Reducing nitrogen leaching losses from paddy field under water-saving irrigation by water table control

Abstract: Effects of controlled drainage (CD) on nitrogen leaching losses from paddy field under controlled irrigation (CI) were investigated. Water table control levels were managing with the use of a lysimeter equipped with an automatic water table control system. Three drainage treatments were implemented, namely, controlled water table 1, controlled water table 2, and controlled water table 3. For controlled water table 1, the water table control levels were adjusted daily based on the actual water table depths that were measured by using a water table observation well. Water table control levels in controlled water table 2 were controlled based on the rice root zone depths in different stages according to the water table management that was tested in the humid regions of Eastern Canada and Midwestern United States. For controlled water table 3, the water table control levels in different stages were selected based on previous studies in paddy field of Southeast China. The water table control levels in the later tillering stage and milk stage were also adjusted depending on the characteristics of rice growth and cultivation needs. Experiments were conducted in nine drainage type lysimeters with a mobile shelter and gallery. Each lysimeter had an area of 2.5 m × 2 m and a depth of 1.3 m. Influence of rainfall was avoided using the mobile shelter to strictly regulate the soil moisture in CI. Each lysimeter was individually irrigated and drained using a pipe installed with a water meter and a tube (40 mm in inner diameter) installed at 1.2 m below the soil surface, respectively. Subsurface drainage was conducted based on the water table control levels by using an automatic water table control system, which was installed on each drain tube in the gallery. Subsurface drainage water were collected twice at 2d intervals after each fertilizer application followed by 4d intervals. A 7d sampling interval was used during the rest time. NH4+-N and NO3?-N concentrations in the water samples were analyzed using an ultraviolet-visible spectrophotometer. NH4+-N or NO3?-N leaching loss was calculated by multiplying the subsurface drainage water volume between the two dates by the NH4+-N or NO3?-N concentration in the sample taken at the latter date. The results showed increasing water table control levels resulted in less subsurface drainage water from CI paddy field. The subsurface drainage water from controlled water table 1 was 179.4mm, which was 8.4% and 37.2% lower than those from controlled water table 2 (195.9 mm) and controlled water table 3 (285.8 mm), respectively. As the water table control levels increased, NH4+-N concentration in subsurface drainage water increased, however, NO3?-N concentration decreased. Average NH4+-N concentration in the subsurface drainage water from controlled water table 1 was 9.3% and 27.3% higher than those from controlled water table 2 and controlled water table 3, respectively. Average NO3?-N concentration in the subsurface drainage water from controlled water table 1 was 32.6% and 1.8% lower than those from controlled water table 2 and controlled water table 3, respectively. Significantly, less NO3?-N leaching loss from CI paddy field was observed as the water table control levels increased. NO3?-N leaching loss from controlled water table 1 was 0.27 kg/hm2, with a significant reduction of 0.16 and 0.61 kg/hm2, respectively, compared with controlled water table 2 and controlled water table 3. NO3?-N leaching loss from controlled water table 2 was significantly reduced by 0.45 kg/hm2 compared with controlled water table 3. The application of CD with suitable water table control level could effectively reduce NO3?-N leaching loss from CI paddy field.