REDOX CONDITIONS AND IRON CHEMISTRY IN HIGHLAND SWAMPS OF BURUNDI

Iron toxicity is a major soil constraint to rice,(Oryza sative L.) cropping in highland swamps of Burundi. These swamps have a wide range of carbon content. This study aims at determining the influence of carbon content and redox conditions on the release of iron from Fe-bearing minerals. The pe-pH pairs distribution and oxalate dissolution data strongly suggest a control of Fe2+ activity by a pool of poorly crystallized ferric oxides. Flooding results in high values of KCl-extractable Fe (up to 22 cmolc kg(-1)) being released from that pool. The iron release is positively correlated with organic matter. On the other hand, highly organic, peaty soils have large CEC and their adsorbed Fe fraction remains relatively low. As the exchangeable Fe fraction has previously been correlated with Fe toxicity to rice, we may conclude that very organic (> 25% C), peaty soils exhibit a lower Fe toxicity hazard than soils with intermediate carbon content (10-25%).