Mario Tenuta, Senior Industrial Research Chair in 4R Nutrient Stewardship, Department of Soil Science, University of Manitoba
CLIMATE SCIENTISTS HAVE an overwhelming consensus that greenhouse gases (carbon dioxide, methane and nitrous oxide) emitted from human activity has resulted in an unprecedented increase in global mean atmospheric and ocean temperatures. The consequences to Manitoba of warmer winters, shorter duration of northern ice roads, extremes in moisture and drought-increased forest fires seem less severe than for other regions where sea level rise will inundate 10s of millions of homes if not protected. However, this article is not about doom and gloom but rather about what we can do to reduce emissions in growing food.
Canada, among many other countries, is committed to reducing greenhouse gas emissions to help lessen the impact of future warming. The commitment is 40% by 2030 of 2020 levels. Canada’s approach to reducing emissions is to have all economic sectors contribute, some more than others. The commitment for agriculture is less than for other sectors being a 30% reduction of nitrous oxide (N2O) emissions from nitrogen fertilizer applied to the soil. Nitrous oxide emissions from manures and crop residues are not on the table, as aren’t methane emissions from livestock and manure. The reduction of N2O is voluntary target for the agriculture sector. Nitrous oxide emissions comprise about 5% of greenhouse gases emitted in Canada. It seems like a small amount, but the majority of those emissions come from nitrogen addition to soil.
Nitrous oxide emission from soil is a natural process produced by native soil bacteria utilizing any form of available N from synthetic fertilizers, manure, residues and decaying fresh plant roots and matter. Nitrogen fertilizers are targeted because their consumption increases steadily with better varieties and hybrids, and there are many management options for farmers to reduce the amount of emissions from their use.
Canadian farmers are good stewards of land and among the best in the world in adapting and meeting challenges to improve the sustainability of their farms. I see achieving N2O reductions from fertilizer use to the advantage of growers where our grains are marketed globally. Nitrogen fertilizers are the largest, if not among the greatest operational cost of crop production, losses of nitrogen mean lost profit, and reducing N2O emissions signals to the Canadian public and global markets that our commodities are sustainably produced.
Increasing grain and forage legume production is a key strategy, such as using advanced 4R practices to achieve emission reductions. Let’s review the past and future benefits of pulse, soy and forage legume production to reduce nitrous oxide emissions.
Teams of researchers with Agriculture and Agri-food Canada across the Prairies using long-term
field studies have shown total grain production, protein production and efficiency of N utilization in crop production to be improved by including pulses such as field pea, lentil and chickpea. The benefits stem from pulses contributing to less summer fallow, free N from the legumes, and increasing rotation lengths to reduce disease issues. In particular, the introduction of pulse production and canola and direct seeding/no-till led to a dramatic decrease in intentional summer fallow starting around 1970 and really picking up steam by 1980. In Manitoba, soy acreage has risen since the turn of the new millennium. The Manitoba crop insurance data (2011-2020) shows that the major field crops, navy beans (10% increase) and soybeans (6%) in rotation, increase the yield of subsequent crops the most.
An example of the soil carbon gains from the removal of fallow and replacement by pulses is a ten-year study from Agriculture and Agri-food Canada colleagues at Swift Current. From 1995 to 2005 removal of fallow in a three-year rotation with pulses (field pea, lentil and chickpea) increased soil organic carbon by 200 kg/ha in the top 6” of soil. That’s a considerable increase for a dry climate. In Manitoba, an increase in soil organic carbon is most prominent in West Man, where no-till and field pea are more common.
The work of colleagues at Agriculture and Agri-food Canada Lethbridge shows that grain and forage legumes biologically produce a considerable amount of nitrogen. In total, 0.9 million metric tonnes of N are produced by legumes across Canada annually. That is equivalent to 90% of the synthetic nitrogen fertilizer used in Saskatchewan in a year, Canada’s largest consumer of N fertilizer. The greenhouse gas emission reduction from having bacteria produce the N compared to manufacturing using natural gas is astounding, being 8 million metric tonnes of CO2 per year and reducing greenhouse gas emissions in Canada by 1.2%.
Grain and forage legumes that produce their own N also reduce greenhouse gas emissions. Our research sponsored in part by Manitoba pulse & soybean growers, has shown soybean, fababean and alfalfa to emit little more N2O than field crops such as spring wheat grown without any N fertilizer added. In essence, emissions are very near background or what the soil produces from mineralization
and subsequent N transformation in soil organic matter. In fact, of many management practices examined to date by our team, soybean, pulse and forage legumes that produce their own N reduced N2O emissions from soil by 61% compared to N-fertilized field crops. The legumes’ emissions that still amount to 39% are actually that background level from soil organic matter. The next best practice to reduce emissions is split application of fertilizer N (part at plant and part at early vegetative stages) of field crops for a 48% reduction with enhanced efficiency fertilizers such as nitrification inhibited and polymer- coated urea products coming in with around a 30% reduction.
So soybean, forage legumes and pulses that don’t receive N fertilizer are important for reducing nitrous oxide emissions. Recent work by Kristen MacMillan at the University of Manitoba strongly indicates dry beans are better N producers than previously thought, where it doesn’t pay off to apply N. That is an additional 182,000 acres of cropland for Manitoba in dry beans that could not be producing greenhouses from manufacturing fertilizer and nitrous oxide emissions from soil to add to the already 1.3 million acres of soybean and 170,000 acres of field peas not receiving N.
The benefit of reducing N2O emissions by growing legume crops was evident as N2O emissions from field crops in Manitoba levelled off the mid- last decade. My modelling for major field crops in Manitoba (wheat, barley, oats, canola, corn and soybean) showed that nitrous oxide emissions levelled as soybean acres peaked in 2017. The drop in soybean acreas from 2017 resulted in a big increase in nitrous oxide emissions. In effect, the increased soybean acres replacing cereals balanced an increase in emissions from more N being applied to canola in the province.
Can we achieve a 30% reduction in nitrous oxide emissions from field crop production in Manitoba from fertilizer use? Yes, we can, but a few approaches will be taken together. Clearly, 4R practices such as split application, enhanced efficiency products, and emerging technologies such as variable rate N and enhanced efficiency products applied to management zones are important. Particularly reducing barriers to their use, such as costs. However, I believe even more reductions are possible to offset possible increasing N rates are crop yields continue to increase from increasing the area of soybean and pulse production in Manitoba. Market demand for plant proteins and edible oils will need to drive that increase in pulse production with decisions by growers to decrease production costs by lowering their fertilizer bills.
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