Nitrogen removal in an ecological ditch receiving agricultural drainage in subtropical central China

•A novel ecological ditch was designed and evaluated for its nitrogen removal capacity.•Nitrogen was effectively removed from agricultural runoff by the eco-ditch.•This ecologically-engineered system will be expanded locally and to similar areas.

Nitrogen (N) loss from agricultural field can cause eutrophication in downstream freshwater systems. Ecological ditches (eco-ditches) which are engineered to mitigate N loss from agricultural runoff. This study presents an analysis of water quality data for a 200 m long eco-ditch treating agricultural drainage in subtropical central China. Inflow concentrations of total N (TN) ranged from 2.3 to 3.1 mg l−1 and typically contained 1.1–1.7 mg l−1 nitrate (NO3−-N) and <0.4 mg l−1 ammonium (NH4+-N). Mean concentration removal efficiencies (η) in the eco-ditch for TN, NO3−-N, and NH4+-N were 75.8, 63.7, and 77.9%, respectively. Mean area-based first-order removal rate constant (J) and removal rate constant (k) of TN were 942.1 mg-N m−2 d−1 and 1.11 m d−1, respectively. Outflow from the eco-ditch typically contained <0.85 mg l−1 TN and <0.70 mg l−1 NO3−-N. Rate of N loss in the eco-ditch was highly seasonal, generally peaking in the summer months (May–August). Results show that the eco-ditch can be effective at reducing transport of non-point source of N in-situ, particularly in warm environments. Its use should be expanded to similar areas, although managing the eco-ditch to maximize N removal in the long-term will require dynamic management.