Organic carbon stabilisation mechanisms in estuarine marsh soils: Effect of salinity and flooding frequency

The capability of coastal wetland soils to store large amounts of organic carbon (OC) has been increasingly recognised. Stabilisation mechanisms (e.g. aggregation or mineral association) and stability of organic matter (OM) (recalcitrant vs. labile) are important features for the long-term storage of soil organic carbon (SOC). In estuarine marshes, SOC storage is dominated by a complex and dynamic interaction of abiotic conditions such as tidal inundation or changes in salinity. However, little is known on OC stabilisation and stability in these transitional ecosystems and how they are affected by system-specific characteristics. Therefore, our aim was to assess the effect of flooding and salinity on (i) OC stabilisation by aggregation and mineral association and (ii) the stability of the OC pool in estuarine marsh soils.

We analysed topsoil (0 – 10 cm) and subsoil (10 – 30 cm) samples from 9 marsh zones along the salinity gradient (salt, brackish and freshwater) and flooding gradient (pioneer zone, low and high marsh) of the Elbe Estuary for their SOC contents, OC stabilisation mechanisms (density fractionation), OC stability (incubation with one- and two-compartment model fits) and dissolved organic carbon (DOC) concentrations.

Total SOC contents were highest in the freshwater marsh and decreased towards topsoils with higher salinity. Flooding frequency had no uniform effect on SOC contents: While there was a positive tendency with decreasing flooding frequency, subsoils of the freshwater marsh showed the opposite trend. Total SOC contents were positively correlated with mineral-associated OC (C_MAOM) and pedogenically unprotected particulate OM (C_fPOM). The highest proportion of C_MAOM was found in topsoils of freshwater marshes and it decreased towards higher salinities in topsoils of high marshes and pioneer zones. The OM protection by aggregation (C_oPOM) increased in topsoils of high marshes. The proportion of C_fPOM was less directly affected by salinity and flooding than by the CN ratio of the aboveground biomass (CN_litter). Furthermore, C_fPOM correlated positively with the potential mineralisable C (C_pot) and labile C (C_labile) and negatively with the recalcitrant C pool (C_recalcitrant) that were derived from the one- and two-compartment models. Labile C, C_pot and C_recalcitrant were also strongly influenced by CN_litter. Moreover, C_recalcitrant was linked to the proportion of C_MAOM. Concentrations of DOC increased with total SOC and C_pot but decreased with C_oPOM.

We conclude that SOC stabilisation in the Elbe Estuary is mainly related to mineral association of OM. With increasing terrestrial influence, physical protection in aggregates becomes more important. Besides these pedogenic stabilisation mechanisms, recalcitrance is strongly determined by vegetation characteristics.