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

Abstract

Context or ProblemFlooded 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 QuestionThe 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. MethodsA 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. ResultsGrain 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. ConclusionsGiven 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 SignificanceThis 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.