Irrigation-induced nitrogen and phosphorus fluxes via lateral subsurface flow in a typical field of the third largest irrigation district in China

Irrigation-driven nutrient dynamics in groundwater is important for regional water resources utilization and ecological environmental security, especially in arid and semi-arid agricultural areas. The Hetao Irrigation District is the third largest irrigation district in China, and the annual irrigation schedule repeats the same sequence each year. The quantification of irrigation-induced lateral nutrient fluxes in the drainage channel remains poorly understood. In this study, we established a typical field in the Hetao Irrigation District in Bayannur, conducted groundwater samplings during the whole year drainage period from April to November 2023, and estimated lateral subsurface seepage of water and nutrients. Results indicated that nitrate-nitrogen was the dominant form of total nitrogen (TN), while dissolved organic nitrogen was the second dominant form. Dissolved organic phosphorus was the dominant form of total phosphorus (TP), while dissolved inorganic phosphorus was the least significant component of TP. Irrigation amount was the highest in May, and decreased from May to November. Lateral subsurface seepage increased from April to July and fell gradually from July to November, showing a lag relative to irrigation. In the typical field, TN and TP fluxes via lateral subsurface seepage were 8.773~9.654 kg/ha and 0.306~0.334 kg/ha in the whole growing season, respectively. Nutrient fluxes showed fluctuations, which were related to irrigation amount, precipitation and evaporation, and fertilizer applications. The research provides methodological guidance for estimating agricultural non-point source pollution in similar irrigation districts.