Soil nitrous oxide emissions from a soybean-wheat succession under different tillage systems in Southern Brazil

ABSTRACT No-tillage (NT) has been largely adopted in Brazil as a strategy for soil conservation, but for the last decade, there have been governmental incentives for its adoption arising from its potential for soil C accumulation. Notwithstanding, the soil mulch formed from crop residues favors the maintenance of soil moisture and nutrients in the upper soil layers, which stimulates soil microbial activity and may increase the potential for nitrous oxide (N2O) emissions. In addition, double-cropping systems in the same year are typical in Brazil and the impact on the fraction of fertilizer N lost as N2O needs to be evaluated. This study aimed to assess the influence of soil tillage and N fertilization on N2O emissions in a wheat-soybean succession system as commonly practiced in southern Brazil. The experiment was carried out at Embrapa Soybean research station located in Southern Brazil. Treatments were conventional tillage (CT) and no-tillage (NT), with and without nitrogen fertilization for the wheat and no N fertilizer for the soybean. Closed-static chambers were used to monitor N2O fluxes for two consecutive years. Together with gas monitoring, soil samples were also taken and analyzed for mineral N, soil moisture and labile carbon. Soybean yields were higher under NT, which seemed to be the result of a higher soil water availability that helped to overcome extended periods without rainfall. Soil N2O emissions were similar between CT and NT, with just a tendency for higher emissions under NT. The highest emissions occurred from the soybean crop. In the second year under NT, the emissions from the soybean crop were higher when preceded by N-fertilized wheat, but the converse was true under CT. None of the soil variables consistently correlated with N2O emissions, with mineral-N as the best predictor in the second wheat cycle and soil moisture in the first soybean cycle. Calculated emission factors were not statistically different between CT and NT and consistently lower than the IPCC default of 1 %. The calculated N2O emission intensity by relating N2O emission to grain yield showed an environmental advantage of NT compared to CT by presenting a 44 % reduction in soybean and similar values for fertilized wheat.摘要：免耕（No-tillage, NT）作为土壤保护策略已在巴西广泛应用，近十年来，因其具备土壤碳固存潜力，巴西政府陆续出台多项政策鼓励推广免耕技术。尽管免耕具有土壤保护优势，但由作物残体形成的土壤覆盖物可维持表层土壤的水分与养分水平，进而刺激土壤微生物活动，可能提升一氧化二氮（nitrous oxide, N₂O）的排放潜能。此外，巴西当地普遍采用年度复种制度，因此亟需评估肥料氮以N₂O形式流失的占比所受影响。 本研究旨在针对巴西南部常见的小麦-大豆轮作系统，探究土壤耕作方式与施氮措施对N₂O排放的影响。试验设于巴西农牧业研究公司（Embrapa）大豆研究站，该站位于巴西南部地区。试验设置常规耕作（Conventional Tillage, CT）与免耕（NT）两种耕作模式，其中小麦季分别设置施氮与不施氮处理，大豆季则不施加氮肥。采用静态密闭箱连续两年监测N₂O通量，同步采集土壤样品并分析矿质氮（mineral N）、土壤含水量与活性碳（labile carbon）含量。 免耕处理下的大豆籽粒产量更高，这一结果可能源于更高的土壤水分可用性，帮助作物克服了长时间无降雨的逆境。常规耕作与免耕的N₂O总排放量相近，仅免耕处理表现出排放略高的趋势。N₂O排放峰值出现在大豆种植季。在免耕处理的第二个试验年度，前茬为施氮小麦的大豆季N₂O排放更高；而在常规耕作处理下则呈现相反结果。 所有土壤变量均未与N₂O排放呈现稳定的相关性：矿质氮是第二个小麦季的最佳预测因子，而土壤含水量则是第一个大豆季的最佳预测因子。计算得到的排放因子在常规耕作与免耕处理间无统计学差异，且均显著低于政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）默认的1%阈值。通过将N₂O排放与籽粒产量关联计算得到的N₂O排放强度显示，免耕处理相比常规耕作具有环境优势：大豆季的排放强度降低了44%，而施氮小麦季的排放强度则与常规耕作相近。