Daily soil surface CO2 flux during non-flooded periods in flood-irrigated rice rotations

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Carbon dioxide (CO2) emitted from the soil, as a result of root and microorganism respiration, is a major process in the global carbon cycle. Since CO2 production is dependent on oxygen availability, prolonged saturated soil conditions in rice (Oryza sativa) can decrease the quantity of soil carbon released in the form of CO2 over time. At present, a deficiency exists in the scientific literature on soil surface CO2 flux in well-established, flood-irrigated rice systems, which are flooded for approximately 3 months a year during the rice growth period. Plenty of studies have examined soil surface CO2 flux in dryland cropping systems and methane emissions in paddy-grown rice, but flood-irrigated rice does not easily fall into either of these categories due to the cyclic nature of seasonal flooding. Therefore, this is the first study to examine daily soil surface CO2 flux during non-flooded periods in well-established, flood-irrigated rice rotations. For a comprehensive analysis, soil surface CO2 flux was measured for 2 years on 10 different rice-based rotations, which had been managed using conventional tillage or no-tillage for 10 and 11 years. Rotations included continuous rice and various combinations of rice rotated with soybean (Glycine max), corn (Zea mays), and/or winter wheat (Triticum aestivum) and were located on a silt-loam soil in the Mississippi River Delta region of Arkansas in the USA. Results showed that 7 of the 16 measurement dates differed in daily soil surface CO2 flux among tillage and/or crop rotations. However, these differences were determined to be from crop maturity, in relation to early- or late-season planting, instead of patterns of long-term flood irrigation. Years that rice was grown reduced the cumulative CO2 emissions, but substantial differences over time were minimized in rotations with soybean or corn. Findings from this experiment are valuable in the scientific understanding of carbon gas cycling in rice-based cropping systems because the aerobic periods between flooding were evaluated, which is the time period often ignored when examining carbon gas emissions in rice. Overall, this study provides evidence that the commonly used rice-based cropping systems reach a somewhat equilibrium state in daily CO2 fluxes over time, regardless of the frequency in periodic soil saturation.