Legumes reduce greenhouse gas emissions from agriculture

Legumes, e.g. pea, bean, soybean, clovers and alfalfa, deliver several important services to society. They provide important sources of oil, fibre 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.

But do they also contribute to the mitigation of climate change induced by human activity? An international author consortium led by Erik Steen Jensen, SLU, has collated and synthesized the current knowledge regarding the capacity of legumes to:

• lower the emissions of the key greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) compared to N fertilized systems
• reduce the fossil energy used in the production of food and forage
• contribute to the sequestration of carbon (C) in soils
• provide a viable source of biomass for the generation of bio-fuels and other materials in future bio-refinery concepts.

Data collated from a diverse range of studies of experimental measures of total N2O fluxes from legumes and N fertilized systems were found to vary enormously, due to different rates of N inputs, as well as the large number of climatic, soil and management variables known to influence N2O fluxes.

Averages across sites and years, soils under legumes, N-fertilized crops and pastures and unplanted soils emitted a total of 1.29, 3.22 and 1.20 kg N2O-N ha-1 during a growing season, respectively.

– 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, says Erik Steen Jensen.

Legume crops and legume-based pastures use 35 to 60 % less fossil energy than N fertilized cereals or grasslands. 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. N-fertilizer production results in significant net emissions of CO2 and N2O.

Data collected from pasture, cropping and agro-forestry 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 and chemicals, if legume biomass is used to generate bio-fuels such as biodiesel, bio-ethanol or biogas.

– The attraction of using leguminous material feedstock is that they do not need the inputs of N fertilizer, says Erik Steen Jensen.

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 bioenergy feed-stocks. The use of legume biomass for bio-refineries needs careful thought as there will be significant trade-offs with the current role of legumes in contributing to the fertility of soils.
 
– Legumes should be considered as important components in the development of future agro-ecosystems, concludes Erik Steen Jensen. 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 bio-refineries are proof enough.