Actual and potential salt-related soil degradation in an irrigated rice scheme in the Sahelian zone of Mauritania

Abstract

Salt-related soil degradation due to irrigation activities is considered a major threat to the sustainability of rice cropping under semi-arid conditions in West Africa. Rice productivity problems related to soil salinity, alkalinity and topographic position were observed in an irrigated rice scheme in southern central Mauritania. Detailed study of soils in a toposequence revealed that highest topsoil salinity and alkalinity were found at the shallow soils (<1.2 m) of the middle and upper slopes. Here, soils have formed in situ from the schist parent rock, which releases carbonate rich salts upon weathering. Within these soils large differences in salinity and alkalinity level occur at short distances, indicating minimal groundwater flow and a strong variation in the geochemical composition of the vertically positioned bedrock. Further downslope, soils have a (partly) colluvio–alluvial origin. Here, sedimentation during annual floods increased soil depth (>2.5 m) and salinity levels remained low due to leaching. Foum Gleita’s irrigation water used is amongst the most alkaline in the Sahel. However, no clear indications of secondary salinization or alkalinization due to irrigation activities were observed. A comparison of historical data revealed no significant changes of topsoil salinity and pH over the last 30 years. The PHREEQC 2.0 model was used to study actual and potential development of soil salinity and alkalinity problems, by simulating excessive concentration of the irrigation water through evaporation. The evolution into a strongly sodic-alkaline solution due to precipitation of Mg-calcite and -silicate minerals did not fit with current composition of ground and surface water, which showed geochemical control of alkalinity at high concentrations. Incorporation of cation exchange processes, using a small (1.0 mmol c per 100 g dry soil) but calcium saturated CEC, resulted in a better fit with field data. Results indicate that the soil’s buffer capacity to counteract alkalinization processes is large. However, the soil water and salt balance needs to be quantified in order to determine development rate and equilibrium levels of soil salinity and alkalinity for different soil type × water management combinations. This study does neither reject the hypothesis that salt-related soil degradation jeopardizes the sustainability of rice cropping in the Sahel, nor does it provide evidence for its verification. However, our results are in line with other studies in west Africa, in that current salt-related production problems are inherited, rather than being induced by irrigated rice cropping.