Measuring methane flux from irrigated rice fields by eddy covariance method using open-path gas analyzer

Highlights

• This is the first CH₄ flux measurement from an irrigated rice field using LI-7700 open-path CH₄ analyzer, in South and Southeast Asia.

• The diurnal patterns were influenced by temperature, surface energy flux, and net ecosystem CO₂ exchange.

• The seasonal variations were controlled by water management and the growth of the rice plants.

• The irrigated rice field has taken up 1.88 kg CO₂ equiv. per kg of grain produced.

• The irrigated rice field had a net biome productivity of 17.22 g C m⁻² and it is a C sink.

Abstract

The newly developed LI-7700 open-path methane analyzer was used to measure methane (CH₄) fluxes from irrigated rice fields using the eddy covariance technique. The diurnal and seasonal variations of CH₄ emissions over the whole cropping period of 2013 dry season were characterized. Clear diurnal cycles of CH₄ fluxes were observed during the different growth stages of the rice plant (vegetative, reproductive, and ripening). Methane flux started to increase at around 0800H, reached a peak at around 1300–1500H, and then decreased to low values after 1900H. Peak CH₄ flux (mean ± standard deviation) was 0.082 ± 0.048 μmol CH₄ m⁻² s⁻¹ during the vegetative stage (0–37 days after transplanting, DAT); 0.063 ± 0.021 μmol CH₄ m⁻² s⁻¹ during the reproductive stage (38–72 DAT); and 0.060 ± 0.033 μmol CH₄ m⁻² s⁻¹ during the ripening stage (73–103 DAT). The diurnal cycles were influenced by temperature (air, floodwater, and soil), surface energy flux (net radiation, soil heat flux, sensible heat flux, and latent heat flux), and ecosystem CO₂ exchange (photosynthesis and respiration). The seasonal variations in daily CH₄ emissions were primarily controlled by water management and the growth of the rice plants. This study has shown that intermittent irrigation during the vegetative stage was an effective water management strategy to lower the seasonal CH₄ emissions to about 3.26 g C m⁻². The irrigated rice field sequestered 306.45 g C m⁻² of CO₂ from the atmosphere, released 3.03 g C m⁻² of CH₄ to the atmosphere during the growing period, and produced a grain yield of 5.44 Mg ha⁻¹. Considering a global warming potential (GWP) of 25 over a 100-year horizon, we accounted for the C footprint during the growing period: the irrigated rice field has taken up 1.88 kg CO₂ equiv. per kg of grain produced. Additionally, the irrigated rice field had a net biome productivity of 17.22 g C m⁻² and it is a C sink.

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.