Integration of urea deep placement and organic addition for improving yield and soil properties and decreasing N loss in paddy field

Few studies have examined nitrogen (N) loss and crop yield under urea deep placement combined with organic addition in highly fertilized paddy field. Therefore, a three-year paddy field trial was conducted in Taihu Lake region of China to gain insight into yields, NH3 volatilization and N leaching. Six treatments were used: control (CK, zero N application), conventional (CT, broadcasting of 300 kg ha−1 N), recommended N (RT, broadcasting of 225 kg ha−1 N), organic addition (RTM, broadcasting of 225 kg ha−1 N and organic N), N deep placement (RTD, deep placement of 225 kg ha−1 N) and organic addition combined with N deep placement (RTDM, organic N combined with deep placement of 225 kg ha−1 N). Compared to the CK treatment, the RTM and RTDM treatments increased soil organic matter by 8%-11% and TN by 9%-14%. NH3 loss was dominated by floodwater NH4+-N concentration and increased with the increase of chemical N use. The RTM treatment had obviously higher floodwater NH4+-N concentration and NH3 loss than the RT treatment. By contrast, NH3 loss in the RTD and RTDM treatment was 84%-89% less than in the RT treatment, only accounting for 0.2%-1% of the total N application. Compared to NH3 volatilization, N leaching was low; and NO3−-N in the percolation water prevailed over NH4+-N and was linearly correlated with floodwater NO3−-N concentration. NO3−-N and TN leaching between the CT and RTDM treatments differed little (p > 0.05). N loss (NH3 loss and N leaching) induced by fertilizer was linearly correlated with N surplus. By comparison with the CT treatment, the RTDM treatment decreased N surplus by 73.1%, and increased grain yield by 18.3% and N recovery efficiency by 63.8%. The RTD treatment also had higher rice yield than the CT treatment, whereas unfavorable delayed senescence occurred in it in 2016 and led to a marked decrease of grain yield. The RTD treatment significantly increased plant N uptake (PNU) by rice straw compared to chemical N treatment and had the lowest N harvest index (NHI) and N physiological efficiency (NPE) among the fertilizer treatments. In short, urea deep placement combined with organic addition was superior to N deep placement or organic addition combined with N broadcasting to decrease N loss and increase grain yield, representing an effective and promising practice to achieve food and environmental security.