Limited N removal by denitrification in agricultural drainage ditches in the Taihu Lake region of China

Abstract

Agricultural drainage ditches constitute corridor wetlands that facilitate agricultural non-point nitrogen (N) load transportation into river systems. Quantifying sediment denitrification in ditches provides relationships between N losses from agriculture and water quality. However, high denitrification rate potential and limited N residence time make the total denitrification removal capacity in ditches uncertain. The purpose of this work was to identify N removal by denitrification in agricultural ditches in the Taihu Lake region of China. A field investigation and laboratory analyses were conducted to investigate the sediment denitrification rate of ditches in areas under different crops, including vegetable, rice-wheat fields, and a peach orchard, between June 2014 and October 2015. At each sampling, concentrations of dissolved inorganic nitrogen (DIN, NO3 −-N, NH4 +-N), water temperature, pH, Eh, dissolved oxygen, and dissolved organic carbon of the overlying water and the DIN and total carbon of the sediment were examined. Sediment denitrification rates in all the ditches exhibited high spatial and temporal heterogeneity. Concentrations of DIN and temperature of the overlying water were key factors controlling denitrification in the ditch system. The sediment denitrification rate in the ditches could be estimated by a nonlinear mixed-effect model. Based on estimating data on N concentrations and temperature of overlying water and our established nonlinear mixed model, less than 1% of N was removed by denitrification annually in the ditches. The limited N removal by sediment denitrification was attributed to short retention times and the low area of the ditch system. Dissolved inorganic nitrogen concentrations and temperature of the overlying water were key factors controlling denitrification in the ditch system. High density of ditches did not lead to high N removal by denitrification due to short retention times and low areal coverage in the Taihu Lake region.