Effects of dredging on nutrients in sediments in Xiashan Daxi Basin of Lianjiang River

To reveal the effects of dredging on the temporal and spatial distribution and the release process of nutrients in surface sediments, the contents of organic matter (OM), total nitrogen (TN) and total phosphorus (TP) of the surface sediments as well as the concentrations of ammonia nitrogen (NH₄ ⁺-N), TN and TP of the interstitial water and the overlying water at 10 points from Xiashan Daxi Basin of Lianjiang River were analyzed before and after dredging, and the pollution status of surface sediments after dredging was also evaluated. The results showed that TP, TN, and NH₄ ⁺-N concentrations in interstitial water and overlying water decreased significantly after dredging. The average concentrations of OM and TN of surface sediments decreased by 53.3% and 54.5%, respectively, compared with those before dredging. TP concentration of surface sediment was higher than that before dredging at nearly half of the points, and the ability of dredging to remove nutrients in surface sediments was TN>OM>TP. The pollution of OM, TN and TP in surface sediments after dredging improved compared with that before dredging, but the pollution level was still high. After dredging, the nitrogen in the sediments displayed as "source", and phosphorus changed from "source" to "sink", indicating that this dredging promoted the internal release of nitrogen in the interstitial water of surface sediment and inhibited the internal release of phosphorus. This was related to the dredging depth, the changes in the physicochemical characteristics of the sediments and the occurrence environment.