Variability of effects of compost on nodulation, N acquisition and yield of cowpea in sub-Saharan areas of Burkina Faso

Legumes/cereals intercropping systems occupy the largest part of the surface area and are the primary food source of people in semiarid tropical regions. Because of drought and low soil fertility, the average grain yields rarely surpass 0.2 t ha-1 for the cowpea and 0.3 t ha-1 for the millet or sorghum. In the northern region of Burkina, farmers have developed the practice of sowing in bowls of about 15 cm depth and 20 cm diameter, referred to as zai practice. The zai bowls are made manually with a hoe. This long and tedious work aims to increase the storage of rainfall-water around the seedlings, and concentrate organic compost application. However, very little data is available on the effects of this organic amendment on the biological N2 fixation, N acquisition and grain yield of the legume. This study aimed at quantifying over three year cycles (2012-2014), the effects of an application of around 9 t ha-1 of compost (0.3 kg-1 zai bowl) on nodules number per plant and grain yields of cowpea intercropped with sorghum in 12 farmers’ fields. The selected fields represented the various soil types found in this area, and the years of study presented contrasted rainfall patterns. The results showed that the compost application effect on the nodules number per plant varied from highly negative to highly positive depending on the field and the year. The impacts of compost application on the nutrients uptake and yield of the legume were generally positive but also very variable between situations. We will present our analysis of the causes of such variability of the compost effect and will draw the practical consequences for improving these cropping systems in a context of climate change.