Can Cover Crops Reduce Nitrous Oxide Emissions from Agricultural Soils?

尽管加拿大大多数经济部门最近减少了温室气体(GHG)的排放，但农业仍然落后。这是因为在过去几十年里，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。作为一个国家，如果我们要履行减少温室气体排放的国际承诺，那么减少加拿大农业土壤的排放就势在必行。此外，作物生产必须现代化，以限制氮素对环境的损失，并增加作物对氮肥的利用。归根结底，我们必须努力创建不排放温室气体和保护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量。然而，如果土壤饱和，将豆科牧草作为绿肥种植的普遍做法会导致硝态氮在冬季携带，并在随后的解冻期间排放非常大的N2O。融化排放也出现在常规作物生产中，占加拿大寒冷冬季气候下农业土壤N2O年排放量的15%-50%。农民们正在采用覆盖种植，即在一年一度的谷物或油籽收获后播种，提供从秋季到春季的土壤覆盖。覆盖作物有多种好处，包括进入冬季后降低土壤硝酸盐水平。对于我们的气候，硝酸盐水平的降低对N2O融化排放的影响尚不清楚。过去在较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，重要的是要知道日益流行的覆盖种植做法是减少还是增加了N2O的排放。作为我的实验室实现其长期目标的一部分，即改变粮食的生产方式，使其成为土壤中温室气体的净非排放者，这个发现资助计划侧重于回答以下问题：a)覆盖在有机生产中的紫花苜蓿后，覆盖是否减少或增加了融化的N2O排放？b)覆盖是否减少或增加了常规生产中的融化排放？c)覆盖更低的硝酸盐和土壤水分以限制融化排放吗？d)覆盖土壤生物和降低硝酸盐的净结果是什么；湿度影响平衡吗？e)产生N2O的微生物基因如何与融化排放以及覆盖、管理和土壤条件的影响有关？f)广泛使用的模拟模型DNDC能否预测覆盖对排放的影响？该计划的结果是促进实践，以减少作物生产中的N2O排放。其影响是有助于扭转最近农业土壤N2O排放增加的趋势，并履行实际减少N2O排放的承诺。

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.