Arsenic Speciation in Rice and the Environment, 2018

 

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

The goal of this continuing project is to study how arsenic moves through rice systems and how management practices can be used to reduce arsenic uptake during cultivation. The objectives in 2018 were:

  • Investigate how iron speciation and redox levels in the soil impact arsenic levels in rice under alternating wet/dry (AWD) conditions.
  • Evaluate the impact of AWD on plant nutrition and grain quality by analyzing phosphorous, nitrogen, iron, and zinc in rice grains.
  • Elucidate the role of AWD in the uptake of nutrients and contaminants by rice plants.

    • Four irrigation treatments—one continuously flooded and three AWD of varying dry-down lengths—were conducted at the Rice Experiment Station. All three AWD treatments received only one dry-down event, two had an early season dry down for five and 11 days and one received a late dry-down for 11 days. Soil and plant samples were analyzed in a lab.

    Results showed that soil redox levels monitored in the field through the season remain low with prolonged flooding and increase following a dry-down period. As expected, this impacts arsenic accumulation in rice grains. Concentration of arsenic in both white and brown rice decreased with the longer AWD treatments. Cadmium concentrations follow an opposite trend, increasing with the higher severity AWD treatments, although levels remain low and are not of concern. Analysis of plant tissue is ongoing.

    Grain samples from 2015, 2016, 2017, and 2018 were analyzed for nutrient content among the different AWD treatments applied in those years. This included concentrations of phosphorous, iron, zinc, and potassium in both white and brown rice.

    The analysis showed that AWD does not negatively impact the concentrations of these nutrients in rice. There are significant increases of some elements in white rice between the control (continuously flooded) and the treatment of highest severity. Phosphorous and potassium showed a 40% increase in 2016 grain, iron increased by 30% in 2015 and 2018, and zinc showed a 20% increase in 2016.

    The nutrient concentrations were consistently higher in brown rice than white rice, which is expected from higher accumulation of nutrients in bran. Concentrations vary from year to year. The same trend was observed for every element, though the iron levels varied more within each year.

    Researchers also analyzed total and speciation of arsenic in shoots and grains, as well as total cadmium concentration in grain. Results confirm trends from prior years. The AWD treatments of higher severity decrease concentrations of arsenic consistently through the growing season. Cadmium levels increased with AWD treatments but remained below levels of concern.

    In summary, the potential for AWD treatments to minimize arsenic uptake in rice is evident in results from 2018 and from previous years. The analysis of nutrient concentrations in grain was an important step in assessing possible effects of AWD on grain quality. AWD had no negative impact on concentrations of phosphorous, iron, zinc, and potassium in grain, and it possibly increases these concentrations. These results indicate a positive finding for recommending AWD treatments to growers.