Paddy rice (Oryze Sativa, L.) is one of the major crops grown in the Sacramento Valley of California. The historical practice of burning rice straw after harvest has now been severely restricted due to California air quality legislation. One of the alternatives to burning is to incorporate the straw into soil. However, straw incorporation may enhance soil reduction processes, resulting in unfavorable soil conditions (such as sulfide accumulation and toxicity) for rice plant development. The sulfide toxicity problem may be exacerbated by increasing soil and water salinity, caused by reduction and diversion of supply water from the Sacramento River. The River was diverted in an effort to protect salmon from being sucked into pumps. Additionally, the river floodwaters had to be retained in paddy fields for a specified time period after chemical application to reduce chemical residue. Both of these constraints have resulted in extensive drainwater reuse in the middle and lower portions of the Colusa Basin, which has given rise to water and soil salinity.

The objective of this study was to obtain data on sulfide accumulation from rice straw treatments under controlled conditions, utilizing greenhouse pot experiments, in the summer of 2000. The main variables investigated (all in triplicate) were straw (addition of straw equivalent to 0, 0.6, and 2.3% by mass) and sulfate (addition of SO₄ equivalent to 0, 168, and 824 mg kg^-1) and their interactions. The soil used was obtained from rice plots in Maxwell, California, which had electrical conductivity (EC) of 2.5 dS m^-1 in 1:1 soil:water extract. Changes in redox conditions were monitored by TEAPs (terminal electron-accepting processes) and OXC (oxidative capacity) methods.

The greenhouse pot experiment was still underway at the time of this report, with plans for termination in late September/early October after grain harvest. The results thus far indicate that higher levels of straw incorporation (about 8 tons ha^-1) had the greatest impact on rice plants (resulting in low average plant height and more than 50% fewer final standing plants) compared to lower levels of straw incorporation (about 2 tons ha^-1) or zero straw incorporation. Redox status, defined by TEAPs and OXC, indicates that reducing conditions developed more rapidly in the high straw treatment than in the low straw treatment, and were slowest to develop in the zero straw treatment. The results of the controlled green house study are more detailed than those from the field plot studies. When the experiment is completed, additional details will be available.

In the past three years, we have conducted field plot studies on redox conditions in rice straw treatments at the Maxwell site, in conjunction with other UC rice researchers. We have monitored redox conditions in paddy soils during the rice growing seasons, utilizing TEAPs (terminal electron accepting processes) and OXC (oxidative capacity) methods. The advantages of using these two methods over conventional redox potential measurements were discussed in the previous report. The field studies at the Maxwell straw management site revealed that straw incorporation was not likely to cause an adverse effect on rice plants on a large scale. However, sulfide toxicity symptoms on rice (such as black roots, retarded growth, and fewer standing plants) were observed in localized sites, mostly in the downstream ends of the plots.