Fast migrations of nitrogen and phosphorus are driven by microorganism in freshwater lake sediments

The migrations of nitrogen and phosphorus in lake sediment are often driven by various factors, while migration after anthropogenic disturbance with microorganism forces has rarely been discussed in previous work. Therefore, this study aimed to investigate the migrations of nitrogen and phosphorus in lake sediments with different status in terms of determining the migration pattern under human activities.

The diffusive equilibration in thin film gel (DET) probe was used to characterize porewater concentrations of ammonium-nitrogen (NH₄⁺-N) and phosphate-phosphorus (PO₄³⁻-P), while total nitrogen (TN) and total phosphorus (TP) contents were measured in sediments from three areas of Lake Taihu when lake sediments were disturbed or undisturbed and after organic matter was added.

The results showed that the porewater NH₄⁺-N and PO₄³⁻-P concentrations increased with increasing sediment depth. Conversely, the TP and TN contents decreased with increasing sediment depth in the mixing sediments after 10 days of rebalancing time. The inflection point of their contents was at a mixed sediment depth of approximately 10 cm. Both microorganisms and organic matter played significant roles in the distributions of TN and TP in lake sediment, which eventually increased the differences in the porewater NH₄⁺-N and PO₄³⁻-P concentrations among different sediment depths by decreasing the pH and Eh. Moreover, the vertical migration of the soluble NH₄⁺ and PO₄³⁻ in porewater continuously migrated from the deep layer to the upper sediment layer, which caused higher TN and TP contents in the upper sediment layer.

The profiles of nitrogen and phosphorus contents in lake sediment could be reconstructed quickly by complicated biogeochemical processes even if the lake sediment was disturbed.