Soil tillage, residue management and site interactions affecting nitrogen use efficiency in maize and cotton in the Sudan Savanna of Africa

Nitrogen (N) fertilizers applied to agricultural lands are often lost due to improper management practices. It is important to manage applied N fertilizers efficiently in order to promote sustainable crop production. In a two year (2013–2014) on-farm trial in the Sudan savanna of Benin (Dassari village) and Burkina-Faso (Dano village), we assessed the single and interactive effects of tillage and crop residue management on nitrogen uptake (NU) and nitrogen use efficiency (NUE) of cotton and maize in four different sites [S1 (Ferric Lixisol, footslope in Dano), S2 (Eutric Plinthosol, upslope in Dano), S3 (Haplic Lixisol, footslope in Dassari), and S4 (Plinthic Lixisol, upslope in Dassari)]. Two tillage practices, i) contour ridge tillage, and ii) reduced tillage, were considered as main plot factors, while the sub-plots consisted of two levels of crop residue management, i) with residue, and ii) without residue, and two levels of N fertilizer, i) control, and ii) recommended level of N fertilizer (45 kg ha⁻¹ for cotton and 60 kg ha⁻¹ for maize). We selected three indices, nitrogen fertilizer recovery efficiency (NFR), agronomic efficiency (AE), and partial factor productivity (PFP_n) to describe the NUE of crops. Compared to reduced tillage, contour ridge tillage significantly improved NU and NUE indices of both cotton and maize on all sites except S3. When averaged across sites, implementation of contour ridge tillage instead of reduced tillage resulted in increase in NFR, AE and PFP_n of cotton by 32.6%, 18.6%, and 20%, respectively. Also, NFR, AE, and PFP_n of maize were increased by 29.4%, 25.8%, 1.7%, respectively under contour ridge tillage. Additionally, retention of crop residue significantly affected mainly NFR of cotton and maize, when averaged across sites. It increased NFR of cotton and maize by 14.6% and 14%, respectively, and enhanced NFR of cotton, yet not significantly on all sites. We concluded that increased soil N availability due to restricted horizontal soil water movement and improved soil water content is the primary contributor to increased NUE of both cotton and maize under the contour ridge tillage and crop residue retention across sites. However, such an effect was not notable on S3 as this site possesses improved soil hydrological properties a priori.