Greenhouse gas flux monitoring in ecosystems is mostly conducted by closed chamber and eddy covariance techniques. To determine the relevance of the two methods in rice paddy fields at different growing stages, closed chamber (CC) and eddy covariance (EC) methods were used to measure the methane (CH4) fluxes in a flooded rice paddy field. Intensive monitoring using the CC method was conducted at 30, 60 and 90 days after transplanting (DAT) and after harvest (AHV). An EC tower was installed at the centre of the experimental site to provide continuous measurements during the rice cropping season. The CC method resulted in CH4 flux averages that were 58%, 81%, 94% and 57% higher than those measured by the EC method at 30, 60 and 90 DAT and after harvest (AHV), respectively. A footprint analysis showed that the area covered by the EC method in this study included non-homogeneous land use types. The different strengths and weaknesses of the CC and EC methods can complement each other, and the use of both methods together leads to a better understanding of CH4 emissions from paddy fields.

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Although bubble ebullition through water in rice paddy fields dominates direct methane (CH4) emissions from paddy soil to the atmosphere in tropical regions, the temporal changes and regulating factors of this ebullition are poorly understood. Bubbles in a submerged paddy soil also contain high concentrations of carbon dioxide (CO2), implying that CO2 ebullition may occur in addition to CH4 ebullition. We investigated the dynamics of CH4 and CO2 ebullition in tropical rice paddy fields using an automated closed chamber installed between rice plants. Abrupt increases in CH4 concentrations occurred by bubble ebullition. The CO2 concentration in the chamber air suddenly increased at the same time, which indicated that CO2 ebullition was also occurring. The CH4 and CO2 emissions by bubble ebullition were correlated with falling atmospheric pressure and increasing soil surface temperature. The relative contribution of CH4 and CO2 ebullitions to the daily total emissions was 95–97% and 13–35%, respectively.

Separate evaluation of methane (CH 4) emission dynamics (e.g., oxidation, production, and transportation) at the soil-plant-atmosphere and soil-water-atmosphere interfaces has been limited in tropical rice paddies, but it is crucial for comprehending the entire CH 4 cycles. We investigated CH 4 oxidation, production, and transportation through plant and water pathways during the reproductive stage in a tropical Thailand rice paddy field using natural abundance carbon stable isotope ratios (δ 13 CH 4 and δ 13 CO 2). Mass balance equations using δ 13 CH 4 and δ 13 CO 2 in soil gases indicated that CH 4 oxidation in the planted soil exceeded those in the interrow soil due to oxygen supply through rice roots. In addition, at 1-11 cm depth acetate fermentation was the dominant process in the planted soil, whereas in the interrow soil the dominant process was H 2 /CO 2 reduction. The water pathway showed a significant negative correlation between CH 4 flux and released δ 13 CH 4 over 24 hr, driven by a diel change in episodic ebullition, steady ebullition, and diffusion, all due to diel changes in soil temperature and atmospheric pressure. In contrast, the plant pathway showed a significant positive relationship between CH 4 flux and emitted δ 13 CH 4 throughout one day. A comparison of the diel change in emitted δ 13 CH 4 between the water and plant pathways showed that the rice plants transported CH 4 in soil bubbles without any large isotopic fractionation. The diel change in the plant-mediated CH 4 transportation was mainly controlled by diel changes in soil bubble expansion and CH 4 diffusion through plants, which were probably regulated by diel changes in soil temperature and atmospheric pressure. Plain Language Summary Methane (CH 4) emissions from paddy soil are mainly controlled by three processes: CH 4 production, CH 4 oxidation (consumption), and CH 4 transportation from soil to plant, water, and ultimately the atmosphere. There are two emission pathways, the soil-plant-atmosphere and soil-water-atmosphere interfaces, but there has not been much detailed evaluation of the different characteristics of the three processes in the two pathways. Here we evaluated CH 4 production, oxidation, and transportation at the soil-plant-atmosphere and soil-water-atmosphere interfaces during the reproductive stage of rice in a tropical Thailand paddy field. We found that in planted soil there was more CH 4 production by acetate fermentation and more CH 4 oxidation, due to more organic matter and oxygen supply, respectively, through plant roots. Methane was transported through water by three modes: episodic bubble ebullition, steady bubble ebullition, and diffusion. Diel variation in the rates of these three transportation modes was related to diel changes in soil temperature and atmospheric pressure. Methane in soil bubbles was also transported into the atmosphere through rice plants. Diel variation in the CH 4 transportation through rice plants was related to diel change in bubble expansion, which was also mainly regulated...

Abstract. The recent development and improvement of commercial laser-based spectrometers have expanded in situ continuous observations of water vapour (H2O) stable isotope compositions (e.g. δ18O and δ2H) in a variety of sites worldwide. However, we still lack continuous observations in the Amazon, a region that significantly influences atmospheric and hydrological cycles on local to global scales. In order to achieve accurate on-site observations, commercial water isotope analysers require regular in situ calibration, which includes the correction of H2O concentration dependence ([H2O] dependence) of isotopic measurements. Past studies have assessed the [H2O] dependence for air with H2O concentrations of up to 35 000 ppm, a value that is frequently surpassed in tropical rainforest settings like the central Amazon where we plan continuous observations. Here we investigated the performance of two commercial analysers (L1102i and L2130i models, Picarro, Inc., USA) for measuring δ18O and δ2H in atmospheric moisture at four different H2O levels from 21 500 to 41 000 ppm. These H2O levels were created by a custom-built calibration unit designed for regular in situ calibration. Measurements on the newer analyser model (L2130i) had better precision for δ18O and δ2H and demonstrated less influence of H2O concentration on the measurement accuracy at each concentration level compared to the older L1102i. Based on our findings, we identified the most appropriate calibration strategy for [H2O] dependence, adapted to our calibration system. The best strategy required conducting a two-point calibration with four different H2O concentration levels, carried out at the beginning and end of the calibration interval. The smallest uncertainties in calibrating [H2O] dependence of isotopic accuracy of the two analysers were achieved using a linear surface fitting method and a 28 h calibration interval, except for the δ18O accuracy of the L1102i analyser for which the cubic fitting method gave the best results. The uncertainties in [H2O] dependence calibration did not show any significant difference using calibration intervals from 28 up to 196 h; this suggested that one [H2O] dependence calibration per week for the L2130i and L1102i analysers is sufficient. This study shows that the cavity ring-down spectroscopy (CRDS) analysers, appropriately calibrated for [H2O] dependence, allow the detection of natural signals of stable water vapour isotopes at very high humidity levels, which has promising implications for water cycle studies in areas like the central Amazon rainforest and other tropical regions.