Measuring methane flux from irrigated rice fields by eddy covariance method using open-path gas analyzer
Introduction
Methane (CH4) is one of the important long-lived greenhouse gases (LLGHGs), aside from carbon dioxide (CO2) and nitrous oxide (N2O), which has a significant effect on atmospheric temperature and chemistry. Methane has the second-largest radiative forcing of the LLGHGs after CO2 (Forster et al., 2007). Wetland rice fields, however, represent globally one of the main sources CH4. It accounts for about 12–26% of the world's total anthropogenic CH4 emissions (IPCC, 2007). Flooding of irrigated rice fields produces anaerobic soil conditions which are conducive to the production of CH4 (Neue, 1993). Irrigated rice is not only the largest source of CH4, but also the most promising target for mitigating CH4 emissions from rice (Wassmann et al., 2000b). Methane emission is mitigated by water management that decreases the length of flooding time, such as intermittent irrigation and mid-season drainage (Wassmann et al., 2000b, Cai et al., 1997, Bronson et al., 1997, Yagi et al., 1996).
Methane is usually measured with the closed chamber technique in rice paddy fields (Wassmann et al., 2000a, Schutz et al., 1989a, Schutz et al., 1989b, Holzapfel-Pschorn and Seiler, 1986). Closed chambers have the advantage of detecting low fluxes, of being easy to manipulate and of low cost (Meijide et al., 2011, Song et al., 2009, Tuittila et al., 2000). However, closed chamber measurements are discrete in time and space and may not capture the dynamics of CH4 fluxes on varying time scales (hours to annual estimates) (McDermitt et al., 2011). Alternatively, the eddy covariance (EC) technique provides continuous measurements over a large area, without interfering with the processes of gas exchange between the sources and the atmosphere (Baldocchi et al., 2001, Aubinet et al., 2000, Denmead, 1995). The eddy covariance technique is commonly used for the estimation of CO2 and H2O and energy fluxes; but several studies have already shown that it is adequate for the estimation of CH4 fluxes (Hatala et al., 2012a, Baldocchi et al., 2001, Dengel et al., 2011, Detto et al., 2011, Herbst et al., 2011, McDermitt et al., 2011, Hendriks et al., 2007, Hendriks et al., 2008, Hendriks et al., 2009, Hendriks et al., 2010, Kroon et al., 2010, Kroon et al., 2009, Kroon et al., 2007, Long et al., 2010, Schrier-Uijl et al., 2010, Smeets et al., 2009, Zona et al., 2009, Rinne et al., 2007).
However, there are only few studies where the eddy covariance technique has been used to measure CH4 emissions from rice paddy fields in temperate countries (Detto et al., 2011, Hatala et al., 2012a, Hatala et al., 2012b, Meijide et al., 2011, Werle and Kormann, 2001); but none in the tropics. The last three studies made use of closed-path CH4 analyzers where Hatala et al., 2012a, Hatala et al., 2012b used a tunable diode laser fast methane sensor while Meijide et al. (2011) employed RMT-200 fast methane analyzer and Werle and Kormann (2001) used a high frequency modulation spectrometer with a lead-salt diode laser. On the other hand, Detto et al. (2011) compared a closed-path CH4 analyzer and an open-path CH4 analyzer to provide advances in understanding the performance and limitations of the eddy covariance method applied to CH4 measurements, from an instrumental and flux processing point of view. Closed-path CH4 analyzers require high flow rates at significantly reduced optical cell pressures to provide adequate response time and sharpen absorption features (Detto et al., 2011, McDermitt et al., 2011). Such methods, when used with the eddy covariance technique, require a vacuum pump and a total of 400–1500 Watts (W) of grid power. The weight of such systems often exceeds 45 kg, restricting practical applicability for remote portable field studies. As a result, spatial coverage of eddy covariance CH4 flux measurements is limited (Detto et al., 2011, McDermitt et al., 2011). The recently developed open-path CH4 analyzer (LI-7700) is lightweight (5.2 kg) and low-powered (8 W). It employs a tunable diode laser in an open Herriott cell configuration using wavelength modulation spectroscopy; it uses a 0.5 m base path with 60 reflections for a total optical path of 30 m. The instrument provides sensitive and robust measurements suitable for remote solar-powered operation, portable deployment, and easy installation at hard-to-reach sites (McDermitt et al., 2011).
In this paper, we report the dynamics of CH4 emissions from irrigated rice fields in the tropics over the whole cropping period using the eddy covariance method with the LI-7700 open-path CH4 analyzer. Our objectives are to characterize the diurnal and seasonal variations of CH4 fluxes and to identify the environmental and biophysical factors affecting them. We present results for the dry season (DS), from 1 December 2012 to 27 May 2013.
Section snippets
Site description
The study site is located within the Experimental Station of the International Rice Research Institute (IRRI) in Los Baños, Laguna, the Philippines, about 66 km south of Manila (14°8′ 27.7″ N, 121°15′54.98″ E). The site has a slope of 1% with a northeasterly aspect, and an elevation of 27 m above sea level. The soils are Lithic Haplustept (Soil Survey Staff, 2010) varying in texture from loam to clay and overlying volcanic tuff evident at 0.3–1.2 m depth. The top soil (0–0.15 m) has a mean pH of
Diurnal variations in CH4 fluxes and driving factors
The RSSI (relative signal strength indicator) was used as one of the metrics for measurement quality. Of a total of 8544 half-hourly data (178 days), only 20.6% were of low quality, with RSSI values below 10% and with quality flags more than 1. These outliers (due to rain, signal dropouts, instrument malfunction, etc.) were filtered and rejected. To determine the temporal variations of the diurnal patterns of CH4 fluxes, the remaining 79.4% data (not gap-filled) were partitioned into five
Summary and conclusion
Our study is the first CH4 flux measurement from an irrigated rice field using the eddy covariance technique with LI-7700 open-path CH4 analyzer, in South and Southeast Asia. The method was successfully used to characterize the diurnal and seasonal variations of CH4 emissions over the whole cropping period of 2013 dry season. Clear diurnal cycles of CH4 fluxes were observed when the data were partitioned into different growth stages (pre-planting, vegetative, reproductive, ripening, and
Acknowledgements
This research was supported by Bayer CropScience, the Kellogg Company, Lindsay Corporation, and YARA. I express appreciation to Dr. Achim Dobermann (Deputy Director General for Research, IRRI) and Dr. Akira Miyata (Director, Agro-meteorology Division, NIAES, Japan) for their motivations; to Dr. Dario Papale (FLUXNET) for his generous guidance in data processing; to LI-COR staff (Jason Hupp, Dave Fredrickson), Campbell staff (Ryan Campbell, Ed Swiatek, Joel Greene, Owen Davis), Hukseflux staff
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Working as GTZ/CIM integrated expert on temporary leave from Karlsruhe Institute of Technology, Germany.