Influences of free-air CO2 enrichment (FACE), nitrogen fertilizer and crop residue incorporation on CH4 emissions from irrigated rice fields

To investigate the response of methane (CH4) emissions to an elevated atmospheric carbon dioxide (CO2) concentration (200 ± 40 μmol mol−1 higher than the ambient atmosphere), we performed a 4-year multi-factorial experiment at a subtropical rice paddy that contained sandy loam soil in the Yangtze River Delta from 2004 to 2007 using free-air CO2 enrichment (FACE) technology. Our results revealed that the elevated atmospheric CO2 increased the seasonal cumulative CH4 emissions by 15 % on average during the 4-year period. The increase was insignificant and much weaker than the previous studies, which might be primarily attributed to the absence of a significant difference in the rice biomass between the two CO2 levels in half of the field treatments. Crop residue incorporation hindered the stimulatory effects induced by the elevated CO2, which were 37, 14 and 6 % for the fields that were incorporated with none, half or all of the wheat straws that were harvested in the preceding winter wheat season, respectively. Nitrogen fertilizers application also hindered the stimulatory effects of the elevated CO2 on the CH4 emissions. The CO2 stimulatory effect was 39 % for the field without nitrogen fertilizers, and reduced to 17, 7 and 5 % for the field with nitrogen fertilization of 125, 250 and 350 kg N ha−1, respectively. The regulation of nitrogen fertilizers on the CO2 effects in this experiment does not well agree with the previous studies, which might because the soil type was different from those of the previous studies. Thus, further studies are necessary to evaluate the role of soil properties in regulating the effects of elevated atmospheric CO2 on CH4 emissions from managed and natural wetlands. There were no significant interactions between the atmospheric CO2 and the incorporations of nitrogen fertilizer and crop residue. Appropriate experiments are necessary for better understanding of the interact influences of the elevated CO2 and farm managements.