Can Cover Crops Reduce Nitrous Oxide Emissions from Agricultural Soils?

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Despite recent emission reductions of greenhouse gases (GHG) by most economic sectors in Canada, agriculture is lagging. This is because doubling in synthetic N fertilizer use in the past several decades for crop production has increased emissions of the greenhouse gas, nitrous oxide (N2O). Lowering emissions from Canadian agricultural soils is imperative if as a nation, we are to meet international commitments to greenhouse gas reductions. Further, crop production must modernize to limit nitrogen loss to the environment and increase fertilizer N utilization by crops. Ultimately, we must strive to create new agricultural systems that don't emit greenhouse gases and conserve nutrients in crops. Organic food production systems are thought to decrease net GHG emissions compared to conventional systems. However, the common practice of incorporating legume forages as green manures results in carry over of nitrate nitrogen over winter and very large emissions of N2O during the following thaw, if the soil is saturated. Thaw emissions also occur in conventional crop production accounting for 15-50% of annual N2O emissions from agricultural soils in the cold winter climates of Canada. Farmers are adopting cover cropping where plants sown following harvest of annual grains or oilseeds provide soil cover from fall through spring. There are multiple benefits to cover cropping including lowering soil nitrate levels going into winter. What the impact of this lowering of nitrate levels has on thaw emissions of N2O is not known for our climate. Past studies in warmer climates indicate that cover crops can increase emissions by feeding soil organisms producing N2O. Thus, it is important to know if the increasingly popular practice of cover cropping is decreasing or increasing N2O emissions. As part of the journey of my laboratory toward its long-term goal of changing how food is produced to be a net non-emitter of GHGs from soil, this Discovery Grant program focuses on answering the following questions; a) do covers reduce or increase thaw N2O emissions following alfalfa ploughdown in organic production?, b) do covers reduce or increase thaw emissions in conventional production?, c) can covers lower nitrate and soil moisture to limit thaw emissions?, d) what is the net result of feeding soil organisms and lowering nitrate with covers on N2O emissions; does moisture affect the balance?, e) how do microbial genes producing N2O relate to thaw emissions and effects of covers, management, and soil conditions?, and f) can the widely used simulation model, DNDC, predict cover effects on emissions?. The outcome of this program is to foster practices to decrease N2O emissions from crop production. The implications are to help in reversing the recent trend for increasing N2O emissions from agricultural soils and meet commitments to actually reduce them.