Arsenic (As) is a toxic element that exists naturally in the environment in several inorganic and organic forms. Some agricultural activities, such as pesticide application or irrigation with contaminated water have significantly increased As levels in soil in many parts of the world, affecting rice-producing countries. In flooded paddy soils As mobility is high and is readily taken up by rice and accumulated in grain, posing a potentially significant health risk. Water management in rice fields is one of the best approaches in controlling arsenic bioavailability in paddy soils. The water drying events induce redox fluctuations, affecting iron (Fe) and As transformations. Under flooded conditions, rice oxygenates its rhizosphere which precipitates iron oxides on the root surface, forming an iron plaque. The iron minerals present in this plaque have a high affinity for binding to cations and anions, thus having the potential to sequester arsenic and mitigate its uptake into grain.
The implications of water management practices on As uptake in rice and mobility in the soil-root interface of have not been thoroughly characterized. In this study we performed a field-scale experiment during the 2017 and 2018 summer growing seasons, as well as plant growth bioassays with three single-drain irrigation treatments of different soil drying severities (high, medium and low), and a continuously flooded (CF) control, with the objective of reducing As accumulation in rice and determining changes in the rhizosphere. Single drain intermittent irrigation (II) demonstrated potential for on-farm use at field-scale. Yields were maintained and As concentrations decreased by an average of 45%. Cadmium was controlled with a medium severity treatment.
Moreover, to understand the mechanisms involved in arsenic immobilization in the soil-root interface with II, rice root plaque formation and trends in Fe and As accumulation were studied. The introduction of oxic conditions with II favored the formation of ferrihydrite, which possesses a great affinity for binding As and is known to be present consistently in samples of II treatments. Although this mineral may serve as a sink for As, higher concentrations and accumulation of As was attributed to the CF treatment, elucidating that, under II, the main mechanism for reducing As accumulation in grain is the oxidation and immobilization of As in bulk soil.
Lastly, our analysis of the microbial 16S rRNA gene in rhizosphere samples revealed that the introduction of oxic conditions increases the abundance of aerobic bacteria that contribute to the oxidation and precipitation of Fe in the rhizosphere, and consequently As immobilization. Increasing the severity of II treatments induces a greater shift in the rhizosphere bacterial community of rice.
Understanding the chemical and biological interactions affected by water management treatments in rice systems is a step forward towards defining an effective strategy to minimize As mobility in rice fields that is potentially acceptable by rice growers, which may positively impact food security and human health globally.
