A synthesis of nitric oxide emissions across global fertilized croplands from crop-specific emission factors

Nitrogen (N)-fertilizer application to agricultural soils results in
substantial emissions of nitric oxide (NO), a key substance in
tropospheric chemistry involved in climate forcing and air pollution.
However, estimates of global cropland NO emissions remain uncertain due to
a lack of information on direct NO emission factors (EFds) of applied N
for variours cropping systems at seasonal or annual scales. Here we
quantified the crop-specific seasonal and annual-scale NO EFds through
synthesizing 1094 measurements from 125 field-based studies worldwide. The
global mean crop-specific seasonal EFd was 0.53%, with the highest for
vegetables (0.75%). Among cereal crops, the EFd of maize (0.45%) or wheat
(0.47%) was about three-times higher than for rice (0.12%). At annual
scale, the mean EFd across all cropping systems was 0.58%, with tea
plantations having the highest (1.54%). For other cropping systems, the
annual-scale EFds ranged from 0.02% to 1.07%. Besides crop type, also soil
organic carbon, total N and pH as well as N fertilizer type were the main
factors explaining the variations of NO EFds. Based on obtained specific
EFds for each crop type, we estimated that NO emissions due to the use of
synthetic fertilizers from global croplands are about 0.42–0.62 Tg N yr−1.
Our budgets are relatively lower if compared to estimates derived by the
use of IPCC defaults for NO emissions (0.72–1.66 Tg N yr−1) or reported
elsewhere (0.67–1.04 Tg N yr−1). In our estimates, cash crops (vegetable,
tea and orchard), which cover only 9% of the world cropland area,
contributed about 31% to total NO emissions from global fertilized
croplands. Overall, our meta-analysis provides improved crop-specific NO
EFds reflecting current stage of knowledge. The work also highlights the
relative importance of cash crop production as sources for atmospheric NO,
i.e., agricultural systems on which mitigation efforts may focus.

向农田土壤施用氮肥（N肥）会导致大量一氧化氮（NO）排放，而一氧化氮是对流层化学中的关键物质，参与气候强迫与大气污染过程。然而，由于缺乏全球范围内不同种植系统在季节或年尺度下的施用氮肥直接NO排放因子（EFds）相关数据，全球农田NO排放的估算仍存在较大不确定性。本研究通过整合全球125项田间研究的1094组实测数据，量化了不同作物专属的季节与年尺度直接NO排放因子。全球作物专属季节尺度直接NO排放因子的平均值为0.53%，其中蔬菜类的排放因子最高，达0.75%。在谷类作物中，玉米（0.45%）与小麦（0.47%）的排放因子约为水稻（0.12%）的3倍。年尺度下，所有种植系统的平均直接NO排放因子为0.58%，其中茶园的排放因子最高（1.54%）。其余种植系统的年尺度直接NO排放因子范围为0.02%至1.07%。除作物类型外，土壤有机碳、总氮、pH值以及氮肥类型也是解释NO排放因子变异的主要影响因素。基于各作物类型的专属排放因子，本研究估算全球农田施用合成肥料产生的NO排放量约为0.42–0.62太克氮每年（Tg N yr⁻¹）。本研究得到的排放估算值相对偏低，相较于采用政府间气候变化专门委员会（IPCC）默认排放参数得到的估算结果（0.72–1.66 Tg N yr⁻¹），或是其他文献报道的结果（0.67–1.04 Tg N yr⁻¹）。在本研究的估算中，仅占全球农田面积9%的经济作物（蔬菜、茶园与果园）贡献了全球施肥农田总NO排放量的约31%。总体而言，本荟萃分析提供了更贴合当前学术认知水平的作物专属NO排放因子。本研究同时强调了经济作物生产作为大气NO排放源的相对重要性，即减排工作可重点聚焦于这类农业生产系统。