The impact of super-typhoon Mangkhut on sediment nutrient density and fluxes in a mangrove forest in Hong Kong.

Cyclone disturbance results in mangrove foliage loss, tree mortality and other changes in ecosystem processes. However, the impact of cyclones on mangrove sediment nutrient density, sediment-air CO2 and CH4 fluxes and their isotopes remains largely unknown. Super-typhoon Mangkhut (maximum gust 256 km h-1) hit Hong Kong in September 2018. We investigated the influence of the cyclone by comparing pre- and post-cyclone sediment carbon cycling processes as well as nitrogen density during an 8-month period in a mangrove forest at Ting Kok, Hong Kong. Time and/or nitrogen density are the dominant drivers of the variation of dark sediment-air CO2 fluxes (Rd) and sediment nutrient density. Significant changes in Rd and their δ13CO2 values, sediment organic carbon density (SOC) and nitrogen density (SON) occurred after the cyclone. Rd were highest one month after the cyclone (0.05 ± 0.01 mmol m-2 min-1) and lowest before the cyclone (8.32 ± 2.84 µmol m-2 min-1). δ13CO2 of pre-cyclone Rd (-18.2 ± 1.7‰) was significantly higher than that of all post-cyclone fluxes (-22.9 ± 1.5‰ to -23.6 ± 1.8‰). Both SOC and SON were highest one month after the cyclone (23.05 ± 1.92 kg C m-3, 2.42 ± 0.11 kg N m-3, 20-40 cm). A significant positive relationship exists between Rd and SON. Sediment-air CH4 fluxes did not show significant changes over time but along the sea-land gradient (0.28 ± 0.21 to 0.61 ± 0.22 µmol m-2 min-1). Cyclone disturbance results in the pulse input of litter, which may explain the significant increase in post-cyclone Rd and lower δ13CO2 of Rd. With anticipated climate change-driven effects on cyclone occurrence and intensity, our data underscores the significance of incorporating the influence of cyclone disturbance in constraining the global nutrient budgets in mangroves.