Changes in concentration and δ13C value of dissolved CH4, CO2 and organic carbon in rice paddies under ambient and elevated concentrations of atmospheric CO2

Changes in concentration and δ13C value of dissolved CH4, CO2 and organic carbon (DOC) in floodwater and soil solution from a Japanese rice paddy were studied under ambient and elevated concentrations of atmospheric CO2 in controlled environment chambers. The concentrations of dissolved CH4 in floodwater increased with rice growth (with some fluctuation), while the concentrations of CO2 remained between 2.9 to 4.4 and 4.2 to 5.8 μg C mL−1 under conditions of ambient and elevated CO2 concentration, respectively. The amount of CH4 dissolved in soil solution under elevated CO2 levels was significantly lower than under ambient CO2 in the tillering stage, implying that the elevated CO2 treatment accelerated CH4 oxidation during the early stage of growth. However, during later stages of growth, production of CH4 increased and the amount of CH4 dissolved in soil solution under elevated CO2 levels was, on average, greater than that under ambient CO2 conditions. Significant correlation existed among the δ13C values of dissolved CH4, CO2, and DOC in floodwater (except for the samples taken immediately after pulse feeding with 13C enriched CO2), indicating that the origins and cycling of CH4, CO2 and DOC were related. There were also significant correlations among the δ13C values of CH4, CO2 and DOC in the soil solution. The turnover rate of CO2 in soil solution was most rapid in the panicle formation stage of rice growth and that of CH4 fastest in the grain filling stage.