Can Cover Crops Reduce Nitrous Oxide Emissions from Agricultural Soils?

尽管加拿大大多数经济部门最近都在减少温室气体（GHG）的排放，但农业却落后了。这是因为在过去的几十年里，用于农作物生产的合成氮肥使用量翻了一番，增加了温室气体一氧化二氮（N2O）的排放。如果作为一个国家，我们要履行减少温室气体排放的国际承诺，降低加拿大农业土壤的排放是必不可少的。此外，作物生产必须实现现代化，以限制氮肥流失到环境中，增加作物对氮肥的利用。最终，我们必须努力创造不排放温室气体并保存作物养分的新农业系统。有机食品生产系统被认为与传统系统相比可以减少温室气体净排放。然而，如果土壤饱和，将豆科牧草作为绿色肥料的常见做法会导致硝酸盐氮在冬季的携带和N2O在随后的解冻期间的大量排放。解冻排放也发生在传统作物生产中，占加拿大寒冷冬季气候下农业土壤年N2O排放量的15-50%。农民正在采用覆盖种植，在一年生谷物或油籽收获后播种，从秋天到春天提供土壤覆盖。覆盖作物有很多好处，包括降低进入冬季的土壤硝酸盐水平。在我们的气候中，硝酸盐水平的降低对N2O融化排放的影响尚不清楚。过去在温暖气候下的研究表明，覆盖作物可以通过喂养产生N2O的土壤生物来增加排放。因此，了解日益流行的覆盖种植是减少还是增加N2O排放是很重要的。作为我的实验室实现其长期目标的一部分，改变食物的生产方式，使其成为土壤中温室气体的净不排放者，这个发现资助计划侧重于回答以下问题；a)覆盖物是否减少或增加有机生产中苜蓿犁耕后解冻的N2O排放？b)在传统生产过程中，覆盖是减少还是增加了融雪排放？c)是否可以覆盖较低的硝酸盐和土壤水分来限制解冻排放？d)饲喂土壤生物和覆盖降低硝酸盐对N2O排放的净结果是什么；湿气会影响平衡吗？e)产生N2O的微生物基因如何与融化排放和覆盖、管理和土壤条件的影响相关？f)广泛使用的模拟模型DNDC能否预测覆盖对排放的影响？该项目的成果是促进减少作物生产过程中一氧化二氮排放的做法。其含义是有助于扭转最近农业土壤中N2O排放增加的趋势，并履行实际减少它们的承诺。

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.