Altitudinal and Agroecological Control on Soil Chemical and Physical Variability in Rwanda’s Marshland Rice Systems

This study assessed the relative influence of agroecological zones (AEZs) and altitude on soil nutrient dynamics to guide site‑specific management. Soil samples from 20 marshlands across seven AEZs, spanning 944 to 1900 m a.s.l., were collected from 0 – 30 cm and analysed for pH, exchangeable bases (Ca, Mg, K, and Na), organic carbon (OC), total nitrogen (TN), available phosphorus (P), and texture using ANOVA, Welch’s tests, and PCA. Fertility varied markedly among AEZs: the Imbo zone exhibited near‑neutral pH and higher base cations, while Mayaga and the Congo Nile Watershed Divide were acidic and nutrient‑poor. Low‑elevation sites (944 – 1044 m a.s.l.) had higher pH (6.32) and base cations, whereas high‑altitude soils (>1800 m a.s.l.) were acidic (pH 4.92) and depleted. Effect size analysis confirmed altitude’s stronger explanatory power for Ca ( η² = 0.53 vs. η² = 0.49), Mg ( η² = 0.54 vs. η² = 0.37), K ( η² = 0.47 vs. η² = 0.29) and pH ( η² = 0.45 vs. η² = 0.33), while AEZs better explained TN ( η² = 0.27 vs. η² = 0.09), though altitude produced a notable mid‑elevation TN peak (1345 – 1544 m a.s.l.), and OC ( η² = 0.31 vs. η² = 0.10). Integrating altitude with AEZ‑based frameworks and applying organic amendments in high‑altitude acidic zones can improve nutrient use efficiency, rice yield, and sustainable marshland productivity in Rwanda.