Mobilization and geochemistry of nutrients in sediment evaluated by diffusive gradients in thin films: Significance...

Investigation of in-situ mobilization of both nitrogen (N) and phosphate (PO₄^3-) in sediment is important for lake management strategy. In this paper, diffusion gradients in thin films (DGT) and DGT induced flux in sediments (DIFS) model are newly designed for in-situ measurement of iron (Fe), PO₄^3-, nitrate (NO₃-N) and ammonium (NH₄-N), and nutrients' mobility in sediment in Lake Nanhu (China). According to DGT profiles together with physicochemical properties in sediment, (I) PO₄^3- is released from (i) Fe-bound P plus loosely sorbed P in anoxic sediment and (ii) the loosely sorbed P in oxic sediment; (II) anoxic sediment inhibits nitrification and NO₃-N release, but it favors denitrification and dissimilatory nitrate reduction to ammonium (DNRA), leading to NH₄-N release; (III) Eh and organic matter are two key influence factors on mobility of PO₄^3-, NO₃-N and NH₄-N. According to DIFS calculation, the dynamics of desorption and diffusion at two sites belong to (i) slow rate of resupply and (ii) fast resupply cases, respectively. Internal loadings are estimated to be 92.74 (PO₄^3-), 268.1 (NH₄-N) and -2466 kg a^-1 (NO₃-N), which reflects sediment mainly acts as a source for PO₄^3- and NH₄-N, and a sink for NO₃-N in water. Based on sediment P release risk index (SPRRI), P release risks in lake sediments are estimated, ranging from light to relative high level. DGT and SPRRI aid choice of restoration methods for sediment, including sediment dredging, phytoremediation and in-situ inactivation.