Soil Texture Effects on Methane Emissions From Direct-Seeded, Delayed-Flood Rice Production in Arkansas

Rice (Oryza sativa L.) is a semiaquatic plant produced globally as a staple food crop under flooded-soil conditions for the majority of the time it is actively growing. Prolonged flooding results in the rapid depletion of oxygen and renders the soil highly anaerobic and reduced, both of which are precursor conditions for methane (CH4) production. Thus, rice production is unique among agricultural crops in its susceptibility to atmospheric CH4 emissions. Among the numerous soil, plant, and agronomic factors known to affect CH4 fluxes and emissions, few direct, replicated, plot-scale comparisons among soil textures have been conducted and little direct in situ quantification has been conducted in the production system common to Arkansas, the top rice-producing state in the United States. Therefore, a study was conducted in the Mississippi River Delta region of eastern Arkansas during 2012 to evaluate the effect of soil texture (i.e., silt loam and clay) on weekly CH4 fluxes and seasonal emissions from the direct-seeded, delayed-flood rice production system common to Arkansas. Chamber-based enclosures, 30 cm in diameter, were capped; gas samples were extracted with a syringe at 0, 20, 40, and 60 min; and samples were analyzed by gas chromatography. Results showed that significant CH4 fluxes began sooner (P < 0.05), increased more rapidly (P < 0.05), and achieved a significantly greater peak flux earlier (P < 0.05) from the optimally N-fertilized rice grown on the silt-loam than that on the clay soil. Seasonal emissions from N-fertilized rice produced on the silt-loam soil averaged 159.6 kg CH4-C ha(-1), which was the same as the EPA-estimated 160 kg CH4-C ha(-1) season(-1) from primary-crop rice production. However, clay soil seasonal emissions from N-fertilized rice were less than 23% (P < 0.05; 35.6 kg CH4-C ha(-1)) that of silt-loam emissions using the same production system, rice cultivar, and N fertilization scheme. These significant soil textural effects on CH4 emissions clearly demonstrate that additional research is needed to better quantify the other known factors affecting CH4 emissions, particularly soil texture, before widespread regulation and policies are set that could have inappropriate, widespread, negative consequences on US rice production.