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 (? _transport) in paddy fields were introduced. The results indicated that the closed chamber + syringe method was much better for the determination of ? _transport during the non-rice-growing season. Presently, ? _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 ? _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.