Expression of denitrification genes in response to a waterlogging event in a Fluvisol and its relationship with large nitrous oxide pulses.

The contributions of large N2 O pulses following waterlogging to the annual cumulative N2 O productions were significant in a Fluvisol. To uncover the mechanisms underlying these large N2 O pulses, a Fluvisol sampled from an agricultural field in Japan was subjected to waterlogging during incubation. Larger N2 O emissions were observed in intact soil cores when compared to emissions from sieved soils, indicating the importance of soil properties. The most important factor controlling the magnitude of the N2 O pulses after waterlogging was the soil moisture prior to waterlogging. The major pathway for N2 O production was denitrification. Quantitative PCR and quantitative RT-PCR analyses showed that the denitrification genes (nirS, nirK, and nosZ) correlated with N2 O emissions at the mRNA level but not at the DNA level. The change in denitrification gene mRNA levels was more prominent in the 0- to 1-cm soil compared with the 1- to 3-cm soil. Water-soluble and hot-water-soluble carbon contents also showed the highest amount in the 0- to 1-cm soil. These indicate that there was a strong variation in soil microbial properties over very small changes in soil depth, and this variation is important in determining the magnitude of N2 O emissions.