Impact of rice straw incorporation on soil redox status and sulfide toxicity

Incorporation of rice (Oryza sativa L.) straw into the soil has become an alternative to straw burning to improve air quality in the Sacramento Valley, California. However, straw addition to paddies could promote reducing conditions that may lead to sulfide accumulation and plant toxicity. Sulfide toxicity has been observed in randomly localized field sites and is characterized by blackened roots, retarded growth, fewer standing plants, and even death in severe cases. The objective of this study was to investigate the impact of straw incorporation on soil redox status and sulfide toxicity to rice in a greenhouse pot study. Treatments included straw incorporation (0, 6, and 23 g straw kg-1 soil) and sulfate additions (0, 160, and 800 mg SO4 kg-1 soil). Redox status was evaluated by identifying dominant terminal electron-accepting processes and geochemical redox classes based on oxidative capacity. Higher straw incorporation rates led to more reducing conditions at earlier times. The most reducing conditions (methanic) were observed within 3 wk for the 23 g straw kg-1 soil treatment and in about 6 wk for the 6 g straw kg-1 soil treatment and were not observed till the end of the experiment when no straw was added. Straw incorporation significantly reduced grain yield (p < 0.0001), number of tillers (p < 0.0001), and plant height at 4 wk (p = 0.01). Sulfate addition only showed significant reduction on the number of tillers (p = 0.0028). Soluble sulfide concentrations were very low, mainly due to precipitation with Fe. The higher straw incorporation rates induced sulfide toxicity symptoms and reduced rice yield significantly. It is not clear, however, if other causes, such as organic acids and salinity, may have contributed to the adverse impact of straw incorporation on rice.