Long-term fertility experiments for irrigated rice in the West African Sahel: effect on soil characteristics

Abstract Long-term fertility experiments (LTFEs) are a tool to investigate the sustainability of cropping systems. The present study analyzed two LTFEs for intensive rice-based irrigated systems in the Senegal River valley at Ndiaye and Fanaye (Sahel savanna). The trials were established in 1991, contain six different fertilizer treatments and rice is grown two times per year. Soil types are a typical Orthithionic Gleysol and an Eutric Vertisol at Ndiaye and Fanaye, respectively. The objectives of the presented study were to analyze the effect of intensive irrigated rice cropping on the soil resource base by studying the changes of soil characteristics over time and by comparing soil N, P and K pools in different fertilizer treatments. In the LTFE at Ndiaye, topsoil pH values increased significantly from 5.5 to about 6.5 and electrical conductivity was high but remained stable. Soil organic carbon (SOC) and total soil nitrogen (TSN) dropped slightly after 16 consecutive seasons but the difference was statistically not significant. At both sites, exchangeable N ranged between 1.6 and 2.8% of TSN and fixed N accounted for 5.5–8.2% of TSN, with slightly higher values in Fanaye. Treatment differences in N dose had no significant effect on these parameters. Results of δ13C analysis showed a decrease due to rice cropping at both sites, and the measurements indicate high turnover rates of soil organic matter. Soil analyses of total soil P and K and of different pools indicated only small changes when these elements were applied at medium quantities. In contrast, treatments with N application only showed considerable soil P and K depletion, and rice cultivation without P and/or K application cannot maintain soil fertility. The soil mining process is relatively quick for P due to the naturally low soil P status, whereas the high soil K reserves buffer even important negative K balances for decades. It is concluded that irrigated rice cultivation in the region can maintain soil fertility if at least medium P doses are applied together with nitrogen.