Potential Nitrogen Contribution of Climbing Bean to Subsequent Maize Crop in Rotation in South Kivu Province of Democratic Republic of Congo

Abstract

Nitrogen has become the most limiting nutrient in the Eastern Highlands of DR Congo characterized by high-potential soils on volcanic deposits. However, because of high population density which imposes intensive and continuous cropping without external inputs, smallholder farmers are facing declining land productivity in the region. Among low-input technologies to increase production, climbing bean is promoted and accepted by farmers because of its high production potential. Besides, this bean type is perceived as a potential crop to contribute to soil fertility and sustainable cropping system through its high biomass production and N-fixing capacity. On-station and on-farm farmer participatory trials were conducted to assess the beneficial effects of the climbing bean on the subsequent maize crop in rotation compared to bush bean and continuous maize cropping systems. Maize response to mineral nitrogen applied at rates of 0, 33, and 66 kg N ha^–1 was evaluated in three different rotations, climbing bean–maize, bush bean–maize, and continuous maize, over three cropping seasons. Maize grain yield was generally higher in climbing bean–maize rotation compared to bush bean–maize and continuous maize cropping system. Without applied nitrogen fertilizer, the average maize grain yield increase over three cropping seasons in response to the preceding climbing bean effect was 489 and 812 kg ha^–1 compared to bush bean and maize as preceding crops, respectively, which is 17.5 and 33.8% increase. However, better yield advantage of climbing bean over continuous maize was obtained in the long-rain cropping season, 43.2% compared to 24.2% in short-rain season. The residual effect measured that the second season following climbing bean was even better, and the yield difference was 1,300 kg ha^–1, i.e., 81% yield increase. Nitrogen contribution from the climbing bean to the system estimated as N fertilizer replacement values varied from 15 to 42 kg N ha^–1 in the first season of rotation. The potential of climbing bean to improve soil fertility was confirmed by farmers’ evaluation in on-farm trials.