Diffusive gradients in thin films for transfer of phosphorus, nitrate and ammonium in lake sediments

Abstract

There is a lack of the understanding of the transfer of phosphate (PO4-P), ammonium (NH4-N) and nitrate (NO3-N) at the millimeter scale in sediments related to internal loadings in the semi-arid and cold Lake Hulun in China. In this paper, a passive sampling technique-diffusion gradients in thin films (DGT) has been used for the biogeochemical behaviors and dynamic transfer of PO4-P, NH4-N and NO3-N in sediments in this lake. The two-sided DGT probe (AMP-TH&ZrO-Chelex) and ZrO DGT piston are designed for sediment/water interface. Based on change characters of DGT concentration profiles (nutrients, Fe and Mn) related to redox status and chemical properties, N and P mobilizations are derived. The reductive dissolution of Mn(IV) or Fe(III) (oxyhydr)oxides accounts for subsequent P release is corresponded with the oxic or anoxic status in sediment layers; Fe(III)-reduction is the main reason for P remobilization. The breakdown of algae biomass deposited on the uppermost sediment and phosphatase enzymes activity enhanced by NH4-N also lead to P remobilization. In sub-anoxic or anoxic layer, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) induce exchangeable ammonium (NH4ex) enrichment and NH4-N release, while, anammox depletes NH4-N mobility. Aerobic nitrification results in NO3-N remobilization. Organic matter favors NH4-N release. DGT induced fluxes in sediments (DIFS) model is used to derive the labile P pool (51.5–465.5 cm3 g−1), resupply constant (0.186–0.734), response time (54–11,070 s) and Dspt rate (32–382,517 nmol cm−3 d−1) for describing dynamic P transfers with two opposite types of “slow” and “fast” resupplies. The diffusive fluxes across sediment/water interface in ranges of 67.9–498.7 μg m−2 d−1 (PO4-P), −1416–106.8 (NH4-N) and –2147–301.1 μg m−2 d−1 (NO3-N) and DIFS parameters can be used for release risks in lake regions and the sediment remediation strategies are recommended.