Pulses are a low carbon footprint food

Food production, along with transportation and energy production have large impacts on greenhouse gas concentrations in our atmosphere. Agriculture alone accounts for 10-12% of global greenhouse gas emissions1.

As explained in "Pulses use half the non-renewable energy inputs of other crops", pulses require little to no nitrogen fertilizer, due to their ability to biologically fix nitrogen from the air. The manufacturing of essential nitrogen fertilizer is energy intensive, and natural gas is used to drive this process2. Knowing this, it is obvious why growing nitrogen-fixing pulses would result in less greenhouse gas emissions to the atmosphere. But this is only half the story.

When soil is fertilized with nitrogen in the form of fertilizer, manure or crop residues, soil microorganisms convert some of this nitrogen to nitrous oxide, a gas which can escape to the atmosphere3. Nitrous oxide is a powerful greenhouse gas; with 298 times the global warming potential of carbon dioxide4. Nitrous oxide represents 60% of the greenhouse gas emissions from Canadian agriculture and the application of nitrogen fertilizer represents the largest source of nitrous oxide from Canadian agricultural soils (35% of direct emissions)5.

Since crop production greenhouse gas emissions are largely driven by nitrogen fertilizers, farmers that grow nitrogen fixing pulse crops are doing their part to reduce global greenhouse gas emissions!6,7,8,9 But consumers have to do their part as well. Next time you are at the grocery store, look for pulses, and products with pulse ingredients.

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1. Smith, P., Martino, D., Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, O. Sirotenko, 2007: Agriculture. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
2. Lal, R. 2004. Carbon Emissions from Farm Operations. Environment International. 30: 981-990.
3. Bouwman, A.F., Boumans, L.J. and Batjes, N.H. 2002. Emissions of N2O and NO from fertilized fields: Summary of measurement data. Global Biogeochemical Cycles. 16: 1058-1071.
4. Snyder, C.S., Bruulsema, T.W., Jensen, T.L. and Fixen, P.E. 2009. Review of greenhouse gas emissions from crop production systems and fertilizer management effects. Agriculture, Ecosystems and Environment. 133: 247-266.
5. Rochette, P., Worth, D.E., Huffman, E.C., Brierley, J.A., McConkey, B.G., Yang, J., Hutchinson, J.J., Desjardins, R.L., Lemke, R. and Gameda, S. 2008. Estimation of N2O emissions from agricultural soils in Canada. II. 1990-2005 inventory. Canadian Journal of Soil Science. 88: 655-669.
6. Lemke, R.L., Zhong, Z., Campbell, C.A. and Zentner, R. 2007. Can Pulse Crops Play a Role in Mitigating Greenhouse Gases from North American Agriculture? Agronomy Journal. 99: 1719–1725.
7. 1.Gan, Y., Liang, C., Wang, X. and McConkey, B. 2011. Lowering carbon footprint of durum wheat by diversifying cropping systems. Field Crops Research. 122: 199–206.
8. 1.Lemke, R. and Farrell, R. Nitrous Oxide Emissions and Prairie Agriculture.
9. Janzen, H. Agriculture and Greenhouse Gases.
In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change

This document is a peer-reviewed chapter from a book written for the Intergovernmental Panel on Climate Change. The chapter describes the proportion of global anthropogenic greenhouse gases that are emitted by agriculture, and how greenhouse gas emissions from agriculture have risen and are projected to continue rising in various regions of the world.

Agricultural greenhouse gas mitigation options are cost competitive with non-agricultural options in achieving long term greenhouse gas policy goals. Different mitigation options are presented for both crop and livestock production, including the use of rotations with legume crops to reduce the reliance of cropping systems on external nitrogen inputs.

Carbon Emissions from Farm Operations

This article summarizes research on energy use in agriculture and converts the different measurements in each study to the same units (kilograms of carbon). Converting measurements to the same units makes it easier to compare energy use in different crop production systems. Research from around the globe was used; no one specific region was targeted. The article was published in the peer-reviewed journal Environment International in 2004.

The carbon emissions from different farm operations and inputs were calculated. These include tillage, irrigation, seeding, spraying, harvesting, transportation, fertilizers and pesticides. The effect of soil erosion on carbon emissions was also considered. Nitrogen fertilizer was highlighted as a very carbon-intensive input and alternatives such as the use of legumes and pulses are recommended to reduce carbon emissions. The article also discusses some of the ways in which sustainability can be measured and summarizes research that compares energy use in different crop production systems. Many studies have found that reducing the amount of tillage, pesticide and fertilizer use has the most potential to reduce energy use.

Emissions of N2O and NO from fertilized fields: Summary of measurement data

This Dutch study, published in the peer-reviewed journal Global Biogeochemical Cycles in 2002, summarizes over 800 measurements of nitrous oxide emissions from fertilized agricultural fields around the world. The goal of the study was to provide information so that more accurate models to predict nitrous oxide emissions can be developed. The study looked at three factors that affect nitrous oxide emissions: soil conditions, crop and fertilizer management, and measurement techniques.

The study found that fertile soils tend to have higher nitrous oxide emissions, as do clay soils, acidic soils, and poorly drained soils. Because soil conditions vary over the landscape, so do nitrous oxide emissions, which creates “hot spots” of emissions. The study found that there is a strong increase in nitrous oxide emissions when nitrogen fertilizer or nitrogen-containing manure is used. Nitrogen fixing legume crops were found to have similar nitrous oxide emissions as fertilized non-legume crops, although most studies were focussed on soybeans or alfalfa. The way in which nitrous oxide is measured in terms of frequency and length of time is also important. For example, nitrous oxide emissions resulting from fertilizer use will continue for more than one growing season, so measurements that include only one growing season will underestimate nitrous oxide emissions.

This study adds to the information needed to continuously improve estimates of nitrous oxide emissions.

Review of greenhouse gas emissions from crop production systems and fertilizer management effects

This detailed article, published in 2009 in the peer-reviewed journal Agriculture, Ecosystems and Environment, reviews current research on the effect of nitrogen fertilizer use on greenhouse gases.

Greenhouse gas emissions from agriculture, especially due to fertilizer use, are described and compared with emissions from other sources. The ways that specific farming practices affect greenhouse gas emissions are then discussed. In particular, the type, amount, timing, and placement of nitrogen fertilizer are considered. Farming practices that reduce greenhouse gas emissions without reducing crop yields are suggested.

One section explores nitrous oxide emissions from legume crops (which include pulse crops). Nitrous oxide emissions from legume crops are low while the crop is actively growing. However, there is uncertainty about whether or not legumes result in higher nitrous oxide emissions than other crops when the crop residues decompose. Another article in this science library (Zhong et al., 2011) explores this issue further and provides evidence that pulse residues do not increase nitrous oxide emissions.

Estimation of N2O emissions from agricultural soils in Canada. II. 1990-2005 inventory

This peer-reviewed article, published in the Canadian Journal of Soil Science in 2008, estimates nitrous oxide emissions from Canadian agricultural land from 1990 to 2005. The study was done because international agreements and negotiations on climate change require that countries determine their national production of greenhouse gases.

Approximately two-thirds of nitrous oxide emissions from agriculture in Canada were from agricultural soil (68%), with the remainder from animal waste management and the indirect loss of nitrogen from land (e.g. through leaching or erosion). Of the emissions from agricultural soils, application of nitrogen fertilizers was the largest source (35%). Other important sources were crop residues, grazing animals, and manure applied to soil.

Can Pulse Crops Play a Role in Mitigating Greenhouse Gases from North American Agriculture?

This article summarized many different experiments investigating the impact of pulse crops on greenhouse gas emissions. It was published in the peer-reviewed Agronomy Journal in 2007. Pulse crops affect the amount of carbon dioxide, nitrous oxide, and methane emitted from the soil they are grown in.

Research shows that crop rotations containing a pulse crop have lower overall greenhouse gas emissions than those that do not include a pulse crop. This is because up to 70% of the non-renewable energy used in Western Canadian cropping systems is due to the use of fertilizers, particularly nitrogen. Pulses supply their own nitrogen, reducing the need for added nitrogen fertilizer. Research on nitrous oxide emissions specifically is limited, but shows that emissions tend to be lower for pulse crops compared to fertilized cereal crops. The more often a pulse crop is grown, the more greenhouse gas emissions are reduced. For example, a 17-year study at Swift Current, SK showed that greenhouse gas emissions were decreased by 31% annually when lentils were included in rotation with spring wheat. A similar study at Indian Head, SK showed an 18% reduction in yearly greenhouse gas emissions when peas were included in rotation with spring wheat, winter wheat, and flax.

Lowering carbon footprint of durum wheat by diversifying cropping systems.

This 2011 article in the peer-reviewed Field Crops Research Journal studied the carbon footprint of durum wheat. Durum wheat was grown under zero tillage conditions in nine different crop rotations. Each three-year rotation included two years of pulses, oilseeds, or spring wheat. This was followed by durum wheat in the third year. The pulses studied were lentils, chickpeas, and peas.

The carbon footprint of each rotation was calculated by adding the greenhouse gases from: 1) farming operations including seeding, spraying, and harvesting; 2) the production, transportation, storage, delivery, and application of fertilizers and pesticides; and 3) the decomposition of crop residues.

The study concluded that including pulses in crop rotations can substantially lower the carbon footprint of a subsequent durum wheat crop by up to one-third. When durum was grown after a pulse crop, the carbon footprint was 28% less than when grown after spring wheat. When durum was grown after two consecutive years of pulse crops, the carbon footprint was 34% lower than when durum was grown after two years of cereal crops. The lower carbon footprint was primarily due to the reduction in nitrogen fertilizer requirement when durum was grown after a pulse crop. Pulses can also lower the carbon footprint by diversifying the crop rotation, leading to fewer weed and pest problems.

Nitrous Oxide Emissions and Prairie Agriculture

This brief article was published in 2008 in the peer-reviewed online journal Prairie Soils and Crops. It provides background information on the properties and sources of nitrous oxide, which is a potent greenhouse gas that also causes destruction of the ozone layer. Agriculture is the largest source of human-created nitrous oxide emissions. Methods of reducing nitrous oxide emissions in agriculture are discussed, as are areas where more research is needed.

In this article, the chemical processes that cause nitrous oxide emissions are described in a straightforward way. The greatest nitrous oxide emissions occur when high soil moisture coincides with high levels of soil nitrogen (in the form of nitrate). This suggests that there are opportunities to reduce nitrous oxide emissions with better farm management. Any practice that uses nitrogen more efficiently will reduce nitrous oxide emissions because it reduces excess nitrogen in the soil. Pulse crops can reduce nitrous oxide emissions because they don’t require nitrogen fertilizer. Innovations in fertilizer technology that may reduce nitrous oxide emissions are also described.

Nitrous oxide emissions are extremely variable over space and time, making measurements challenging. More research is needed to understand this variability so that the impact of different management practices can be understood.

Agriculture and Greenhouse Gases.

Henry Janzen is a research scientist with Agriculture and Agri-Food Canada, specializing in agricultural greenhouse gases. In this article, he provides a very readable summary of agricultural greenhouse gases that is intended to generate discussion among farmers, consumers and policy makers. This article appeared in 2008 in the peer-reviewed online journal Prairie Soils and Crops. It provides background information on how agriculture affects greenhouse gases, particularly methane and nitrous oxide. Carbon sequestration and carbon trading are also briefly summarized.

Several perspectives on the problem of agricultural greenhouse gases are discussed. Measuring greenhouse gases has many uncertainties and requires continuous improvement. Because farms are complex ecosystems, changing any one agricultural practice will affect the whole farm ecosystem. Therefore, the “big picture” of farm-level emissions should be the focus of attempts to reduce greenhouse gases. Farms provide important “ecosystem services” (such as food, employment, and wildlife habitat) that may be affected by reducing greenhouse gases. For example, reducing greenhouse gases may have additional benefits such as decreasing soil erosion. Alternately, negative effects such as lower crop yields may occur. Greenhouse gas emissions indicate inefficient use of energy, nutrients, or resources. Reducing greenhouse gases is therefore an opportunity to improve farm efficiency.

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