Potential effect of nitrate applied during flood period on N2O emissions during subsequent drainage in a paddy soil

Application of nitrate (NO3−) in flooded soil promotes denitrification activity and N2O emissions, accompanied by NO3− depletion. However, whether the stimulating effect of NO3− on flooded soil still affects the response of nitrifiers and denitrifiers to subsequent water drainage remains unclear, as does the N2O emission dynamics. We conducted a microcosm experiment using intact paddy soil columns with three levels of NO3− treatment under flooded conditions: the application of NO3− solutions with 0, 60 and 200 kg KNO3-N per hectare (N0, N60, and N200, respectively). The N2O emission rate was measured using a closed chamber method, and the community composition and abundance of nitrifiers and denitrifiers were determined using the terminal restriction fragment length polymorphism (T-RFLP) and real-time quantitative polymerase chain reaction (qPCR) methods, respectively. The results showed that all the added NO3− was exhausted during the flood period and below 0.50 mg kg−1 at the end of flooded incubation. Arch-amoA and narG gene abundance were significantly elevated by NO3− application before drainage. Then water drainage promoted the narG gene numbers of N60- and N200-treated soils increase by 109 copies g−1 more than N0 treatment, and the community compositions of narG- and nirK-containing denitrifiers differed significantly among the three NO3− treatments. In addition, draining resulted in clear shifts in the community structures of Arch-amoA- and Bac-amoA-containing nitrifiers in the N60-treated soils, and also resulted in higher increase of Arch-amoA gene abundance in the N60- and N200-treated soils than in N0 treatment. These results indicate that NO3− application on flooded soil enhance the response of Arch-amoA and narG gene abundance to subsequent water drainage. However, the differences in the denitrifier community had few correlations with N2O emissions during draining (p > 0.05), while the Arch-amoA gene abundance in the N0- and N60-treated soils increased about 6.45 and 3.70 times respectively after drainage (p < 0.05) and strongly correlated with higher N2O emissions (r > 0.92, p < 0.01). Considering the NH4+-N consumption and the NO3−-N production, we could speculate that the changes in the ammonia-oxidizing archaea community after drainage are only important regulators of N2O emission rates in this paddy soil columns.