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.