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.

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

The response to shade in growth and nutritive quality of buffalo grass (Stenotaphrum secundatum) was compared with that of two other stoloniferous grasses, mat grass (Axonopus compressus) and kikuyu grass (Pennisetum clandestinum). The grasses were grown outdoors in Brisbane, Australia, in soil in pots over two growing seasons in 1985/86. Treatments in the first season (Expt 1) were shading at 0 (full sun), 42, 59 and 68 % with moderate N (two spaced applications of 50 kg/ha) and, in the second season (Expt 2), 0 and 59 % shade with high (50 kg/ha every 2 weeks) and low (5 kg/ha every 2 weeks) N. Unusually for tropical grasses, the top yield of all species was higher under shade than in full sun, except for S. secundatum at high N in Expt 2. 5. secundatum and A. compressus showed an increase in top yield up to 68 % shading whereas P. clandestinum reached a maximum at 42 % shading. Shade increased shoot: root ratio and specific leaf area in all species but had little effect on leaf: stem ratio and the proportion of dead material in the tops. Stubble yield (stem bases and stolons) was reduced under shade but to a much smaller extent than root yield. S. secundatum and A. compressus had similar morphological characteristics. Their yield was lower than that of P. clandestinum in Expt 1 (autumn growth) but was similar to that of P. clandestinum in Expt 2 (summer growth). These grasses had a higher leaf: stem ratio and lower proportion of dead material in tops than P. clandestinum. Shade generally increased herbage nutritive value through an increase in N cncentration and dry-matter digestibility. The latter response was somewhat variable and mostly small (1-3% units). Under shade, the general trend was towards a decrease in total nonstructural carbohydrates, cell-wall content and insoluble ash, and an increase in lignin content. A. compressus was higher in digestibility and soluble carbohydrates than the other grasses. S. secundatum was not clearly superior in response to shade and its nutritive quality was lower than that of A. compressus.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600072154

Growth, morphology and nutritive quality of shaded Stenotaphrum secundatum, Axonopus compressus and Pennisetum clandestinum

The soil organic carbon (SOC) pool is the key indicator of soil health and quality which in turn plays a vital role to soil sustainability. The continuous uses of unsustainable agricultural approaches have depleted most of the SOC pool of global agricultural lands. Promoting cultivation of leguminous crops, grasses, shrubs, and trees offers multiple advantages, e.g., augmenting crop and soil productivity and adapting to climate change by increasing resilience of agroecosystems. As per model-based prediction by World Bank, the cumulative soil carbon (C) sequestration of pulses in Asia and Africa is expected to be 33.0 and 35.12 Mg ha−1, respectively, by 2030. Legumes have the potential to reduce the CO2 emitted during the manufacturing of chemical nitrogenous fertilizers through their biological nitrogen fixation (BNF) capacity. Therefore, the main advantage of using legumes is to ensure that the BNF which in turn reduces the amount of nitrogen (N) fertilizer required for the succeeding crop. A meta-analysis study suggested that the legumes have the capacity to store 30% higher soil organic carbon (SOC) when compared to other species; this is because of their N-fixing ability. The leguminous vegetation improves soil health and soil C content as per the nature of the specific crop. The C sequestration potential and the amount of organic C returned by leguminous species to soil depend largely on specific legume species, growth behavior, root morphology and physiology, leaf morphology, climatic conditions, structure and aggregation, prevailing cropping system, and agronomic interventions during crop growth period. The aboveground plant biomass (e.g., plant leaves, branches, stem, foliage, fruits, wood, litter-fall) and the belowground plant biomass (e.g., dead roots, carbonaceous substances from root exudates, rhizospheric deposition, and legume-promoted microbial biomass C) directly contribute to the SOC pool.

Role of Legumes in Soil Carbon Sequestration

The relationships between land uses, soil management practices, and soil carbon fractions in South Eastern Australia

Livestock have long been integral to food production systems, often not by choice but by need. While our knowledge of livestock greenhouse gas (GHG) emissions mitigation has evolved, the prevailing focus has been-somewhat myopically-on technology applications associated with mitigation. Here, we (1) examine the global distribution of livestock GHG emissions, (2) explore social, economic and environmental co-benefits and trade-offs associated with mitigation interventions and (3) critique approaches for quantifying GHG emissions. This review uncovered many insights. First, while GHG emissions from ruminant livestock are greatest in low- and middle-income countries (LMIC; globally, 66% of emissions are produced by Latin America and the Caribbean, East and southeast Asia and south Asia), the majority of mitigation strategies are designed for developed countries. This serious concern is heightened by the fact that 80% of growth in global meat production over the next decade will occur in LMIC. Second, few studies concurrently assess social, economic and environmental aspects of mitigation. Of the 54 interventions reviewed, only 16 had triple-bottom line benefit with medium-high mitigation potential. Third, while efforts designed to stimulate the adoption of strategies allowing both emissions reduction (ER) and carbon sequestration (CS) would achieve the greatest net emissions mitigation, CS measures have greater potential mitigation and co-benefits. The scientific community must shift attention away from the prevailing myopic lens on carbon, towards more holistic, systems-based, multi-metric approaches that carefully consider the raison d'etre for livestock systems. Consequential life cycle assessments and systems-aligned 'socio-economic planetary boundaries' offer useful starting points that may uncover leverage points and cross-scale emergent properties. The derivation of harmonized, globally reconciled sustainability metrics requires iterative dialogue between stakeholders at all levels. Greater emphasis on the simultaneous characterization of multiple sustainability dimensions would help avoid situations where progress made in one area causes maladaptive outcomes in other areas.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/gcb.15816

Carbon myopia: The urgent need for integrated social, economic and environmental action in the livestock sector

The tree legume Leucaena leucocephala (leucaena) is a high quality ruminant feed, vitally important for livestock production in the tropics, despite the presence of mimosine in the leaves. This toxic non-protein amino acid has the potential to limit productivity and adversely affect the health of animals. In the 1980s, the ruminal bacterium Synergistes jonesii was discovered and subsequently distributed in Australia as an oral inoculum to overcome these toxic effects. However, in recent times, a number of factors, including: surveys of the status of toxicity worldwide; improved understanding of the chemistry and mode of action of the toxins; new techniques for molecular sequencing; and concerns about the efficacy of the in vitro inoculum; have cast doubt on some past understanding of leucaena toxicity and provide new insights into the geographical spread of S. jonesii . There is also confusion and ignorance regarding the occurrence and significance of toxicity in many countries worldwide. Ongoing research into the taxonomy and ecology of the Synergistetes phylum, improved methods of inoculation, and improved management solutions, along with aware-ness-raising extension activities, are vital for the future success of leucaena feeding systems.

http://tropicalgrasslands.info/index.php/tgft/article/viewFile/32/5

Leucaena toxicity: a new perspective on the most widely used forage tree legume

Soil organic carbon (OC) and total nitrogen (TN) accumulation in the top 00.15m of leucaenagrass pastures were compared with native pastures and with continuously cropped land. OC and TN levels were highest under long-term leucaenagrass pasture (P0.05). For leucaenagrass pastures that had been established for 20, 31, and 38 years, OC accumulated at rates that exceeded those of the adjacent native grass pasture by 267, 140, and 79kg/ha.year, respectively, while TN accumulated at rates that exceeded those of the native grass pastures by 16.7, 10.8, and 14.0kg/ha.year, respectively. At a site where 14-year-old leucaenagrass pasture was adjacent to continuously cropped land, there were benefits in OC accumulation of 762kg/ha.year and in TN accumulation of 61.9kg/ha.year associated with the establishment of leucaenagrass pastures. Similar C:N ratios (range 12.714.5) of soil OC in leucaena and grass-only pastures indicated that plant-available N limited soil OC accumulation in pure grass swards. Higher OC accumulation occurred near leucaena hedgerows than in the middle of the inter-row in most leucaenagrass pastures. Rates of C sequestration were compared with simple models of greenhouse gas (GHG) emissions from the grazed pastures. The amount of carbon dioxide equivalent (CO 2-e) accumulated in additional topsoil OC of leucaenagrass pastures ≤20 years old offset estimates of the amount of CO2-e emitted in methane and nitrous oxide from beef cattle grazing these pastures, thus giving positive GHG balances. Less productive, aging leucaena pastures 20 years old had negative GHG balances; lower additional topsoil OC accumulation rates compared with native grass pastures failed to offset animal emissions.

Soil organic carbon and total nitrogen under Leucaena leucocephala pastures in Queensland

Concern about mimosine toxicity and its management has contributed to the restricted adoption of leucaena as a forage for ruminants. The toxicity is a function of the antimitotic effects of mimosine, which is rapidly converted to isomers of hydroxypyridone (DHP), also toxic compounds, by plant and microbial enzymes. Work by R.J. Jones and colleagues (1960-1994) identified a rumen bacterium (Synergistes jonesii) capable of degrading DHP, and rumen fluid containing this bacterium was subsequently made available in Australia as a commercial inoculum for cattle producers.Research by University of Queensland and CSIRO over 15 years, commencing in 2003, found evidence for another pathway of toxin management in Indonesia, where hundreds of Balinese farmers had fed uninoculated Bali bulls (Bos javanicus) up to 100% leucaena without experiencing toxicity symptoms, apart from an initial 1-2 week period while their cattle became adapted to the new diet. Tests showed that the Indonesian cattle were not degrading all DHP, as it appeared in high concentrations in urine samples, predominantly as 2,3-DHP and almost all (>97%) in a conjugated form. The conjugating compounds (glucuronic acid and sulfate compounds), produced in the liver, appeared to be the major pathway for neutralizing the toxicity of DHP. Other work revealed that S. jonesii was a ubiquitous organism in the rumen fluid of animals in all countries but always as a minor population, just detectable using new PCR-based assays, and sometimes not detected in all animals studied.Since the Indonesian cattle fed leucaena suffered symptoms of mimosine toxicity for only a short time before quickly recovering, we hypothesize that conjugation of DHP by the liver was the major detoxification pathway for these animals. This detoxification pathway is also operative in Australia and other countries but further studies are needed to determine its significance.

/pdf/an-update-on-leucaena-toxicity-is-inoculation-with-26bv9g2jd4.pdf

An update on leucaena toxicity: Is inoculation with Synergistes jonesii necessary?

The contribution of West Nusa Tenggara Province to domestic beef supply in Indonesia is relatively small; however, beef cattle are very important for the livelihoods of smallholder farmers in the region. Bali cattle (Bos javanicus) are the predominant breed, as they are adapted to harsh nutritional conditions, are highly fertile and have low calf mortality (Toelihere 2003). While these cattle are genetically capable of achieving a growth rate of 0.85 kg/d (Mastika 2003), this is rarely achieved as poor nutrition is a severe limitation to animal growth in traditional village systems (Panjaitan 2012). Improving feed quality and supply is vital to increasing growth rates and product quality. Forage tree legumes such as leucaena (Leucaena leucocephala) offer the best chance of providing high quality feed to fatten Bali bulls in village systems, where leucaena is well-adapted. Leucaena has been fed for about 2 decades in Sumbawa district of West Nusa Tenggara, although the practice is limited to specific villages, mostly Balinese, even though farmers nearby have similar biophysical conditions and livestock nutrition problems. The objective of this work was to document the practices employed by farmers in Sumbawa to maximize growth rates by feeding leucaena, so that their detailed knowledge can be passed onto other villagers in a pilot roll-out program (Kana Hau 2014).

Growth of Bali bulls fattened with Leucaena leucocephala in Sumbawa, Eastern Indonesia

Leucaena has been fed to cattle by the Balinese community in Sumbawa and West Sumbawa districts on Sumbawa Island since the 1980s. However, prior to 2011, this practice was not adopted by the local Sumbawanese farmers. Since then, a model leucaena-based cattle fattening system was developed in Sumbawa and West Sumbawa districts in a collaborative research project between the Assessment Institute for Agricultural Technology (BPTP), University of Mataram and The University of Queensland (UQ) funded by the Australian Centre for International Agricultural Research (ACIAR), followed by a scaling-out project involving collaboration between the University of Mataram and CSIRO (Applied Research and Innovation Systems in Agriculture - ARISA project) funded by DFAT (Department of Foreign Affairs and Trade) promoting public-private partnerships. Further promotion of leucaena-based fattening systems occurred in Dompu, Sumbawa, through a project with the University of Mataram and Massey University funded by the New Zealand Ministry of Foreign Affairs and Trade (MFAT). By the end of October 2018, more than 2,500 farmers on Sumbawa Island were practicing leucaena-based cattle fattening. The main drivers of adoption of cattle fattening with leucaena were: (1) The high growth rates achieved (0.4-0.6 kg/d for bulls fed 100% leucaena and 0.66 kg/d when maize grain was added to the leucaena basal diet) compared with 0.16 kg/d for the traditional system, combined with high profitability; (2) the needs of farmers being met in terms of relevance and cultural appropriateness; (3) field extension staff being well trained and mentored, and respected by the farmers; (4) the local government being highly supportive of leucaena-based cattle fattening; and (5) additional benefits being increased dressing percentage and high carcass quality. The rapid increase in the use of leucaena for cattle fattening in eastern Indonesia is expected to have a significant positive impact on household incomes as well as on regional economic growth.

/pdf/adoption-of-leucaena-based-feeding-systems-in-sumbawa-36h7mwxfw9.pdf

H. Max Shelton

Papers

Leucaena toxicity: a new perspective on the most widely used forage tree legume

Soil organic carbon and total nitrogen under Leucaena leucocephala pastures in Queensland

An update on leucaena toxicity: Is inoculation with Synergistes jonesii necessary?

Growth of Bali bulls fattened with Leucaena leucocephala in Sumbawa, Eastern Indonesia

Adoption of leucaena-based feeding systems in Sumbawa, eastern Indonesia and its impact on cattle productivity and farm profitability