Processes driving soil solution chemistry in a flooded rice-cropped vertisol: analysis of long-time monitoring data

Abstract

Senegal river water has positive alkalinity (0.55 meq l −1) and positive calcite-residual alkalinity (0.30 meq l −1). Without leaching, this water presents a possible alkalinization and sodication hazard for the soil. The effect of flooding on soil solution chemistry in an irrigated nondrained vertisol under rice was monitored during 3 years in northern Senegal. During flooding, soil solution redox potential dropped to about −0.15 V, and alkalinity increased to 15 meq l −1. Mg content increased as well, from about 1 to 5 meq l −1, whereas Cl and SO 4 contents decreased to very low levels at the end of the growing season (0.05 meq l −1). After the fallow, nonflooded period between two crop cycles, the soil solution composition returned to its initial neutral sulfate and chloride composition. The observed cycle in soil solution chemistry could not be explained by water and solute transfer because infiltration rates were very low, due to air entrapment in the soil profile. Geochemical control by calcite and gypsum was excluded during irrigation. Mössbauer spectroscopy showed that iron oxides were both well- and poorly crystallized goethite. Goethite dissolution was assessed by pH–Eh stability diagrams. We explained the increase in alkalinity and Mg concentration in the soil solution partly by the composition of the irrigation water, and partly by the reduction and dissolution of Fe oxides, and Fe 2+ fixation on exchange sites of the clay minerals. These processes were reversed as soon as the soil oxidized. The decline in Cl concentration was attributed to geochemical control through the formation of a GR-Cl − mineral around the porous cups. In that case, Cl should not be used as a natural tracer. Carbonates accumulate in soil solution not only due to iron reduction, but also because of the positive residual alkalinity of the irrigation water. When the soil dries at the end of crop cycle, the carbonate concentration of topsoil will increase and calcite will precipitate. This ongoing process may result in calcium control and ultimately soil alkalinization over the years.