Study on the mechanism of Phoslock's impact on nitrogen migration at the sediment-water interface

Phoslock has a wide range of applications in regulating the release of endogenous phosphorus. Nevertheless, the knowledge regarding its influence on the nitrogen migration process at the sediment-water interface (SWI) remains rather limited. Therefore, this study explores Phoslock's impact on NO₃^--N and NH₄⁺-N migration at the SWI in Lake Xuanwu and Lake Li using DGT and Rhizon samplers. The results revealed that at 30 °C, Phoslock increased bioavailable NO₃^--N concentrations in the overlying water compared to the control group in both lakes. However, there were no significant differences in bioavailable NH₄⁺-N concentrations between experimental and control groups in the overlying water at different temperatures. At 30 °C, the downward diffusion intensity of bioavailable NO₃^--N in the overlying waters of Lake Xuanwu was about 41.59% and 45.76% higher under aerobic and anaerobic conditions (p < 0.05), respectively, and 10.64% and 184.33% higher in Lake Li, compared to the control group. At 15 °C, compared to the control groups, the overall diffusion intensity in the experimental groups of both lakes showed an increasing trend, but the differences were not significant. Phoslock reduced the upward diffusion flux of bioavailable NH₄⁺-N at the SWI of Lake Xuanwu and Lake Li. The inhibitory effect on the release flux of bioavailable NH₄⁺-N increased as the temperature decreased, but the impact on dissolved oxygen (DO) was minimal. For example, at 30 °C, the addition of Phoslock in Lake Xuanwu reduced the upward diffusion flux of bioavailable NH₄⁺-N by approximately 23.4% and 22.01% under aerobic and anaerobic conditions, respectively. At 15 °C, the reduction was approximately 33.52% and 36.25%, respectively. Phoslock also decreased the abundance of sediment nitrification functional genes (AOA and AOB) and enhanced the ability of sediments to replenish bioavailable NO₃^--N into interstitial water, while the ability to replenish bioavailable NH₄⁺-N decreased. According to the diffusion direction and intensity of bioavailable NO₃^--N and NH₄⁺-N at the SWI, the addition of Phoslock makes more bioavailable nitrogen in the overlying water accumulate in the sediments.