Effects of crab disturbance on nitrogen migration and transformation in a coastal tidal flat wetland

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.