Solutions containing dissolved organic carbon (DOC) from a depressional wetland that receives agricultural drainage water were incubated in pure quartz sands with FeCl3 added as the main electron acceptor. The effects of three factors (the water, DOC, and Fe contents) on the CO2 and CH4 production rates, integrated cumulative C emissions and Fe-OC co-precipitation were studied. The results showed that CO2 production during the DOC mineralization process was facilitated by Fe(III) reduction at the expense of CH4 production. Additionally, the cumulative CO2 emissions determined by integrating the temporal curves of CO2 production were negatively correlated with those of CH4 (r = -0.48, p < 0.01). Extremely large ratios of the CO2:CH4 production rate (13,762 and 44,885 under two soil water conditions: saturation and flood conditions of twice the saturation level) were observed. These ratios were likely caused by simultaneous anaerobic fermentation, microbial respiration, and methanogenesis suppression triggered by iron reduction. The effects of exogenous Fe(III) inputs on total C emissions (as the sum of integrated cumulative CO2 and CH4) were only dependent on soil water conditions during the initial period of the experiment, and flooded conditions increased total C emissions by as much as double. Increased ratios of Fe inputs to C contents were found to proportionally increase the total C emissions (R-2 = 0.32, p < 0.01). Under saturated conditions, the co-precipitation of Fe-OC complexes prevented the remainder of the DOC from undergoing mineralization. In terms of DOC, we concluded that wetland saturation with introduced Fe(III) can reduce total C emissions from anaerobic respiration and promote C sequestration.