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

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 (eta) 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. (C) 2015 Elsevier B.V. All rights reserved.