Land Management Effects on Biogeochemical Functioning of Salt-Affected Paddy Soils1

Most lowlands in Northeast Thailand (Isaan region) are cultivated with rice and large areas are affected by salinity, which drastically limits rice production. A field experiment was conducted during the 2003 rainy season to explore the interactions between salinity and land management in two fields representative of two farming practices: an intensively managed plot with organic inputs and efficient water management, and one without organic matter addition. Field measurements, including pH, Eh, electrical conductivity (EC), and soil solution chemistry, were performed at three depths, with a particular focus on Fe dynamics, inside and outside saline patches.High reducing conditions appeared after flooding particularly in plots receiving organic matter and reduction processes leading to oxide reduction and to the release of Fe and, to a lesser extend, Mn to the soil solution. Oxide reduction led to the consumption of H+ and the more the Fe reduction was, the higher the pH was, up to 6.5. Formation of hydroxy-green rust were likely to be at the origin of the pH stabilization. In the absence of organic amendments, high salinity prevented the establishment of the reduction processes and pH value remained around 4. Even under high reduction conditions, the Fe concentrations in the soil solution were below commonly observed toxic values and the amended plot had better rice production yield.