In this study,to further promote the application of the stable carbon isotope natural abundance(SCINA)method to the study of CH4in paddy fields in China,field experiments were carried out to investigate carbon isotope fractionation during CH4transport in both rice-and non-rice-growing seasons.More importantly,two new methods for the measurement of the CH4transport fractionation factor(εtransport)in paddy fields were introduced.The results indicated that the closed chamber+syringe method was much better for the determination ofεtransport during the non-rice-growing season.Presently,εtransport was calculated using theδ13C value of the CH4emitted from a rice field minus that of the CH4in the floodwater(–6.7‰to–3.0‰).In addition,there were three methods available for estimatingεtransport during the rice-growing season:deduction of theδ13C value of the CH4in the floodwater from that of the CH4emitted from the field(–16.6‰to–15.2‰);deduction of theδ13C value of the CH4in the soil pore water from that of the CH4emitted from the field(–13.2‰to–1.1‰);and deduction of theδ13C value of the CH4in the aerenchyma of plants from that of the CH4emitted 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.
CH 4 emission and the concentration of dissolved CH 4 in soil solution and floodwater in a rice field and their stable carbon isotopic signatures as affected by straw application were investigated in 2009 in a field experiment at Jurong,Jiangsu Province,China.Straw application increased CH 4 emission and CH 4 concentration in the soil solution and floodwater.A positive seasonal correlation was also observed in the variation between CH 4 flux and CH 4 concentration in soil solution.The seasonal total CH 4 emission(51.6 g CH 4 m 2) in Treatment WS(straw applied) was about 168% higher than that in Treatment CK(without straw).The emitted CH 4 and CH 4 in soil solution were initially relatively enriched,then depleted and finally enriched again in 13 C in both treatments,while CH 4 in floodwater became isotopically heavier.The carbon isotopic signature of emitted CH 4 and CH 4 in floodwater averaged around 62‰ and 45‰ for both treatments,respectively,and was not significantly influenced by the application of straw.However,straw application caused the CH 4 in soil solution to be significantly depleted in 13 C during the middle of the rice season,and the mean δ 13 C value was lower in WS(57.5‰) than in CK(49.9‰).Calculation from the isotopic data showed that straw application increased the fraction of CH 4 oxidized,causing no significant difference in the δ 13 C value of the emitted CH 4 between the two treatments.
