<b>Mineral-microbial synergy prolongs carbon turnover in coastal wetlands through the burial of pre-aged allochthonous materials</b>

Coastal margins are critical sites for carbon (C) storage, yet our understanding of how pre-aged, allochthonous C is stabilized in these environments remains incomplete. The interplay between mineral association, microbial processing, and the ¹⁴C-age of stored C represents a significant knowledge gap in blue carbon science. Here, we investigated C storage mechanisms in Chinese mangrove and saltmarsh soils by analyzing topsoils and cores across a 20-degree latitudinal transect. We found that C-depleted saltmarshes exhibited significantly longer soil organic C (SOC) turnover times than C-enriched mangroves (2200 vs. ~500 years in topsoils, respectively). The longer turnover times in saltmarshes corresponded to higher proportions of pre-aged (~50%) and petrogenic (~20%) SOC, as well as a greater fraction of mineral-associated organic C (MAOC). Furthermore, our results revealed a crucial, depth-dependent synergy between microbial processes and long-term stabilization. While microbial necromass accumulation was a key driver of C turnover in surface soils, the degree of lignin degradation emerged as the primary control on C persistence at depth in multi-millennial time scale. Our findings provide compelling evidence that coastal wetlands effectively store old, allochthonous C through a mineral-microbial synergy.