Rice-soil interactions in Vietnamese acid sulphate soils: impacts of submergence depth on soil solution chemistry and yields

The aim of this study was to investigate the effects of water submergence d epth on radial oxygen loss (ROL), soil solution chemistry and rice growth p erformance in acid sulphate soils in southern Vietnam. ROL was measured in a solution culture. In a separate pot experiment the impact of water submer gence depth on rice growth and soil solution chemistry was studied. Three s ubmergence depths were used in the two experiments (5, 10 and 15 cm). ROL d eclined with submergence depth and was significantly greater in young roots (with no root hairs) than in older roots. In the pot experiment rice growt h and soil solution chemistry were clearly affected by the submergence dept h. During the first crop at 5 cm submergence, there was a significantly hig her yield and a higher oxidation state (pe+pH) compared to 10 or 15 cm subm ergence. The Fe concentration was significantly greater at the 5 cm depth c ompared to the 10 or 15 cm depth. SO42- reduction was delayed at the 5 cm d epth. Rice yield was c. 25% less at the 15 cm than at the 5 cm depth. Durin g a second crop, there was a substantial SO42- reduction and H2S formation and almost no significant effects of submergence depth on either soil solut ion chemistry or crop yield. In a field experiment with a dry-season rice c rop, yield and Fe, Al and SO42- concentrations were higher at a shallow sub mergence depth than at greater depths in the same field, showing similar de pth trends to those found during the first crop in the pot experiment. Farm ers should be advised to use a shallow submergence depth and, if possible, avoid deep-rooted rice varieties. A conceptual model is suggested, which su mmarizes the relationships between ROL and soil solution chemistry.