Soil organic carbon stocks and stabilization mechanisms in tidal marshes along estuarine gradients
Tidal marshes store large amounts of soil organic carbon (SOC), however, little is known on SOC stabilization mechanisms in these ecosystems. In estuarine marshes, SOC storage is dominated by a complex interaction of tidal inundation and salinity with biotic ecosystem components, leading to strong spatio-temporal variations within estuaries. Our aim was to assess (i) SOC stocks, (ii) SOC stabilization mechanisms (aggregation and mineral-association), and (iii) their environmental drivers along estuarine gradients. We analyzed SOC stocks and SOC density fractions in topsoil (0-10 cm) and subsoil (10-30 cm) of three marsh zones representing three flooding regimes (daily, monthly, yearly) in three marsh types along the salinity gradient (salt, brackish, freshwater) of the Elbe Estuary, Germany. Increasing salinity and flooding reduced SOC stocks 0-30 cm (9.3-74.6 t ha-1), which was related to decreasing plant biomass and soil texture. Mineral-associated organic matter (CMAOM) was the largest SOC fraction (59% of total SOC), followed by aggregate-occluded organic matter (CoPOM) (24%) and free particulate organic matter (CfPOM) (16%). The CMAOM amount in topsoils decreased downstream with increasing salinity, reflecting decreasing fine-texture along the estuary. The amount of CoPOM was higher in topsoils and high marshes, indicating negative effects of flooding on aggregation. The relative proportion of CfPOM (% of total SOC) increased with increasing flooding frequency and reducing soil conditions. Our results underline the importance of estuarine gradients as drivers of SOC storage and stabilization. Climate-change induced sea-level rise and variations in salinity might reduce SOC storage and stabilization in estuaries.