Arsenic Speciation in Rice and the Environment, 2017

 

Project Leader

Sanjai Parikh, professor, Dept. of Land, Air, and Water Resources, UC Davis

The goal of this research is to provide realistic recommendations to help farmers minimize arsenic uptake during rice cultivation. The objectives in 2017 were:

  • Evaluate the impact of straw management on arsenic uptake into rice grains.
  • Compare arsenic levels in rice fields under alternate wetting and drying (AWD).
  • Chemically analyze arsenic availability to rice in the root zone.
  • Optimize AWD management to minimize arsenic accumulation in rice grains.

Straw management

The potential for straw management to minimize arsenic uptake in rice was evaluated at the Rice Experiment Station. Winter straw treatments included burned, incorporated, fallowed, bailed, and bailed/burned. The treatment with the highest concentration of arsenic in paddy rice was the bailed/burned treatment. The levels of arsenic measured in rice grains are consistent with the arsenic concentrations in soil in each location.

Importantly, many grain samples measured had inorganic arsenic levels above U.S. Food and Drug Administration proposed action levels for infant rice cereal. No general trends were observed by straw management treatments.

AWD impacts

Plants and soil subjected to AWD treatments under one dry down were analyzed. The results showed that the longer the dry down period, the greater the likelihood of reducing accumulation of arsenic in rice grains.

Arsenic levels, as well as total cadmium content, were analyzed in shoots, roots, and grains from the 2016 harvest. Results confirm prior research that arsenic uptake decreased with AWD 25% and AWD 35% soil moisture compared to continuous flood. The “safe” AWD treatment, where the soil remains wetter than the other AWD practices, was not effective at reducing arsenic accumulation in rice grains.

Roots sequestered a considerable amount of arsenic because of the presence of iron. Flooded conditions favor iron plaque formation in the root zone compared to aerated conditions. This is an area that needs further research to determine how the presence or absence of iron affects arsenic in rice tissues.

The concentration of cadmium in various plant tissues showed an opposite trend to arsenic. At maturity, continuous flood and safe AWD systems accumulated the least cadmium in roots, shoots, or grains. Cadmium levels in white rice under the AWD 25% practice were about five times that of the continuous flood system. However, AWD 35% reduced cadmium content in brown and white rice compared to AWD 25% by more than a third. Thus, AWD 35% may represent the best management strategy for minimizing both cadmium and arsenic uptake simultaneously.

During the 2017 growing season, three AWD treatments were examined with only one dry down period. Soil redox potential was observed. Longer dry down periods indicated oxidizing conditions that may be related to the immobilization of arsenic in the soil. Grain and plant tissue analysis from 2017 samples is ongoing.