The effects of dredging on nitrogen balance in sediment-water microcosms and implications to dredging projects

Sediment dredging is a controversial technology for eutrophication control. It is necessary to understand its environmental effectiveness and mechanisms. In this study, sediments from un-dredged (UDR), fresh-dredged (FDR) and post-dredged (PDR, 3 years later) regions of Dongqian Lake were collected to compose sediment-water microcosms for evaluating the effects of dredging on nitrogen cycling. From 0 to 19 days, ammonia in the overlying water greatly increased in UDR (603.1%) and FDR (269.3%), but slightly in PDR (88.7%). The increase of ammonia concentration in water was negatively correlated with DO which was higher in two dredged groups than that in un-dredged groups. These results suggest that dredging could lead to ammonia release to water column in a short term, while ammonia release risk is low few years after dredging. In addition to high DO content, high Eh values, low pH and sediment porosity in two dredged groups may weaken the attenuation of nitrate. TN contents in both sediment and overlying water indicate that nitrogen lost in un-dredged group while had no significantly change in two dredged groups, indicating the ability of nitrogen removal in dredged groups was weakened. Our study suggests that dredging may have potentials to weaken nitrate and/or nitrogen removal from lake systems over years. For this, we should pay more attention on dredging area in dredging projects. Moreover, dredging is better to be conducted in low temperature seasons after blocking external nutrient loadings.

► N balances in sediment-water microcosms representing FDR and PDR were similar.
► DO and Eh were higher; pH and porosity were lower in FDR and PDR than in UDR.
► Dredging may cause ammonia release in a short term but reduce the risk in a long run.
► N removal and nitrate attenuation were weakened in FDR and PDR microcosms.