Nutrient Management Research Database

The flooding-drying cycle can cause obvious increases of nitrous oxide (N₂O) emissions from paddy soil. However the relationships between N₂O flux and N₂O concentrations in soil and the microbial driving mechanisms during the flooding-drying process are unclear. In this study, a flooding-drying incubation experiment was carried out with a paddy soil. The topsoil (0–6 cm) was divided into 6 micro-sublayers each of 1 cm depth which were sampled independently. Terminal restriction fragment length polymorphism (T-RFLP) and real-time quantitative polymerase chain reaction (qPCR) were employed to determine the community composition and abundance of nitrifiers and denitrifiers, respectively. Results showed that the dynamics of N₂O flux were more closely related to the N₂O concentrations at 2–3 cm in comparison with that at 4–5 cm depth in the soil profile. During the peak period of N₂O flux, the top three micro-sublayers (0–3 cm) simultaneously harbored significantly higher ammonia oxidizing bacteria (AOB) population sizes, and contained higher nitrate and lower ammonia concentrations. Therefore, the top soil (0–3 cm) possesses a strong ability to produce nitrate substrate for denitrification during the flooding-drying process, and the drying surface soil, with O₂penetration, favoured N₂O generation. In contrast, although the bottom soil (4–6 cm) contained abundant nitrate reductase gene (narG) copy numbers, it maintained low levels of AOB abundance, which could suggest that low nitrifying activity would be the major restriction limiting N₂O production in this layer. In conclusion, the flooding-drying process induced significant N₂O emissions from the paddy soil, which were closely related to the increasing nitrifying capability in the topsoil within 0–3 cm and the dynamics of N₂O concentrations at 2–3 cm depth.

• This laboratory study investigated how the flooding and drainage of rice paddy soil influenced N2O emissions.
• The N2O, ammonium, and nitrate concentrations at varying depths of soil cores were measured over the course of a 36 day laboratory study.
• The total number of nitrifiers and denitrifiers was also measured through the course of the experiment.

• Close to the soil surface, drainage resulted in quick increases in denitrifying bacteria, while it took several weeks for nitrifiers to proliferate.
• Total N2O emissions from the soil surface was closely related to N2O levels at the 2-3cm depth.
• The authors suggested that these observations resulted from three things:

1. Drainage results in increased water loss from the soil surface.
2. Continued drying can bring nitrate from deeper in the soil closer to the surface.
3. N2O produced deeper in the soil is more likely to be converted to N2 by denitrifiers before reaching the soil surface.