Concentration and carbon isotope profiles of CH4 in paddy rice canopy:: Isotopic evidence for changes in CH4 emission pathways upon drainage

Methane concentration ([CH4]) as well as its carbon isotope composition (613 CH4) was measured within and above the rice canopy (5.0, 1.0, 0.6, 0.3, and 0.1 m above the ground) in the late growing season. Both [CH4] and delta(13)CH(4) showed strong vertical stratifications, but with significant diurnal variations in their intracanopy and above-canopy gradients. The diurnal variations were mainly influenced by atmospheric stability under flooded conditions and by soil temperature under drained conditions, respectively. With the disappearance of floodwater, delta(13)CH(4) profiles showed a distinct transition from higher values (-56.1 parts per thousand to -46.0 parts per thousand) to lower values (-68.9 parts per thousand to -47.1 parts per thousand). At the peak emission of CH4 upon drainage, CH4 in the canopy air space showed an identical isotopic composition (-70.1 parts per thousand) to CH4 stored in the soil. CH4 in retarded flushes showed irregular vertical distributions of delta(13)CH(4), implying varying degrees of the oxidation of CH4 released. The results clearly indicate that floodwater drainage initially brings about release of less-oxidized CH4 into the atmosphere in a large quantity. Good linear relationships were found between 1/[CH4] and the corresponding delta(13)CH(4) until the peak emission period. However, the net ecosystem delta(13)CH(4) (delta(E)), estimated for each sampling time, showed large fluctuations during the day, and was lower than expected. delta(E) estimated on a daily scale was -64.2 parts per thousand, -77.8 parts per thousand, and -64.5 parts per thousand for flooded, transient, and drained conditions, respectively. (c) 2005 Elsevier B.V. All rights reserved.