5:15 PM - 6:45 PM
[AGE28-P01] Measurement of methane concentration profiles and inverse estimation of pathway-specific fluxes in rice canopies
Keywords:Methane, bubble
Methane is released from rice paddies into the atmosphere via three pathways: molecular diffusion of dissolved methane across the atmosphere-water boundary, ebullition of gas bubbles, and diffusive transport through the aerenchyma tissue of rice plants. However, in-field measurement methodologies for quantifying these fluxes are not well established. The inverse Lagrangian analysis, a micrometeorological technique for estimating the source/sink strength of gas constituents from concentration profiles within a canopy, can be an alternative approach. This is particularly useful because ebullition and plant-mediated methane are emitted at different heights: ebullition occurs just at the water surface, but plant-mediated emission occurs a few centimeters above the water surface. The aims of this study were to develop a method for measuring the vertical profile of methane concentration within the canopy and to partition methane fluxes by pathway using inverse Lagrangian analysis.
Field monitoring was conducted at a continuously flooded rice paddy in Tsukuba City, Ibaraki Prefecture, Japan, from August 24 to 31, 2023. Two 9 m ×9 m experimental areas were created, and Oryza sativa (cv Koshihikari) was cultivated. Comparative measurements were made under two sets for contrasting conditions: one with and without standing water and the other with and without plastic films. All rice plants in the treatment plot were wrapped with plastic film to alter the emission height of plant-mediated methane to approximately 20 cm above the water surface. To validate the inverse analysis, methane fluxes were simultaneously measured with three 30 cm × 60 cm × 120 cm chambers installed in each control and treatment plot. For the inverse Lagrangian analysis, the following measurements were conducted. A three-dimensional ultrasonic anemometer (CSAT3, Campbell Scientific) was installed at 1.81 m to measure wind speed. The air inside the canopy was sucked at 0.5 L per minute through a 4.5 m tube with eight holes that were extended horizontally in the canopy at 16 heights (every 2 cm from 0 to 16 cm, then at 20, 30, 40, 50, 60, 80, 100, and 150 cm). Methane concentration was measured with a gas analyzer (LI-7810, Licor).
Different methane concentration profiles were observed between the control and treatment (wrapped) plots. In the control plot, a methane concentration decreased smoothly with height in the canopy. In the wrapped plots, the concentration decreased smoothly from the lowest (2cm) to about 4 cm, then remained constant from 6 to 20 cm, which was probably due to the effect of the plastic film application.
Inverse Lagrangian analysis depicted the vertical distribution of methane emissions and reasonable diurnal variation in both plant-mediated and ebullition fluxes. However, the magnitude of total methane flux by the inverse analysis was generally greater than that by the chamber method. During the daytime, methane fluxes via rice plants were overestimated compared to the chamber method, resulting in the overestimation of the total flux by the inverse analysis. On the other hand, ebullition fluxes by the inverse analysis were underestimated compared to the chamber method. It was considered that the dispersion in the canopy was not well reproduced in the inverse analysis, and the contribution of ebullition was not adequately captured by the sampling system. Increasing the accuracy of dispersion within the canopy and the sampling density in the emission layer are needed to investigate the diffusive behavior of ebullitions in the lower part of the canopy.
Field monitoring was conducted at a continuously flooded rice paddy in Tsukuba City, Ibaraki Prefecture, Japan, from August 24 to 31, 2023. Two 9 m ×9 m experimental areas were created, and Oryza sativa (cv Koshihikari) was cultivated. Comparative measurements were made under two sets for contrasting conditions: one with and without standing water and the other with and without plastic films. All rice plants in the treatment plot were wrapped with plastic film to alter the emission height of plant-mediated methane to approximately 20 cm above the water surface. To validate the inverse analysis, methane fluxes were simultaneously measured with three 30 cm × 60 cm × 120 cm chambers installed in each control and treatment plot. For the inverse Lagrangian analysis, the following measurements were conducted. A three-dimensional ultrasonic anemometer (CSAT3, Campbell Scientific) was installed at 1.81 m to measure wind speed. The air inside the canopy was sucked at 0.5 L per minute through a 4.5 m tube with eight holes that were extended horizontally in the canopy at 16 heights (every 2 cm from 0 to 16 cm, then at 20, 30, 40, 50, 60, 80, 100, and 150 cm). Methane concentration was measured with a gas analyzer (LI-7810, Licor).
Different methane concentration profiles were observed between the control and treatment (wrapped) plots. In the control plot, a methane concentration decreased smoothly with height in the canopy. In the wrapped plots, the concentration decreased smoothly from the lowest (2cm) to about 4 cm, then remained constant from 6 to 20 cm, which was probably due to the effect of the plastic film application.
Inverse Lagrangian analysis depicted the vertical distribution of methane emissions and reasonable diurnal variation in both plant-mediated and ebullition fluxes. However, the magnitude of total methane flux by the inverse analysis was generally greater than that by the chamber method. During the daytime, methane fluxes via rice plants were overestimated compared to the chamber method, resulting in the overestimation of the total flux by the inverse analysis. On the other hand, ebullition fluxes by the inverse analysis were underestimated compared to the chamber method. It was considered that the dispersion in the canopy was not well reproduced in the inverse analysis, and the contribution of ebullition was not adequately captured by the sampling system. Increasing the accuracy of dispersion within the canopy and the sampling density in the emission layer are needed to investigate the diffusive behavior of ebullitions in the lower part of the canopy.