Field analysis of water and nitrogen fate in lowland paddy fields under different water managements using HYDRUS-1D

• HYDRUS-1D model was calibrated and validate for lowland paddy soil water and N regime.
• Water and N balances of paddy fields under AWD and CF irrigation were compared.
• AWD irrigation decreases percolation and N leaching losses.
• AWD irrigation increases the N processes of nitrification–denitrification.

With the wide adoption of alternate wetting and drying (AWD) irrigation for rice production, the water and nitrogen (N) fate in lowland paddy fields under AWD irrigation was needed to be investigated for assessing the water saving and environmental effects of AWD, compared to traditional continuously flooded (CF) irrigation. The HYDRUS-1D software package was used to simulate water movement, and N transport and transformations in experimental paddy fields under AWD and CF irrigation during 2007 and 2008. The variation in N transformation between AWD and CF paddy fields due to different water regimes in soil profiles were represented by time-varying boundary conditions and N transformation parameters that are dependent on soil water content. Simulations show that AWD irrigation decreased 27.8 and 19.0% of percolation, 5.0–11.2% and 3.0–23.5% of N leaching losses in 2007 and 2008, respectively, compared to CF irrigation. However, AWD irrigation increased 6.0–22.0% and 2.5–11.7% of N losses of volatilization, and 6.3–9.4% and 4.5–7.6% of nitrification, as well as 6.7–19.8% and 4.1–11.2% of denitrification in 2007 and 2008, respectively. These results reflect the intensified nitrification–denitrification processes that were caused by the relatively high ammonium concentration, and alternate aerobic and anaerobic environment in AWD paddy fields. The increased nitrate, which was formed from nitrification of ammonium in drying (aerobic) phase, can be easily denitrified to N2 or N2O in the wetting (anaerobic) phase. Therefore, the practice of AWD irrigation should consider its side effects on increasing N emissions from paddy fields that may also decrease the N use efficiency. Simulation of water and N regime together using HYDRUS-1D is an alternative system approach to improve water and N management for sustainable rice production.