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A Mechanistic, Numerical Model of Sedimentation, Mineralization, and Decomposition for Marsh Sediments†
Contribution from The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543 and Dep. of Biology, Univ. of South Carolina, Columbia, SC 29208.
This paper describes a computer model that relates measured nutrient profiles to organic matter diagenesis in marsh sediments. Processes included in the model are sedimentation of exogenous and endogenous organic and inorganic matter, decomposition, above- and belowground biomass and production, and N and P mineralization. The shape of organic nutrient profiles in marsh sediments is controlled by the relative rates of nutrient mineralization and inputs of organic and inorganic matter. Sediment inputs were specified for a freshwater, tidal marsh on the North River, Massachusetts, and the model was solved for nutrient mineralization rates by fitting the model to observed nutrient profiles. Model calculations of net annual N and P export from a 1-m sediment column, 9.6 to 17.5 g N m−2 yr−1 and 1.19 to 2.02 g P m−2 yr−1, were sensitive to small changes in estimates of belowground production and the fraction of organic material that is refractory, constants in the model that are poorly known for marsh ecosystems. These export rates are about 55 to 78% and 71 to 85% of the annual sediment inputs of organic N and P, respectively. The fate of nutrients exported from the sediment was not specified, but it could include plant uptake, denitrification, and net loss to the adjacent tidal river. The model demonstrates that mature marshes with deep sediments recycle proportionally more of the nutrients required by primary producers than young marshes with shallow sediments. Model calculations indicated that the greatest changes in C, N, and P occur in the upper 2 cm of sediment. Labile organic C decomposed most rapidly which caused the relative concentrations of organic N and P to increase. Below this thin surface zone C loss became negligibly small, but there was a gradual decline in concentrations of organic N and P as they continued to mineralize slowly within the upper 60 cm of sediment. Apparently, only refractory C, N, and P compounds remained below the 60-cm depth. The predicted ratio of N/P loss from sediment ranged from 8.2 N/1 P to 9.55 N/1 P (g/g) and was constant over depth; this compares to a ratio of about 11 N/1 P in litter of the dominant macrophytes. This model is sufficiently general as to be useful in studies of sediment formation and nutrient cycling in other systems.