Mid-season drain severity impacts on rice yields, greenhouse gas emissions and heavy metal uptake in grain: evidence from on-farm studies

Context or Problem: Flooded rice systems produce an important staple crop but are a source of methane (CH₄) and arsenic (As) exposure. Introducing non-continuous flooding events can decrease seasonal CH₄ emissions and grain As concentration. A single mid-season drain (MD: a 7-to 10-day soil drying period during the mid-season) is easier for farmers to manage than multiple drain events (e.g. alternate wetting and drying). Previous research has shown that a MD can accomplish these goals without yield reduction. However, depending on soils and precipitation, the soil drying severity during a MD can vary substantially. Objective or Research Question: The objectives of this study were to compare a MD to the farmer practice under on-farm conditions with respect to yields, greenhouse gas (GHG) emissions and As and cadmium (Cd – which can increase with non-continuous flooding). Second, to quantify the relationship between soil drying severity during the MD and the reduction in CH₄ emissions and grain As. Methods: A three-year study with seven on-farm trials was implemented. Each trial had two treatments: a MD and the farmer practice. Grain yields, GHG emissions, and grain As and Cd concentrations were quantified for each treatment. Results: Grain yields were similar between treatments. The MD decreased seasonal CH₄ emissions by 20–77% (average of 52%), with the magnitude of reduction being related to soil-drying severity during the MD. Combining previous on-station data with this on-farm data, indicates that for every 1% reduction in soil gravimetric water content (GWC), seasonal CH₄ emissions were reduced by 2.5% (r² = 0.47). With MD, N₂O emissions increased (average = 0.25 kg N₂O-N ha⁻¹), but accounted for only 3% of the global warming potential. The MD decreased grain As concentration (by 20% on average, but not related to soil drying severity), and there was no effect on grain Cd concentrations. Conclusions: Given these results, an MD has similar GHG mitigation potential to other non-continuous flooding practices that require more drain periods and may be more difficult to implement. These results indicate that a MD is a viable on-farm management practice for GHG mitigation and reducing grain As concentration with limited risk of yield reduction. Implications or Significance: This is the first research that has documented the relationship between soil drying severity and reduction in CH₄ emissions across soils. This relationship is a potentially useful tool for on-farm monitoring, but will need to be developed further across more locations.