Nutrient and Water Resource Use Efficiency Under Recommended and a Water Saving Practice in the Sahel: Multi-Season Effects of Mixed Fertilizer and Straw Applications.

Demand for rice in the Sahel is increasing faster than any other grain. Governments are responding by promoting irrigated cultivation in the region’s river basins, although high costs for fertilizers and water cast doubt on the future viability of production. Despite considerable controversy, the System of Rice Intensification (SRI), which is now practiced in six Sahelian countries, is often proposed as an alternative solution to this dilemma. SRI principles include transplanting young seedlings, wide spacing, compost, mechanical weed control, and intermittent irrigation. We studied the effects of these practices on yield and resource use efficiency by comparing them to conventional management practices (CMP) during five seasons on an orthothionic Gleysol soil in the Senegal River Delta. But because compost production in the Sahel is constrained by low biomass availability, we instead studied the impact of rice straw, synthetic fertilizer (FERT) and combined fertilizer and straw (FERT+STRAW) application compared to a control in CMP and this locally adapted version of SRI (ASRI). While no significant yield differences in FERT verses FERT+STRAW treatments were detected during seasons 1-3, significant additive effects of combining straw and fertilizer were observed and ASRI yields were significantly higher than FERT treatments during season 4 (+31%) and 5 (+17%). Across seasons, ASRI used 20-48% less water than RPM for seedbeds, land preparation and field growth, resulting in higher water productivities for grain (FERT: +1-48%; FERT+STRAW: +18-52%). Fertilizer-N recovery efficiencies were similar across systems. While some research suggests that intermittent irrigation could increase NO₃- availability and denitrification risks, the similarities in N recovery and lack of NO₃- build-up in this clayey soil provided little support for this hypothesis. Large water savings were recorded in ASRI without compromising yield. Mixed fertilizer and straw applications appear to be beneficial in ASRI, but only after multiple seasons of continuous application.