Biogeochemical Responses of Rwandan Wetland Rice Soils to Flooding: Implications for Nutrient Management and Methane Emissions

Abstract

Paddy rice farming in Rwandan wetlands is expanding, yet the effects of prolonged flooding on soil biogeochemistry remain poorly quantified across representative wetland soils. We measured soil redox potential (Eh), pH, N, P, Fe, Mn, S, and greenhouse-gas (GHG) fluxes at six contrasting wetland sites over seven sampling dates during 84 days of flooding, using a randomized complete block design (18 plots) and linear mixed-effects models. Soil Eh declined from +316 to −246 mV (p < 0.001), indicating progressive anaerobic reduction. NH₄⁺–N increased 675% (18.4–142.6 mg/kg), supplying an estimated 65–76 kg N ha⁻¹ season⁻¹ and potentially replacing 72–84% of standard fertilizer requirements. Available P increased 212%, associated with dissolution of Fe-bound phosphate. CH₄ flux increased from 0.2 to 41.5 mg C m⁻² d⁻¹, with a time-weighted mean of 19.7 mg C m⁻² d⁻¹ and an illustrative national estimate of ~71 Gg CO₂-eq/season, subject to substantial scaling uncertainty. N₂O flux peaked at 18.6 µg N m⁻² h⁻¹ on Day 7 before declining under strongly reducing conditions. Continuously flooded sites emitted ~13% more CH₄ than intermittently flooded sites, a descriptive difference only, as just two sites were intermittently flooded. Rice yields averaged 4.75–5.15 t/ha, approximately 25% above Rwanda’s national average. These findings provide a spatially and temporally resolved basis for optimizing fertilizer use, improving rice productivity, and strengthening national paddy-rice GHG inventories.