Nutrient removal performance from agricultural drainage by strengthening ecological ditches in hilly areas

Agricultural drainage from catchment greatly impacts water quality in the hilly area due to high nitrogen concentrations in the central basin of Sichuan Province, China. The original ecological ditches have proven effective in removing associated nutrient loads, but their nitrogen removal efficiency is limited by the elevated nitrogen levels of overland flow and subsurface flow. Field experiments were conducted to optimize the drainage network to enhance the performance of ecological ditches and enhance nitrogen removal efficiency in hilly regions. The results suggested that ecologically permeable dam ditches exhibited superior total nitrogen (TN) and total phosphorus (TP) removal efficiencies compared to single dams, multiple dams, and ecological intercept dams. Specifically, in single, multiple, and ecological intercept dams, TN removal was 20.2 %, 12.7 %, and 2.5 % higher, respectively, while TP removal was 18.2 %, 10.3 %, and 3.7 % higher, respectively. The TN removal efficiency by the Hydrocotyle verticillate and Iris pseudacorus L. ditch was 15.8 % and 2.3 % higher than that of the Myriophyllum verticillatum L. and Hydrocotyle verticillata and Hydrocotyle verticillata and Thalia dealbata Fraser, respectively. The TP removal efficiency in Hydrocotyle verticillata and Thalia dealbata Fraser ditch was 11.4 % and 3.5 % higher than that in Myriophyllum verticillatum L. and Hydrocotyle verticillata and Hydrocotyle verticillate and Iris pseudacorus L., respectively. The soil bed ditch demonstrated superior TN and TP removal efficiencies, with rates of 15.2 % and 11.3 %, respectively, compared to the pebble bed and mudstone bed (2.9 % and 3.4 %, respectively). Therefore, the original ecological ditch was reinforced through a combination of fine-textured soil substrates, native emergent plants with large biomass and ecologically permeable dams to enhance the removal efficiencies of TN and TP in agricultural drainage. Following reinforcement and evaluation in 2020, the removal efficiencies of TN, TP, ammonia nitrogen (AN) and nitrate nitrogen (NN) in the strengthened ecological ditches increased to 58.1 %, 56.5 %, 56.8 % and 54.2 %, respectively. In this study, a scientific foundation for enhancing the efficacy of ecological ditches in mitigating nonpoint source pollution from agricultural drainage in mountainous and hilly regions of subtropical and developing countries is established.