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Coastal wetlands are hotspots for nitrogen (N) cycling, and crab burrowing is known to transform N in intertidal marsh soils. However, the underlying mechanisms remain unclear. This study conducted field experiments and used indoor control test devices to investigate the seasonal response of nitrogen to crab disturbance at the sediment–water interface in coastal tidal flat wetlands. The results showed that crab disturbance exhibited significant seasonality with large seasonal differences in cave density and depth. Due to crab disturbance, nitrogen fluxes at the sediment–water interface were much greater in the box with crabs than in the box without crabs. In summer, NH4+-N showed a positive flux from the sediment to the overlying water, but NO2−-N and NO3−-N showed positive fluxes from the sediment to the overlying water only in early stages. In winter, NH4+-N showed a positive flux from the sediment to the overlying water, but NO2−-N and NO3−-N both exhibited positive and negative fluxes. These results indicated that the presence of crab burrows can cause the aerobic layer to move downward by approximately 8–15 cm in summer and directly promote nitrification at the sediment surface.

The influence of crab disturbances on nitrogen migration and the transformations of pore water and overlying water in a coastal tidal flat wetland were investigated at the lab scale, and the nitrogen exchange flux at the sediment-water interface was calculated. The results showed that crabs, combined with tidal effects, had significant effects on the microtopography of the studied crab box. In addition, there was no significant difference in the concentrations of NH4+-N, NO3−-N, or TN between two points in the horizontal direction (P > 0.05), and there were significant differences in the concentrations of NH4+-N and TN in the vertical direction (P < 0.05); the NO3−-N concentration difference was not obvious (P > 0.05). The NO3−-N concentration in the surface pore water of the crab box had a downward trend with time. Furthermore, the NH4+-N and TN contents in the overlying water in the crab box were significantly higher than those of the control box, indicating that crab disturbances also had significant effects on the concentrations of NH4+-N, NO3−-N, and TN in the overlying water. The existence of crab caves greatly promoted the nitrogen exchange flux at the sediment-water interface, and the mean exchange fluxes of NH4+-N, NO3−-N and TN were 51.40 mmol (m2 day)−1, −13.44 mmol (m2 day)−1 and 39.74 mmol (m2 day)−1, respectively (much higher than those measured in the control box), implying that NH4+-N and TN were released from the sediment to the overlying water, while NO3−-N was released from the overlying water to the sediment. [ABSTRACT FROM AUTHOR]

Periodic floods and ebb tides driven by seawater affect arsenic (As) behavior in coastal wetlands, which is not fully understood. In this study, we combined DGT, high-throughput sequencing, and real-time quantitative polymerase chain reaction (qPCR) techniques for the first time to investigate the effects of flood and ebb tide on As transformation and mobility in a typical coastal tidal flat wetland by laboratory experiments. In the flood-tide stage, more As was released into the water solution, and As(III) concentration in water solution was notably higher (51 μg/L) than in the ebb-tide stage, especially in SA (sodium acetate addition) treatment. As-methylation also occurred in this stage. The release of As was attributed to the reductive dissolution of amorphous and crystalline Fe oxides bound As induced by microbes. Fusibacter , Sva1033 , Bradyrhizobium , Ralstonia , and Desulfobacter were primarily involved in the reduction of As, Fe, and S. The copy numbers of arsC were 20–148 times higher than arrA , suggesting that the detoxification reduction pathway was the primary mechanism of direct As-reduction in the flood-tide stage. However, 2-D high-resolution DGT techniques revealed that the concentrations of As(III) and total As in pore water of sediments in the ebb-tide stage were higher than those in the flood-tide stage. The detected aioA gene indicated that As was oxidized in the sediment surface, resulting in As stabilization by crystalline iron minerals, whereas more As(III) were accumulated in pore water. The reduction pathway mediated by arrA may play a prominent role in the ebb-tide stage. This study provides new insights into As release processes in coastal wetlands in seawater environments, which further deepens our understanding of As biogeochemical cycling behavior in different natural environments. [Display omitted] • As Reduction and methylation occurred in coastal tidal flat sediments. • As(III) release in water solution was higher in the flood tide stage. • As Detoxification reduction was the primary mechanism in flood tide stage. • As(III) release in pore water of coastal sediments was higher in ebb tide stage. • As Reduction pathway mediated by arrA played a prominent role in the ebb tide stage. [ABSTRACT FROM AUTHOR]