Diurnal Methane Fluxes as Affected by Cultivar from Direct-Seeded, Delayed-Flood Rice Production

Methane (CH4) emissions are known to differ between rice (Oryza sativa L.) cultivars, where CH4 emissions from pure-line cultivars are often greater than from hybrids. Numerous field studies have shown that CH4 emissions follow a diurnal pattern, typically reaching their maximum during afternoon hours. However, it is unknown whether cultivar affects CH4 fluxes/emissions at various measurement times of day or how those cultivar effects may differ spatially across soil textures and temporally throughout the rice growing season. The objective of this field study was to evaluate the effects of time of day (300, 800, 1200, 1800, and 2300 hours) and cultivar (one hybrid and one pure-line) on CH4 fluxes before and after heading from a silt-loam and clay soil in a direct-seeded, delayed-flood rice production system. Enclosed headspace chambers, 30 cm in diameter, were used for CH4 gas sampling on 22 July and 19 August at a silt-loam site and on 29 July and 26 August, 2014 at a clay-soil site in the Lower Mississippi River delta region of eastern Arkansas. Methane fluxes measured pre- and post-heading ranged from 0.7 to 2.2 mg CH4-C m-2· hr-1 from the clay soil and from 2 to 7 mg CH4-C m-2·hr-1 from the silt-loam soil. Hourly CH4 fluxes and estimated daily emissions differed among measurement times of day (P 4 flux or daily emissions for a given day differs by soil texture and rice growth stage, but conducting CH4 flux measurements around late morning to mid-day appear to be optimum to best capture the mean CH4 emissions for the day.

Chamber Size Effects on Methane Emissions from Rice Production

Quantifying methane (CH4) emissions from cultivated rice (Oryza sativa L.) production in the field has received increased attention recently due to methane’s importance as a greenhouse gas. The enclosed-headspace chamber technique is the standard methodology for field assessments of trace gas emissions. However, to our knowledge, no direct comparisons of measured CH4 fluxes and emissions from field-grown rice among differing chamber sizes have been reported. Therefore, the objective of this study was to evaluate the effect of chamber size [15.2- and 30.4-cm inside diameter (id)] on CH4 fluxes and season-long emissions from rice grown on a clay soil in Arkansas. Chamber size did not affect (P > 0.05) CH4 fluxes on 10 sampling dates during the flooded portion of the rice growing season and only affected fluxes on one of four sampling dates after flood release. Total season-long CH4 emissions from optimally N-fertilized rice were 32.6 and 35.6 kg CH4-C ha&#451&#183season&#451, which did not differ, and from bare clay soil were 0.74 and 1.75 kg CH4-C ha&#451&#183season&#451, which also did not differ, from the 15.2- and 30.4-cm chambers, respectively. Chamber size (i.e., 15.2- or 30.4-cm id) did not result in differences in cumulative CH4 emissions from this flooded-rice study that was conducted on a Sharkey clay soil in northeast Arkansas. Results indicate that both 15.2- and 30.4-cm diameter chambers were similarly adequate for measuring CH4 fluxes and emissions from the clay soil investigated. The similarity in emissions results between chamber sizes also indicates that the 15.2-cm diameter chambers adequately facilitated the quantification of CH4 emissions in this study.