Tailoring fertilizer rates to catena positions improves nutrient use efficiency of rice in inland valleys of West Africa

Context: Rainfed rice production in inland valleys has the potential to improve food security in West Africa, yet the average grain yield ranges only between 1 and 3 t ha−1. Increased nutrient use efficiency through site-specific fertilizer management can enhance the rice yield and make rice production economically attractive. Objectives: This study aimed to assess the effects of catena position and nitrogen fertilizer rates on rice yield, nutrient use efficiency and economic profitability. Methods: On-farm experiments were conducted in three inland valleys from 2019 to 2021, i.e. in Yawtabrikrom, Ghana and in Ouragahio and Pangbabo in Côte d'Ivoire. The experimental design was a split-plot design, with catena positions as the main factor (i.e., valley bottom, lower slope and middle slope), and fertilizer rates as the sub-factor. In the first (2019) and second (2020/2021) years of the experiment, four and eight fertilizer rates were used, respectively. Results: Fertilizer applications consistently, and mostly linearly increased rice yields regardless of the catena positions. Yields at the valley bottom (2.1–7.4 t ha−1) and lower slope (0.9–6.5 t ha−1) were higher than those obtained at the middle slope (1.0–5.6 t ha−1), where water deficits were more pronounced compared to the lower two catena positions. Overall, agronomic N use efficiency was higher in the valley bottom and lower slope (26 kg grain kg−1 N) than at the middle slope (19 kg grain kg−1 N). The higher returns on investment made on fertilizer were achieved at N application rates from 45 to 90 kg N ha−1. At high N levels risks on lodging were observed. Conclusions: Hydro-edaphic conditions, soil fertility distribution across the catena position, rice lodging risks, and economic indicators must be considered to guide site-specific fertilizer use in inland valley rice systems.