Transport and transformation of water and nitrogen under different irrigation modes and urea application regimes in paddy fields

Nitrogen pollution from paddy fields is a main contributor to global non-point source (NPS) pollution and greatly increases the risks of surface water eutrophication and groundwater contamination. This study assesses the comprehensive transport and transformation of water and nitrogen (TTWN) for a 4-year irrigation and urea application in the paddy field using 6 water balance components and 12 nitrogen inputs and outputs. The results show that alternate wetting and drying (AWD) irrigation reduces water and nitrogen for irrigation, drainage, and leaching, but has a minor effect on rice evapotranspiration (ETc). Urea application and mineralization are the most important nitrogen inputs to the paddy field, accounting for 58.7–68.2% and 23.7–35.0% of nitrogen input, respectively. Rice plant uptake, ammonia volatilization (AV), and nitrification-denitrification account for 68.0–75.0%, 11.9–17.1%, and 5.1–9.3% of nitrogen output, respectively. Nitrogen leaching (3.5–11.5 kg·ha−1) is also large while the observed depth of leaching needs to be further investigated. Meanwhile, nitrogen drainage (0.72–1.11 kg·ha−1) is not as large as conventionally accepted. AWD affects the nitrogen accumulation and release of duckweed, which is one of the potential reasons for the increase in rice yields. In the course of four continuous rice seasons, the soil nitrogen content (SNC) of the 0–20 cm and 20–40 cm soil layers continued to decrease and increase, respectively, and the overall nitrogen content did not change significantly. Irrigation modes and fertilization splits may promote or inhibit AV, and it depended on the time when urea was applied. In addition, AWD and three fertilization splits significantly promote nitrification-denitrification and nitrogen uptake by rice. This study presents a theoretical basis for NPS pollution reduction and provides an important framework for establishing an accurate model of TTWN for paddy fields.