Carbon isotope fractionation during CH4 transport in paddy fields

In this study, to further promote the application of the stable carbon isotope natural abundance (SCINA) method to the study of CH4 in paddy fields in China, field experiments were carried out to investigate carbon isotope fractionation during CH4 transport in both rice- and non-rice-growing seasons. More importantly, two new methods for the measurement of the CH4 transport fractionation factor (ɛ transport) in paddy fields were introduced. The results indicated that the closed chamber + syringe method was much better for the determination of ɛ transport during the non-rice-growing season. Presently, ɛ transport was calculated using the δ 13C value of the CH4 emitted from a rice field minus that of the CH4 in the floodwater (−6.7‰ to −3.0‰). In addition, there were three methods available for estimating ɛ transport during the rice-growing season: deduction of the δ 13C value of the CH4 in the floodwater from that of the CH4 emitted from the field (−16.6‰ to −15.2‰); deduction of the δ 13C value of the CH4 in the soil pore water from that of the CH4 emitted from the field (−13.2‰ to −1.1‰); and deduction of the δ 13C value of the CH4 in the aerenchyma of plants from that of the CH4 emitted from plants (−16.3‰ to −10.9‰). Unfortunately, the first two methods showed relatively large uncertainties. Only the last one, the dividing + cutting method, was not only scientific and reliable but also provided accurate measurements.