Introduction Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol102Issue4May2000:

Agricultural and Forest Meteorology 102 2000 287–303 Carbon dioxide and methane fluxes from an intermittently flooded paddy field Akira Miyata a,∗ , Ray Leuning b , Owen Thomas Denmead b , Joon Kim c , Yoshinobu Harazono a a National Institute of Agro-Environmental Sciences, Tsukuba 305-8604, Japan b CSIRO Land and Water, F.C. Pye Laboratory, Canberra, GPO Box 1666, ACT 2601, Australia c National Research Laboratory for Atmospheric Modeling Studies, Department of Atmospheric Sciences, Yonsei University, Seoul 120-749, South Korea Received 5 January 1999; received in revised form 16 December 1999; accepted 27 December 1999 Abstract To assess the role of floodwater in controlling the exchanges of CO 2 and CH 4 from soil, floodwater and the canopy in intermittently flooded rice paddies, an intensive field campaign IREX96 was conducted in Japan during August 1996. Eddy covariance was employed to measure fluxes of heat, water vapor and CO 2 . The flux-gradient method was used to determine CH 4 fluxes from measured profiles of CH 4 concentrations, with the required eddy diffusivity estimated using a modified aerodynamic approach or CO 2 as a reference scalar. When the paddy was drained, net CO 2 uptake from the atmosphere during daytime was 23 less, and nighttime CO 2 emissions were almost twice as great, than when the paddy was flooded. The mean daily CO 2 uptake on the drained days was 14.5 g m − 2 , 50 of the mean for the flooded days. These differences in the CO 2 budget were mainly due to increased CO 2 emissions from the soil surface under drained conditions resulting from the removal of the diffusion barrier caused by the floodwater. Small changes in canopy photosynthesis observed between flooded and drained paddies had little influence on the CO 2 budget and could be explained by sensitivity of stomata to humidity saturation deficit. The CH 4 flux for the drained paddy showed distinct diurnal variation with a maximum of ∼ 1.3 mg CH 4 m − 2 s − 1 in the afternoon, but after reflooding the peak flux decreased to 0.9 mg CH 4 m − 2 s − 1 . Mean daily CH 4 emissions were 28 larger for the drained paddy than when it was flooded. As with the CO 2 flux, the larger CH 4 flux on the drained days can be attributed to reduced resistance of CH 4 transfer from the soil to air by removal of the floodwater. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide flux; Methane flux; Rice; Eddy covariance; Gradient

1. Introduction

Rice paddies in monsoonal Asia have an important role in the global budget of greenhouse gases such as CO 2 and CH 4 IPCC, 1995, but there is still consid- erable uncertainty in the magnitude of the net fluxes from these ecosystems. Many of the factors control- ling gas exchange between rice paddies and the atmo- ∗ Corresponding author. sphere are different from those in dryland agriculture and other ecosystems because rice is flooded during most of its cultivation period. Field studies designed to measure net fluxes and to improve our understanding of the factors controlling the fluxes are thus needed. CO 2 exchange as well as energy balances of rice paddies have been studied intensively in the 1950s and 1960s using conventional micrometeorological techniques such as the aerodynamic and Bowen ratio methods Uchijima, 1976. Since the 1980s, the 0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 0 9 2 - 7 288 A. Miyata et al. Agricultural and Forest Meteorology 102 2000 287–303 development of fast response CO 2 analyzers enabled us to measure CO 2 fluxes over a rice canopy by the eddy covariance method Ohtaki and Matsui, 1982; Ohtaki, 1984, which gave us more reliable flux esti- mates than before. However, the mechanism of CO 2 exchange between rice paddies and the atmosphere is not fully understood. For example, using eddy covari- ance measurements, Tsukamoto 1993 found a sig- nificantly smaller net CO 2 flux from the atmosphere to a rice canopy when the field was drained com- pared to when it was flooded, but the reason for the difference was not clear. The existence of floodwater, anaerobic soil or changes in the micrometeorological environment with flooding will influence root activity, photosynthesis and respiration of rice plants. Activity of aquatic plants such as algae in the floodwater may also affect CO 2 exchange between rice paddies and the atmosphere. Many of the data obtained so far are not sufficiently detailed to examine the influence of these factors on the CO 2 exchange in rice paddies. Paddy fields are also one of largest sources in the global budget of CH