Response of dissolved organic carbon molecular footprints in mangrove tidal flats to extreme rainstorm events

Mangroves, as typical blue carbon ecosystems, store massive amounts of soil organic carbon and serve as both sources and sinks of organic carbon. Under extreme rainstorm conditions, once mangrove ecosystems are damaged, they lead to significant release of greenhouse gases into the atmosphere, causing severe greenhouse effects and resulting in changes to carbon footprints. However, the effects of extreme rainstorms on the spatial distribution, optical properties, and molecular characteristics of soil organic carbon in mangrove sediments remain unclear. This study focused on a typical mangrove tidal flat in Shenzhen Bay, Southeast China, to quantify variations in sediment carbon fractions (total carbon (TC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC)), fluorescent composition, and biomolecular compounds of DOC, and identified their response mechanisms to extreme rainstorms. The results showed that the spatial distribution of sediment TC, DOC, and total nitrogen (TN) followed the order of tidal creek > mudflat > mangrove both before and after a rainstorm. The variation degree of DOC content (5.0%-48.5%) after extreme rainstorms followed the pattern of mangrove > mudflat > creek, while the result for DIC was completely opposite. In the vertical direction, tidal flat sediments exhibited decreasing TC, DOC, and TN concentrations with depth before the extreme rainstorm. After the rainstorm, DOC and TN increased across all sediment layers, whereas IC decreased at all depths. After the extreme rainstorm, TN and DOC in sediments increased significantly. Combined with analysis of the optical and molecular properties of DOC, the results indicated that lignin- and tannin-like compounds with high stability increased in mudflat sediments after the extreme rainstorm, while tryptophan-like compounds with high aromaticity and low humification increased in mangrove sediments, and protein-like organic compounds in tidal creek sediments decreased. Principal component analysis showed that extreme rainstorms mainly affected sediment DOC by influencing the transport of silt and clay. This study not only contributes to a better understanding of how extreme rainstorms regulate organic carbon behavior in mangrove sediments to improve sustainable management of mangrove wetlands under the pressure of extreme weather events but also provides new directions for carbon footprint research.