Establishing a Regional Nitrogen Management Approach to Mitigate Greenhouse Gas Emission Intensity from Intensive Smallholder Maize Production

The overuse of Nitrogen (N) fertilizers on smallholder farms in rapidly developing countries has increased greenhouse gas (GHG) emissions and accelerated global N consumption over the past 20 years. In this study, a regional N management approach was developed based on the cost of the agricultural response to N application rates from 1,726 on-farm experiments to optimize N management across 12 agroecological subregions in the intensive Chinese smallholder maize belt. The grain yield and GHG emission intensity of this regional N management approach was investigated and compared to field-specific N management and farmers' practices. The regional N rate ranged from 150 to 219 kg N ha−1 for the 12 agroecological subregions. Grain yields and GHG emission intensities were consistent with this regional N management approach compared to field-specific N management, which indicated that this regional N rate was close to the economically optimal N application. This regional N management approach, if widely adopted in China, could reduce N fertilizer use by more than 1.4 MT per year, increase maize production by 31.9 MT annually, and reduce annual GHG emissions by 18.6 MT. This regional N management approach can minimize net N losses and reduce GHG emission intensity from over- and underapplications, and therefore can also be used as a reference point for regional agricultural extension employees where soil and/or plant N monitoring is lacking.过去20年间，快速发展中国家的小农户农场过量施用氮肥（Nitrogen Fertilizer），不仅加剧了温室气体（Greenhouse Gas, GHG）排放，还推高了全球氮素消耗总量。本研究基于1726个田间定位试验（on-farm experiments）所得的不同氮肥施用量对应的农业响应成本，构建了一套区域氮肥管理方案（regional N management approach），用于优化中国集约化小农户玉米种植带内12个农业生态亚区（agroecological subregions）的氮肥管理策略。研究对该区域氮肥管理方案的籽粒产量与温室气体排放强度进行了评估，并与田块专属氮肥管理方案（field-specific N management）及农户常规种植模式（farmers' practices）展开对比。针对这12个农业生态亚区，区域推荐氮肥施用量区间为150~219 kg·N·ha⁻¹。相较于田块专属氮肥管理方案，该区域氮肥管理方案下的籽粒产量与温室气体排放强度表现相当，表明该区域推荐施氮量接近经济最优施氮量。若这套区域氮肥管理方案在中国全域大规模推广，可实现年减少氮肥施用量超140万公吨，年均玉米增产3190万公吨，年减排温室气体1860万公吨。该方案可通过调控过量或不足施氮情形，最小化氮素净损失（net N losses）并降低温室气体排放强度，因此在缺乏土壤和/或植株氮素监测（soil and/or plant N monitoring）条件的地区，也可作为区域农业推广人员的参考依据。