Contribution of mud-associated organic matter to marsh blue carbon

Organic matter (OM) in marsh soils is commonly assumed to originate from in situ plant production. We illustrate that the OM associated with mud deposited in marshes is an underestimated source of OM accumulation. Here we assume that the marsh soil is a mixture of three constituents: locally produced plant material, mud with its associated OM, and sand. By using datasets of paired dry bulk density and loss on ignition, we estimate that the mud present in marshes has an organic content of ~0.09, which remains remarkably constant among saline, brackish, and fresh marshes. The mud-associated OM constitutes ~35% of the total marsh OM. The presence of mud-associated OM explains why marsh vertical accretion correlates better with OM accumulation than with mineral accumulation. This framework also implies that mud contributes to ~42% of the marsh vertical accretion, higher than the ~14% previously estimated.