Diel and seasonal nitrous oxide fluxes determined by floating chamber and gas transfer equation methods in agricultural irrigation watersheds in southeast China

Agricultural nitrate leaching and runoff incurs high nitrogen loads in agricultural irrigation watersheds, constituting one of important sources of atmospheric nitrous oxide (N₂O). Two independent sampling campaigns of N₂O flux measurement over diel cycles and N₂O flux measurements once a week over annual cycles were carried out in an agricultural irrigation watershed in southeast China using floating chamber (chamber-based) and gas transfer equation (model-based) methods. The diel and seasonal patterns of N₂O fluxes did not differ between the two measurement methods. The diel variation in N₂O fluxes was characterized by the pattern that N₂O fluxes were greater during nighttime than daytime periods with a single flux peak at midnight. The diel variation in N₂O fluxes was closely associated with water environment and chemistry. The time interval of 9:00-11:00 a.m. was identified to be the sampling time best representing daily N₂O flux measurements in agricultural irrigation watersheds. Seasonal N₂O fluxes showed large variation, with some flux peaks corresponding to agricultural irrigation and drainage episodes and heavy rainfall during the crop-growing period of May to November. On average, N₂O fluxes calculated by model-based methods were 27% lower than those determined by the chamber-based techniques over diel or annual cycles. Overall, more measurement campaigns are highly needed to assess regional agricultural N₂O budget with low uncertainties.