Leguminous plants reduce greenhouse gas emissions

Leguminous plants are unique because they fix nitrogen from the air using symbiotic soil bacteria. This makes them particularly interesting to cultivate. However, their importance for climate-related issues has not previously been widely discussed. Now, an international team, led by Erik Steen Jensen at SLU, has compiled and analysed a large number of studies of the effects of cultivation of leguminous plants on nitrous oxide and carbon emissions, energy use, carbon sequestration in soils and the potential of leguminous plants for bioenergy. Positive effects on all areas were identified.

Legumes such as peas, beans, soybean, clover and alfalfa, can thus contribute to:

• Reduction of the emission of the greenhouse gases carbon dioxide and nitrous oxide (CO2 and N2O) in comparison to emissions from nitrogen-fertilized crops
• Reduction of fossil energy use in the production of food and feed
• Sequestration of carbon in the soil
• Biomass for biofuel production

Big difference in nitrous oxide emissions
A large number of studies on nitrous oxide emissions from cultivation of leguminous plants and from nitrogen-fertilized systems showed a very large variation. The differences may be due to different nitrogen fertilization rates, as well as various climate, soil and cultivation measures. Most often, the nitrous oxide emissions from leguminous plant cultivation do not differ from the background value. The leguminous crops emit an average of 1.29 kg nitrous oxide nitrogen per hectare, the background emission (fallow) is 1.20 kg, while mineral-fertilized crops and pastures on average emit 3.22 kg of nitrous oxide nitrogen per hectare during the growing season.

- The emission of nitrous oxide from soil with leguminous plants is therefore generally lower than from a fertilized system, says Erik Steen Jensen. And there are not many studies that show that biological nitrogen fixation to any greater extent contributes to the total emission of nitrous oxide.

Lower energy use and carbon emissions
It takes 35 to 60 percent less fossil fuel to leguminous crops and to pastures with high content of leguminous plants than fertilized grain crops or leys. This is primarily because there is no need for nitrogen fertilization in legumes and the nitrogen need is less in succeeding crops in the crop rotation. Additionally, the production of nitrogen fertilizers causes significant emissions of carbon dioxide and nitrous oxide.

More carbon sequestrated in the soil
Leguminous plants also play a central role in supplying the extra organic nitrogen, which is required to stimulate the accumulation of carbon in the soil to an extent that exceeds the immobilisation that takes place at nitrogen fertilized cropping of cereals and grasses. This is shown in data from pastures and from both annual and perennial cropping systems.

The biomass of the legumes can be used as raw material for biodiesel or bioethanol, but it also contains a wide range of interesting substances, which can be refined for use as industrial materials and chemicals.

- The uniqueness of legumes as energy crops is that you do not need to fertilize with nitrogen, says Erik Steen Jensen.

Legumes are suitable for sustainable agroecosystems
The large fossil energy used in synthesis, transport and distribution of nitrogen fertilizers often reduces other energy crops net sequestration of soil carbon, and hence the advantage of them. However, there must be a compromise between the use of legumes for biorefineries and letting them to contribute to soil fertility.

- The many ecosystem services conducted by leguminous plants, including their ability to reduce greenhouse gas emissions from agriculture, reducing the use of fossil energy, increase immobilisation of carbon in the soil and provide valuable raw material for biorefineries are making them essential for future sustainable agroecosystems, summarizes Erik Steen Jensen.