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

Investigation of in-situ mobilization of both nitrogen (N) and phosphate (PO43-) 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), PO43-, 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) PO43- 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 PO43-, 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 (PO43-), 268.1 (NH4-N) and -2466 kg a-1 (NO3-N), which reflects sediment mainly acts as a source for PO43- 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.