Yantai Gan, Research Scientist, Agriculture and Agri-Food Canada
The fight against climate change calls upon all economic sectors to do their part. Agriculture accounts for about 8% to 12% of total greenhouse gas emissions worldwide and the way that grain and other food crops are produced on farmland has significant impacts on the amount and type of these emissions. Adopting improved farming practices can increase crop yield while reducing the ‘carbon footprints’ of agricultural products.
Crop production can result in many types of greenhouse gas emissions. Fossil fuels are used in various farming operations, such as planting, applying fertilizers and pesticides, and harvesting. Considerable amount of fossil fuel energy is used during the manufacture, transport, storage, and delivery of agrichemicals and other crop inputs to farm gates. Tillage and other activities trigger release of carbon to the atmosphere from soil and crop residues. Another major contributor to agricultural greenhouse gas emissions is nitrous oxide related to nitrogen fertilizer application to farm soils. Improving nitrogen use efficiency or replacing inorganic nitrogen fertilizer with biological fixation of atmospheric nitrogen by pulse plants can substantially decrease greenhouse gas emissions in crop production.
Nitrogen to share
One practice that has proven to be effective is the use of pulse crops in crop rotation systems. A powerful partnership between pulse plant roots and soil microbes, known as rhizobia, harnesses the sun’s energy to transform inert atmospheric nitrogen into forms that can be used by plants. This energy transformation provides a large part of the nitrogen for pulse plant growth as well as ‘residual’ nitrogen in roots, nodules, and soil that benefits subsequent crops.
Although pulse plants usually have a lower aboveground biomass than wheat, the higher nitrogen concentration in pulse residues provides greater nitrogen benefits. For example, a 25-year field study conducted in southwestern Saskatchewan, Canada, helped researchers to see that, compared to continuous wheat monoculture, rotation of lentils with spring wheat produced a similar amount of wheat grain yield and required 29% less nitrogen fertilizer to be applied. In other words, the efficiency of fertilizer nitrogen use in the wheat-lentil rotation averaged 81% greater than continuous wheat.
Carbon out of the air
Farmers can also help the fight against climate change by sequestering more carbon. Through photosynthesis, atmospheric carbon dioxide is converted into plant residues and eventually stored in the soil. Greenhouse gas emissions from crop production can be entirely or partly offset by increased carbon sequestration. In a long-term crop rotation study at the Agriculture and Agri-Food Canada Research Centre in Swift Current, a lentil-wheat rotation gained an average 1039 kg CO2-equivalent per hectare each year through soil carbon sequestration. This carbon gain out-distanced other cropping systems by 26% (continuous wheat) to 62% (summerfallow-flax-wheat rotation). Consequently, spring wheat grown in rotation with lentil attained carbon neutrality, that is, the carbon fixed by pulse and wheat plants offset the carbon emissions during the production of the crops.
Beneficial microbe loop
Another great positive aspect of pulse-cereal rotations is that pulse roots stimulate soil microbial community activity in the year the pulse is grown. This beneficial microbial activity provides a large positive feedback to the crop in rotation. The feedback mechanism allows the cereal grown following a pulse in rotation to achieve rapid root growth, to become more tolerant to abiotic stress, and to produce higher grain yield, without requiring additional inputs by the farmer.
As we see greater awareness of climate change and energy security, we are also seeing increased investigation of improved farm management systems that can produce high-quality and affordable food in sufficient quantities while minimizing potentially negative impacts on the environment. Studies in western Canada demonstrate that each individual farming practice has its own role in affecting crop yield, but the integration of pulse crops in well-designed crop rotations can boost grain production for the entire farm, while concurrently reducing the carbon footprint of crop production.
Next week, at a meeting of the Intergovernmental Panel on Climate Change, experts will be discussing impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways. Promoting pulses in crop rotations is one of many solutions the world can implement right now to mitigate climate change while also improving agricultural sustainability and food security.