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

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. This study aimed to assess the effects of catena position and nitrogen fertilizer rates on rice yield, nutrient use efficiency and economic profitability. 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. 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. 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. [Display omitted] •N fertilizer increases rice yields similarly across inland valley catena positions.•Rice production at higher inland valley catena positions carries a potential drought risk.•N fertilizer is used more efficiently by the rice crop at lower inland valley catena positions.•The highest returns on investments are achieved with fertilizer rates between 45 and 90 kg N ha−1.