Living root contributions dominate vertical accretion, but not carbon burial, in two SE Australian tidal wetlands

Organic matter derived from allochthonous and autochthonous sources makes an important contribution to the accumulation and burial of “blue carbon” in tidal wetlands. Organic matter accretion is also a mechanism by which tidal marshes and mangroves adjust vertically to the pressure of sea-level rise, through feedbacks between marsh autotrophic productivity and hydroperiod. However, the separate contributions of inorganic matter, detrital organic matter and living roots to marsh accretion have rarely been documented. We used a network of Surface Elevation Table-Marker Horizon (SET-MH) stations SE Australia as a benchmark against which the accumulation of inorganic, organic and living root material was measured. Established in 2000–2002, the SET-MH stations allowed for sampling accretion and elevation gain in mangrove and saltmarsh in fluvial and marine sand geomorphic settings. We found living root material to be the dominant contributor to the volume of accretion above feldspar marker horizons and elevation gain over the 20-year experimental period. However, detrital organic matter, being of lower bulk density than living roots, contributed most to carbon burial. Higher rates of accretion and elevation gain were associated with higher contributions of pore volume, and lower contributions of inorganic matter. The rate of subsidence increased with accretion and was lower where inorganic matter contributions to mass were higher. The results highlight the importance of biological and hydrological contributions to accretion and elevation gain in a region subject to riverine minerogenic inputs.