In East Africa, rainfed lowland rice is primarily produced by smallholders in alluvial floodplain and inland valley wetlands. These wetlands differ in their dominant soil types and water regimes that vary seasonally, inter-annually and between field positions. Yield responses to mineral nitrogen (N) fertiliser thus likely vary between and within wetlands and years, modulating the profitability of N fertiliser use. Therefore, the locally-validated APSIM model was used to study yield responses to N fertiliser rates (0, 30, 60, 90, 120, and 150 kg ha^-1) and supplemental irrigation at different field positions in a floodplain in Tanzania (fringe and middle positions) and an inland valley in Uganda (valley-fringe, mid-valley and valley-bottom positions) over 30-years. Average rainfed yield gains with mineral N and N use efficiencies were high, ranging between 1.7 and 4.5 Mg ha^-1 and 27–70 kg kg^-1 in the floodplain and between 1.0 and 3.2 Mg ha^-1 and 18–34 kg kg^-1 in the inland valley, depending on field position, N rate and year. Consequently, N fertiliser use was generally profitable in both wetlands, with value/cost ratios ≥ 4 and marginal rates of returns > 150%. Profitable N rates in all years were 30–120 kg ha^-1 in the fringe and 30–90 kg ha^-1 in the middle positions of the floodplain, and 60–150 kg ha^-1 in the mid-valley and 90–150 kg ha^-1 in the valley-bottom positions of the inland valley. In the valley-fringe position, N fertiliser use was comparatively riskier and profitable only in 77–90% of years at N rates of 60–150 kg ha^-1. Supplemental irrigation may help boost N fertiliser use efficiencies and use profitability with average yield gains of > 1.5 and > 0.4 Mg ha^-1 in the floodplain and inland valley, respectively, while simulated spatial-temporal water stress pattern may help guide efficient irrigation scheduling.

在东非，雨育低地水稻主要由冲积洪泛区和内陆山谷湿地的小农生产。这些湿地的主要土壤类型和水情因季节、年际和田间位置而异。因此，对矿物氮 (N) 肥料的产量反应可能会在湿地和年份之间和内部有所不同，从而调节氮肥使用的盈利能力。因此，本地验证的 APSIM 模型用于研究产量对氮肥施用量（0、30、60、90、120 和 150 kg ha ^-1) 以及 30 多年来在坦桑尼亚洪泛区（边缘和中间位置）和乌干达内陆山谷（山谷边缘、山谷中部和谷底位置）的不同田间位置进行补充灌溉。使用矿物氮和氮利用效率的雨养平均产量增加很高，洪泛区在 1.7 和 4.5 Mg ha ^-1和 27-70 kg kg ^-1之间，在 1.0 和 3.2 Mg ha ^-1和 18-34 kg kg ^-之间1在内陆山谷中，取决于田间位置、氮含量和年份。因此，在这两个湿地中，氮肥的使用通常都是有利可图的，价值/成本比≥ 4，边际回报率 > 150%。所有年份的盈利氮肥率为 30-120 kg ha ^-1在边缘和 30-90 kg ha^-1位于洪泛区中部，60 ～150 kg ha ^-1位于中谷，90～150 kg ha ^-1位于内陆河谷谷底。在山谷边缘位置，仅在 77-90% 的年份中，在 60-150 kg ha ^-1的 N 施用量下，使用 N 肥料的风险相对较高且有利可图。补充灌溉可能有助于提高氮肥的使用效率和使用盈利能力，在洪泛区和内陆河谷的平均产量增益分别 > 1.5 和 > 0.4 Mg ha ^-1，而模拟的时空水分胁迫模式可能有助于指导有效的灌溉计划。