Comparison of Redox Indicators in a Paddy Soil during Rice-Growing Season

The objective of this study was to compare three methods of evaluating redox status, i.e., conventional redox potential (EH) measurement, terminal electron-accepting processes (TEAPs) and oxidative capacity (OXC) in pore waters of a paddy soil during the rice-growing season. The redox potential can be measured readily which can monitor progressive development of reducing conditions and distinguish oxic from anoxic conditions but with little information on specific redox processes under anoxic conditions. Identifying dominant TEAPs requires intensive data collection and analysis. The sequence of TEAPs development basically followed theoretical predictions but overlapping, typically among Mn, Fe, SO2−4-S reductions and methane production, was featured throughout the season. The measured dissolved H2 gas as the intermediate product, reflected the overlap among electron acceptors. Oxidative capacity integrated all the major oxidized and reduced species to a single conservative parameter and showed clearly the progressive redox status from oxic to postoxic and then to sulfidic conditions in the paddies with no apparent methanic condition during this particular growing season. In OXC computations, a more reliable method to estimate Mn and Fe oxyhydroxide concentrations as electron acceptors needs further testing. The measured EH showed a higher correlation to redox species Mn(II), Fe(II) and methane concentrations (r2 = 0.76, 0.73, and 0.76, respectively) than to dissolved O2 (DO), NO−3-N, and SO2−4-S (r2 = 0.53, 0.37, and 0.16, respectively). Measured EH was also highly correlated to OXC for low sulfate solutions. The three methods for evaluating redox status all indicated that more reducing conditions were developed in straw-incorporated paddies than that with straw-burned.