kg Nlha in addition to the basal treatment was included as a treatment from 1986 to 1988. Across all sites and seasons, grain yields were in the order SB>SM = NT, stubble retention having a greater effect than tillage. In some years at some sites, differences in grain yield and grain N yield were not significant. In others, when significant yield differences occurred, variations in grain yield and grain N yield were highly correlated with differences in soil N available for the crop. The data show that the influence of fallow management interacted with season and crop nutrition, and required long-term study for proper assessment.

A comparison of three fallow management strategies for the long-term productivity of wheat in northern New South Wales

Chickpea has potential as a rotation or break crop in the northern grains region of New South Wales and Queensland. Definition of that potential requires information on chickpea N2 fixation and on effects of chickpea on maintenance of soil N fertility and delivery of mineral N for use by a following cereal crop. Results from 6 experiments in northern NSW in which chickpea and wheat in one season were followed by wheat in subsequent seasons indicated variable N2 fixation by chickpea (mean 73 kg/ha; range 4-116 kg/ha), associated with variable Pfix (percentage of chickpea N derived from N2 fixation) (mean 57%; range 4-79%). There were no consistent differences between chickpea and wheat in effects on soil water, either pre-harvest or after the summer fallow. Chickpea ‘spared’ nitrate, relative to wheat (mean 15 kg/ha; range 1-35 kg/ha), and mineralised additional nitrate during the summer fallow (mean 18 kg/ha; range 5-40 kg/ha). Nitrate-N in the top 1·2 m of the soil profile at sowing of the following wheat crop was on average 89 kg/ha after chickpea (range 63-113 kg/ha) and 56 kg/ha after wheat (range 33-94 kg/ha). Nitrogen mineralisation rates during the summer fallow at 2 sites of 0·17 and 0· 21 kg N/ha · day (after chickpea), although greater than the rates after wheat (0· 07 and 0·12 kg N/ha · day), were not sufficient to meet the N requirements of a moderate to high yielding cereal crop. We concluded that chickpea did not fix sufficient N2 or mineralise sufficient N from residues either to maintain soil N fertility or to sustain a productive chickpea{wheat rotation without inputs of additional fertiliser N.

Chickpea in wheat-based cropping systems of northern new south wales. i. n2 fixation and influence on soil nitrate and water

There is a need to introduce cropping practices in the northern N.S.W. cereal belt that increase N supply for cereal, and in particular wheat, production. Annual crop legumes, grown in rotation with cereal crops, can contribute to the total pool of N in the soil and improve yields of the cereals. Experiments, in 1989 and 1990 at two sites near North Star, N.S.W., are described which aim to (i) assess the effects of chickpea on concentrations of soil nitrate, both in terms of “N sparing” during legume growth and release of N bound in the crop residues; (ii) quantify N2 fixation by chickpea using the natural15N abundance and modified N different techniques; and (iii) explore the concept of water use efficiency (WUE) of N2 fixation for use in the management of N in cropping systems. The two sites and three rates of fertilizer N (0, 50 and 100 kg ha−1) provided six combinations of site, season and nitrate-N fertility. Nitrogen fixed ranged from 29 to 85 kg ha−1, with amounts influenced by method of assessment and fertilizer N treatment. There was general agreement between the natural 15N abundance and modified N difference methods. Soil nitrate spared by chickpea ranged from 6 to 31 kg N ha−1, over sites and treatments, and averaged 11 kg N ha−1 at Windridge and 26 kg N ha−1 at Glenhoma. In the May sampling in the following year, i.e. after the summer and autumn fallow, differences in soil nitrate-N between the chickpea and wheat plots ranged from 29 to 51 kg ha−1, and averaged 44 kg ha−1 (Windridge) and 43 kg ha−1 (Glenhoma). Water use efficiency of chickpea N2 fixation varied between 0.14 and 0.24 kg ha−1 mm−1, with the higher values associated with higher N2 fixation.

Chickpea increases soil-N fertility in cereal systems through nitrate sparing and N2 fixation

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