Using the paddy eco-ditch wetland system to reduce nitrogen non-point source pollution discharge from paddies: A consecutive six-year study in the Yunnan Plateau, China

Abstract

Nitrogen non-point source pollution from paddies occurs in a wide area to the detriment of the environment. Paddies are usually connected with eco-ditches and wetlands in the dominant rice planting regions in southern China, forming a paddy eco-ditch wetland system. This system could effectively reduce nitrogen discharge from the source (paddies) to the process (eco-ditches and wetlands). However, the rules and mechanisms of nitrogen discharge reduction still need to be clarified. Here, a consecutive six-year (2016–2021) study with different irrigation modes and nitrogen application regimes in paddies, different plant types and overflow conditions in eco-ditches and wetlands was conducted. Results showed that applying alternate wetting and drying irrigation, along with 135 kg ha−1 total nitrogen application divided into three times in paddies, reduced 35.4% nitrogen discharge compared to the continuous flooding irrigation along with 193 kg ha−1 total nitrogen application. The average nitrogen discharge reduction rates of Pontederia cordata, Canna indica, and Zizania latifolia ditches under natural overflow were 23.9%, 14.2%, and 18.7%, respectively. Likewise, those of Pontederia cordata, Thalia dealbata Fraser, and Schoenoplectus tabernaemontani wetlands were 33.6%, 17.1%, and 22.9%, respectively. The average nitrogen discharge reduction rate of eco-ditches and wetlands reached more than 50% after 3-day hydraulic retention, which was significantly higher than that under dynamic overflow. Reducing the water depth, overflow velocity, and nitrogen concentration could effectively increase the nitrogen discharge reduction rate of eco-ditches. Furthermore, the discharge reduction effect of ammonium nitrogen in eco-ditches was better than that of nitrate nitrogen. The reduction rate of the paddy eco-ditch wetland system on nitrogen discharge from paddies ranged from 32.3% to 64.9%. Pontederia cordata ditch and wetland had the best performance of nitrogen discharge reduction due to the robust root system. In conclusion, the optimal strategy to reduce the nitrogen non-point source pollution from paddies using the paddy eco-ditch wetland system was applying the alternate wetting and drying irrigation and 135 kg ha−1 nitrogen in paddies, extending the overflow path in eco-ditches, and prolonging the hydraulic retention time in wetlands.