Influences of free-air CO 2 enrichment (FACE), nitrogen fertilizer and crop residue incorporation on CH 4 emissions from irrigated rice fields

To investigate the response of methane (CH 4 ) emissions to an elevated atmospheric carbon dioxide (CO 2 ) 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 CO 2 enrichment (FACE) technology. Our results revealed that the elevated atmospheric CO 2 increased the seasonal cumulative CH 4 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 CO 2 levels in half of the field treatments. Crop residue incorporation hindered the stimulatory effects induced by the elevated CO 2 , 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 CO 2 on the CH 4 emissions. The CO 2 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 CO 2 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 CO 2 on CH 4 emissions from managed and natural wetlands. There were no significant interactions between the atmospheric CO 2 and the incorporations of nitrogen fertilizer and crop residue. Appropriate experiments are necessary for better understanding of the interact influences of the elevated CO 2 and farm managements.