The Spatial Variability of Organic Matter and Decomposition Processes at the Marsh Scale

Accretion rate in salt marshes is governed by inorganic soil deposition and soil organic matter (SOM) accumulation. Existing (limited) observations and modeling results suggest that SOM amounts, biomass production, and decomposition processes should vary widely and systematically at the marsh scale. However, we lack observations aimed at understanding how SOM production is modulated spatially within a marsh, and at elucidating the relative importance of the controlling processes. The little existing data suggest that competing effects between biomass production and decomposition processes determine an approximately spatially constant contribution of SOM to total accretion. Here we investigate this idea using concurrent observations of SOM and decomposition rates from marshes in North Carolina. Our results indicate that systematic spatial variations in SOM are small, possibly as a result of an at least partial compensation of opposing trends in biomass production and decomposed organic matter. Our analyses show that deeper soil layers are, on average, characterized by lower decomposition rates and higher stabilization factors than shallower layers, likely because of differences in the persistence of water‐logged conditions. Overall, decomposition processes are sufficiently rapid that the labile material in the fresh biomass is completely decomposed before it can be sufficiently buried and stabilized. Our findings point to the importance of the fraction of initially refractory material and of stabilization processes in determining the final distribution of SOM within the soil column. Key Points Soil organic matter decomposition rate and decomposable fraction tend to decrease with depth below the marsh surface Observed decomposition rates indicate that labile fraction is fully decomposed before it is buried sufficiently deep to become stable The fraction of refractory organic matter and the stabilization of labile material are main determinants of soil organic matter content