Differential contributions of electron donors to denitrification in the flooding-drying process of a paddy soil

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Flooding-drying (FD) is an important process causing nitrous oxide (N2O) emission in paddy field, which is mainly driven by denitrification. However, information on the contributions of the electron donors to soil denitrification during FD is limited. In this study, the potential contributions of various electron donors to denitrification during FD in a paddy soil were estimated using a 9-d soil incubation experiment without rice plants cultivation in a chamber. The results showed that during continuous flooding (CF), the soil remained very low denitrifying activity, and CH4 was the major electron donor for the denitrification with contribution of 28.85–45.23%. In the FD treatment, the soil N2O concentration and flux in 0–5 d were significantly lower than that in 5–9 d. Therefore, the estimations were based on these two phases. The Fe2+ and Mn2+ contents rapidly decreased, while the NO3−-N content slowly increased during the first 5 d of drying. However, the outstanding feature during 5–9 d drying was that the soil CH4 concentration decreased, and the cumulative oxidation of CH4 increased. The stoichiometry of the electrons showed that the potential contributions of Fe2+, Mn2+, CH4, and other electron donors to the denitrification were 51.29%, 45.88%, 2.82%, and 0% in the first phase, and 12.15%, 1.35%, 6.17%, and 80.33% in the second phase, respectively. Thus, the autotrophic denitrification driven by Fe2+ and Mn2+ would be the main mechanisms for gaseous N-loss during the first phase of drying, whereas, the other electron donors, including organic matter, were the major contributors for denitrification in the second phase. These findings should be considered when implementing measures to mitigate N2O and CH4 emissions from paddy fields.