Salt marshes are areas of intense biogeochemical cycling driven by high rates of primary production. The interactions between macroorganisms and marsh sediment biogeochemistry are complex, and much is still poorly described because of the great temporal and spatial diversity of processes. Understanding biogeochemical processes in salt marshes will help elucidate their role as essential habitats. The objective of my research was to elucidate the influence of macrophyte, benthic infauna and inundation on the biogeochemistry and pathways of organic matter mineralization in salt marsh sediments. Sulfate reduction and Fe(III) reduction processes were focused on since sulfate and Fe(III) minerals are the most abundant electron acceptors available for microbial respiration in salt marsh sediments. Furthermore, the use of commonly applied sampling techniques was evaluated. Studies were conducted in NW European Spartina anglica salt marshes, a North American S. alterniflora marsh and in experimental mesocosms.

Mineralization rate

Spartina avoid the toxic effects of waterlogging by creating an oxygenated zone around the root. Root oxygen loss was observed qualitatively as root iron coatings and high redox potentials deep in the sediment, and measured quantitatively using microelectrodes. Spartina further affect the salt marsh sediment by a large input of refractory as well as newly produced, labile organic matter directly into the sediment, resulting in enhanced (up to 7 times) rates of total benthic mineralization compared to unvegetated sediment. Decomposition is further enhanced by anaerobic cometabolism of the more refractory DOC pool with labile DOC leached from roots.

Mineralization pathways

Salt marsh sediments are rich in Fe, and reoxidation of Fe(II) through the infusion of oxygen by Spartina roots and bioturbating infauna (Uca pugnax and Nereis diversicolor) supply reactive Fe(III) for reduction processes. In sediment directly affected by macroorganisms dissimilatory Fe(III) reduction outcompete sulfate reduction as the most important anaerobic respiration pathway. Sulfate reduction only dominates in the most inundated low marsh and in sediments unaffected by macroorganisms. The relative contribution of microbial Fe(III) reduction to total mineralization rates is governed by the availability of Fe(III), and the rates measured in this study are the highest dissimilatory Fe(III) reduction rates measured in marine sediments.

Sampling methods in vegetated sediment

Application of the commonly used 35S core injection technique for sulfate reduction measurements in vegetated sediments involves cutting of roots and rhizomes when coring. This causes instant leakage of labile dissolved organic matter (DOC) from the roots, which is rapidly utilized by sulfate reducing bacteria, giving rise to enhanced rates of sulfate reduction. Cores collected in vegetated salt marsh sediments should not be used for sulfate reduction measurements before a pre-incubation period. DOC is further leached into the porewater during centrifugation or sediment squeezing rendering these porewater collecting techniques undesirable in vegetated sediments. Instead in situ sippers for extraction of porewater for analysis of dissolved organic or plant-derived constituents is recommended, while high resolution porewater samples for the analysis of redox sensitive elements can be obtained by a newly developed, segmented gel probe.

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