Huicui Yang's research while affiliated with Chinese Academy of Sciences and other places

Paddy soil is submerged under water for most of the rice growing season, but our understanding of the driving mechanisms of nitrogen cycling, N2O production, and N2O consumption under submerged conditions is limited. In this study, intact paddy soil cores were sampled and an incubation experiment was conducted with nitrate amendments under flooding. N2O concentrations in the soil profile and N2O flux rates were measured by gas chromatography. The community compositions and abundances of narG- and nosZ-containing denitrifiers were analyzed by terminal restriction fragment length polymorphism (T-RFLP) and real-time quantitative polymerase chain reactions (qPCR), respectively. The results showed that N2O emissions and N2O concentrations in the submerged soil profile were significantly affected by the nitrate inputs. Higher NO3-N additions stimulated a sharp increase in N2O flux rate, which suggested an obvious stimulation caused by the increasing nitrate input. This effect was closely related to the significant increases in the population size of narG-containing denitrifiers and obvious alterations in its community composition. Therefore, high nitrate concentrations in submerged paddy soil can stimulate much higher N2O production and emissions, and in this process, narG- rather than nosZ-containing denitrifiers are the important drivers. We also observed that N2O concentrations in the 0–5 cm soil layer were clearly lower than those in the 5–10 cm layer, but the nitrate contents in these layers were reversed, indicating large N2O losses occur from the 0–5 cm layer. The emitted N2O is derived mainly from the uppermost soil layer (0–5 cm).

Rice fields are an important source of nitrous oxide (N2O), where rice plants could act as a key factor controlling N2O fluxes during the flooding-drying process; however, the microbial driving mechanisms are unclear. In this study, specially designed equipment was used to grow rice plants and collect emitted N2O from the root-growing zone (zone A), root-free zones (zones B, C, and D) independently, at tillering and booting stages under flooding and drying conditions. Soil samples from the four zones were also taken separately. Nitrifying and denitrifying community abundances were detected using quantitative polymerase chain reaction (qPCR). The N2O emission increased significantly along with drying, but the N2O emission capabilities varied among the four zones under drying, while zone B possessed the highest N2O fluxes that were 2.7~4.5 times higher than those from zones C and D. However, zone A showed N2O consumption potential. Notably, zone B also harbored the highest numbers of narG-containing denitrifiers and amoA-containing nitrifiers under drying at both tillering and booting stages. This study demonstrates that drying caused significant increase in N2O emission from rhizosphere soil, in which the higher abundance of AOB would help to produce more nitrate and significantly higher narG-containing microbes would drive more N2O production and emission.

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 O2 penetration, 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.