Effects of Acetic Acid and Glucose as a Carbon Source for Denitrification on Nitrous Oxide Emissions from Anaerobic Soil Condition

The denitrification process is the main process by which nitrous oxide (N₂O) is generated under anaerobic conditions in rice cultivation soil. The denitrification process under anaerobic conditions is carried out by microorganisms. The denitrification process is influenced by the type and availability of carbon sources in the soil. Among the components of rice root exudate, sugars and organic acids provide essential carbon sources for soil microorganisms, and microorganisms help the physiology and growth of rice. The incubation experiment was conducted to determine the effect of sugars and organic acids contained in rice root exudates on N₂O emissions from paddy soil. The soil was placed in a plastic bottle (capacity 824 cm³) mixed with glucose and acetic acid at the rates of 0.0, 0.1, 0.2, and 0.4% (wt/wt), and saturated with water to maintain anaerobic condition. Then, 0.15g of urea was added as a nitrogen source. The plastic bottles containing mixed soils were placed in the incubation under dark conditions at 25℃ for 30 days. Gas samples were collected daily during the experiment time, and daily N₂O flux and cumulative N₂O emissions were measured. Cumulative N₂O emissions significantly decreased with increasing addition rate of glucose. Cumulative N₂O emissions were 200, 8.51, 4.04, and 2.77 mg/m²/day at the glucose addition rates of 0.0, 0.1, 0.2, and 0.4%, respectively. Cumulative N₂O emissions decreased more rapidly with increasing addition rate of acetic acid than that of glucose. Cumulative N₂O emissions were 225, 3.02, 2.35, and 1.85 mg/m²/day at the acetic acid addition rates of 0.0, 0.1, 0.2, and 0.4%, respectively. These results suggest that acetic acid was rapidly decomposed and provided and available carbon source to microorganisms involved in denitrification, thereby promoting the process of converting nitrate (NO₃^-) into nitrogen gas (N₂) and minimizing N₂O production. In the case of the glucose treatment group, the NO₃^-/ N₂O ratio was 1.18, 33.0, 32.6, and 31.2 and the NH₄⁺/ NO₃^- ratio was 0.16, 0.13, 0.16, and 0.25 at the rates of 0.0, 0.1, 0.2, and 0.4%, respectively. In the case of the acetic acid treatment, the NO₃^-/ N₂O ratio was 1.13, 97.9, 123, and 151 and the NH₄⁺/ NO₃^- ratio was 0.15, 0.40, 0.40, and 0.75 at the rates of 0.0, 0.1, 0.2, and 0.4%, respectively. These results showed that the main emission process of N₂O during the experiment period was the denitrification process rather than the nitrification process. In addition, the higher NO₃^-/ N₂O ratio in the acetic acid treatment group compared to the glucose treatment group indicates that NO₃^- was converted to N₂ more effectively, reducing N₂O emissions.