Table1_Climate Overrides Effects of Fertilizer and Straw Management as Controls of Nitrous Oxide Emissions After Oilseed Rape Harvest.docx

Oilseed rape (Brassica napus L.) is an important bioenergy crop that contributes to the diversification of renewable energy supply and mitigation of fossil fuel CO2 emissions. Typical oilseed rape crop management includes the use of nitrogen (N) fertilizer and the incorporation of oilseed rape straw into soil after harvest. However, both management options risk increasing soil emissions of nitrous oxide (N2O). The aim of this 2-years field experiment was to identify the regulating factors of N cycling with emphasis on N2O emissions during the post-harvest period. As well as the N2O emission rates, soil ammonia (NH4+) and nitrate (NO3−) contents, crop residue and seed yield were also measured. Treatments included variation of fertilizer (non-fertilized, 90 and 180 kg N ha−1) and residue management (straw remaining, straw removal). Measured N2O emission data showed large intra- and inter-annual variations ranging from 0.5 (No-fert + str) to 1.0 kg N2O-N ha−1 (Fert-180 + str) in 2013 and from 4.1 (Fert-90 + str) to 7.3 kg N2O-N ha−1 (No-fert + str) in 2014. Cumulative N2O emissions showed that straw incorporation led to no difference or slightly reduced N2O emissions compared with treatments with straw removal, while N fertilization has no effect on post-harvest N2O emissions. A process-based model, CoupModel, was used to explain the large annual variation of N2O after calibration with measured environmental data. Both modeled and measured data suggest that soil water-filled pore space and temperature were the key factors controlling post-harvest N2O emissions, even though the model seemed to show a higher N2O response to the N fertilizer levels than our measured data. We conclude that straw incorporation in oilseed rape cropping is environmentally beneficial for mitigating N2O losses. The revealed importance of climate in regulating the emissions implies the value of multi-year measurements. Future studies should focus on new management practices to mitigate detrimental effects caused by global warming, for example by using cover crops.

油菜（Brassica napus L.）是一类重要的生物能源作物，可助力可再生能源供应多元化，并缓解化石燃料二氧化碳排放。常规油菜田间管理措施包括施用氮肥（N），以及收获后将油菜秸秆翻压还田。然而，这两类管理措施均存在提升土壤一氧化二氮（N₂O）排放的潜在风险。本研究开展了为期2年的田间试验，旨在明确氮循环的调控因子，重点关注收获后阶段的N₂O排放特征。除N₂O排放速率外，试验同时测定了土壤铵态氮（NH₄⁺）、硝态氮（NO₃⁻）含量、作物残体量与种子产量。
试验设置了氮肥施用梯度（不施肥、90 kg N ha⁻¹、180 kg N ha⁻¹）与秸秆管理模式（秸秆留存、秸秆移除）两个变量。实测N₂O排放数据显示出显著的年内及年际波动：2013年N₂O-N排放量区间为0.5 kg N₂O-N ha⁻¹（不施肥+秸秆还田处理）至1.0 kg N₂O-N ha⁻¹（施180 kg N ha⁻¹+秸秆还田处理）；2014年该区间则为4.1 kg N₂O-N ha⁻¹（施90 kg N ha⁻¹+秸秆还田处理）至7.3 kg N₂O-N ha⁻¹（不施肥+秸秆还田处理）。
累积N₂O排放量分析结果表明，相较于秸秆移除处理，秸秆还田处理的N₂O排放量无显著差异或仅略有降低；而氮肥施用对收获后N₂O排放无显著影响。
本研究采用基于过程的模型（process-based model）CoupModel，结合实测环境数据完成校准后，用以解析N₂O排放的年际差异。模拟结果与实测数据均显示，土壤充水孔隙度与温度是调控收获后N₂O排放的关键因子，尽管该模型对氮肥水平的N₂O响应强度高于实测结果。
综上，油菜种植体系中的秸秆还田在缓解N₂O损失方面具备环境效益。本研究揭示的气候对排放的调控作用，凸显了多年度监测的重要价值。未来研究应聚焦于新型管理措施，以缓解全球变暖带来的不利影响，例如通过种植覆盖作物实现。