Highly reworked subtropical mudflat still provides carbon sequestration due to high sedimentation rate

Abstract

Tidal flats are typically considered areas with large carbon storage potential over time. Yet such insights are mostly from vegetated areas, the carbon dynamics and long-term sequestration capacity of widespread unvegetated areas remain largely unclear. In this study, we investigated carbon sources, stocks, and sequestration rates over a two-year sampling period at a tidal flat within Mai Po Inner Deep Bay Ramsar Site, Hong Kong. To trace the origins of carbon, monthly measurements of carbon and nitrogen content and their stable isotopes (i.e., δ¹³C and δ¹⁵N) were conducted for surface sediment and suspended matter from adjacent rivers. Our results showed that sediment carbon content fluctuated both temporally and spatially, but the stable isotope analysis revealed a consistently terrestrial signature. The estimated carbon stock in top meter of sediment was approximately 23.5 Mg C ha⁻¹, which was notably lower than the global average for unvegetated tidal flat (86.3 Mg C ha⁻¹). However, sediment accumulation rates determined by ²¹⁰Pb dating, varied from 1.0 to 1.6 with a mean of 1.3 cm yr⁻¹. These rates were comparably higher than most of the rates (<1 cm yr⁻¹) reported worldwide for tidal flats. The historical record of the sediment cores indicated a marked increase in sedimentation rate since late 20th century. While the active and high sedimentation rates drove the lower observed carbon stock, our results still highlight the potential for carbon sequestration in dynamic unvegetated tidal flats. The lower stock underscore the vulnerability of this function to coastal development, which could lead to substantial carbon loss if not well managed.