Data from: Initial nitrous oxide, carbon dioxide, and methane costs of converting conservation reserve program grassland to row crops under no-till vs. conventional tillage

Around 4.4 million ha of land in USDA Conservation Reserve Program (CRP) contracts will expire between 2013 and 2018 and some will likely return to crop production. No-till (NT) management offers the potential to reduce the global warming costs of CO2 , CH4 , and N2 O emissions during CRP conversion, but to date there have been no CRP conversion tillage comparisons. In 2009, we converted portions of three 9-21 ha CRP fields in Michigan to conventional tillage (CT) or NT soybean production and reserved a fourth field for reference. Both CO2 and N2 O fluxes increased following herbicide application in all converted fields, but in the CT treatment substantial and immediate N2 O and CO2 fluxes occurred after tillage. For the initial 201-day conversion period, average daily N2 O fluxes (g N2 O-N ha-1 d-1 ) were significantly different in the order: CT (47.5 ± 6.31, n = 6) ≫ NT (16.7 ± 2.45, n = 6) ≫ reference (2.51 ± 0.73, n = 4). Similarly, soil CO2 fluxes in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH4 (-0.69 ± 0.42 to -1.86 ± 0.37 g CH4 -C ha-1 d-1 ) with no significant differences among treatments. The positive global warming impact (GWI) of converted soybean fields under both CT (11.5 Mg CO2 e ha-1 ) and NT (2.87 Mg CO2 e ha-1 ) was in contrast to the negative GWI of the unconverted reference field (-3.5 Mg CO2 e ha-1 ) with on-going greenhouse gas (GHG) mitigation. N2 O contributed 39.3% and 55.0% of the GWI under CT and NT systems with the remainder contributed by CO2 (60.7% and 45.0%, respectively). Including foregone mitigation, we conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion.美国农业部（United States Department of Agriculture, USDA）保护储备计划（Conservation Reserve Program, CRP）的合约中，约440万公顷土地将在2013至2018年间到期，其中部分地块可能恢复农作物种植。免耕（No-till, NT）管理有望在CRP土地转换过程中，降低CO₂、CH₄和N₂O排放带来的全球变暖成本，但迄今为止尚未有针对CRP土地转换的耕作方式对比研究。2009年，我们将密歇根州3块面积为9~21公顷的CRP土地中的部分区域，分别转换为常规耕作（Conventional Tillage, CT）和免耕大豆种植模式，并预留第4块地块作为对照。所有转换地块在施用除草剂后，CO₂和N₂O的排放通量均有所上升；但在常规耕作处理中，耕作操作后立即出现了大量的N₂O和CO₂排放通量。在转换初期的201天内，日均N₂O通量（单位：g N₂O-N·ha⁻¹·d⁻¹）呈现显著差异，排序为：常规耕作（47.5±6.31，n=6）≫免耕（16.7±2.45，n=6）≫对照地块（2.51±0.73，n=4）。类似地，常规耕作地块的土壤CO₂通量为免耕地块的1.2倍，为未转换的CRP对照地块的3.1倍。所有处理组的CH₄均表现为弱吸收汇，通量范围为-0.69±0.42至-1.86±0.37 g CH₄-C·ha⁻¹·d⁻¹，组间无显著差异。转换后的大豆田在常规耕作（11.5 Mg CO₂e·ha⁻¹）和免耕（2.87 Mg CO₂e·ha⁻¹）模式下均呈现正的全球变暖影响（Global Warming Impact, GWI）；而未转换的CRP对照地块因持续开展温室气体（Greenhouse Gas, GHG）减排，呈现负的GWI（-3.5 Mg CO₂e·ha⁻¹）。在常规耕作和免耕系统中，N₂O分别贡献了GWI的39.3%和55.0%，剩余部分由CO₂贡献（占比分别为60.7%和45.0%）。综合考虑原有减排收益的损失后，我们得出结论：在CRP土地转换初期，免耕管理相比常规耕作可降低约60%的温室气体成本。