INTERACTIONS OF WATER, MULCH AND NITROGEN ON SORGHUM IN NIGER

Water and nutrients are the main limiting factors to sorghum (Sorghum bicolor L. Moench) production in the Sahel. Our objective was to quant ify the interactions of irrigation, mulch and N on sorghum yield and w ater use efficiency (WUE) in Sahelian agroecosystems. Sorghum was grow n in eight management systems as a factorial arrangement of irrigation (none and 125 mm), mulch (none and 12 Mg ha(-1)) and N (none and 50 k g ha(-1)) on an Ustifluvent in Niger (13 degrees 17' N, 7 degrees 3' E ). Rainfall during the growth period of sorghum was 283 mm with an unf avorable distribution causing three dry spells. Irrigation and mulch i nduced more intensive root growth at the soil surface. Irrigation incr eased root density by 52% when N was applied. Few and small difference s in sorghum height and leaf area index (LAI) among sorghum management systems indicated little influence of these crop properties on the wa ter balance. Irrigation, mulch and nitrogen as sole amendments increas ed biomass by 55%, 20% and 30%, respectively. Averaged across the sorg hum management systems, grain yield was increased by 41%, 17% and 20% by irrigation, mulch and N, respectively. This study showed that in a Sahelian agroecosystem, nitrogen as a sole amendment increased water u se efficiency of sorghum by 21%. Mulch reduced evaporation from soil b y 28% in both dryland and irrigated treatments, but not all of the sav ed water was subsequently transpired by the crop. Irrigation improved water use efficiency by 9 to 40% but did not greatly increase the tota l transpired water.