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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. • Yield responses to N fertiliser in East African lowlands were studied using APSIM. • NUE a was 27–70 kg kg-1 in the floodplain and 18–34 kg kg-1 in the inland valley. • Hydro-edaphic condition and N rate affected yield and fertiliser use profitability. • N fertiliser use was largely profitable but somewhat riskier in the inland valley. • Supplemental irrigation boosted N fertiliser use efficiency and use profitability. [ABSTRACT FROM AUTHOR]

Lowland rice yields in East Africa remain low despite favourable hydro-edaphic conditions as benefits from improved cultural management vary between and within wetland types and interactions are poorly understood. Hence, multi-year agronomic field experiments were established to assess the differential responses of lowland rice to management (rainfed 0 and 60 kg N ha−1, and irrigated 120 kg N ha−1 + 60 kg PK ha−1) and field position within a floodplain in Tanzania (fringe and middle positions) and an inland valley in Uganda (valley-fringe, mid-valley and valley-bottom positions). We then calibrated and validated the Agricultural Production System Simulator (APSIM), evaluated the importance of external water table data as model input and assessed the relative effects of water and N stress on yield as affected by wetland type and field position. Yields of 3.2–9.2 Mg ha−1 were attained in the floodplain and of 1.9–6.3 Mg ha−1 in the inland valley, highlighting the substantial scope to boost yields beyond current regional means of around 2 Mg ha−1. The model estimated grain yields in both wetlands well within the experimental uncertainty during model validation (n = 12, r2 = 0.76, RMSEa= 0.92 Mg ha−1 in the floodplain; n = 18, r2 = 0.71, RMSEa= 0.72 Mg ha−1 in the inland valley). Results further emphasised the importance of external water table data for sound model performance as they evidently alleviated seasonal droughts. Simulated abiotic stress patterns additionally highlighted hydro-edaphic differences from field positioning within and between both wetlands. While low soil N was generally the main yield constraint, water stress was comparably more pronounced in the inland valley and supplemental irrigation thus more beneficial on yield. Hydro-edaphic field conditions favoured rice production in the floodplain's fringe with comparably lower N stress, while large spatial-temporal variabilities prevented a distinct delineation based on toposequential field positions in the inland valley. • Improved cultural practices can boost East African lowland rice yields substantially. • The APSIM model adequately simulated rice performance using water table input data. • Low soil N was the main yield constraint but fertiliser responses were significant. • Water stress was more pronounced in the inland valley and irrigation more beneficial. • Hydro-edaphic field conditions modulated rice yields, particularly in the floodplain. [ABSTRACT FROM AUTHOR]