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Abstract Salt marsh ponds expand and deepen over time, potentially reducing ecosystem carbon storage and resilience. The water filled volumes of ponds represent missing carbon due to prevented soil accumulation and removal by erosion and decomposition. Removal mechanisms have different implications as eroded carbon can be redistributed while decomposition results in loss. We constrained ponding effects on carbon dynamics in a New England marsh and determined whether expansion and deepening impact nearby soils by conducting geochemical characterizations of cores from three ponds and surrounding high marshes and models of wind‐driven erosion. Radioisotope profiles demonstrate that ponds are not depositional environments and that contemporaneous marsh accretion represents prevented accumulation accounting for 32%–42% of the missing carbon. Erosion accounted for 0%–38% and was bracketed using radioisotope inventories and wind‐driven resuspension models. Decomposition, calculated by difference, removes 22%–68%, and when normalized over pond lifespans, produces rates that agree with previous metabolism measurements. Pond surface soils contain new contributions from submerged primary producers and evidence of microbial alteration of underlying peat, as higher levels of detrital biomarkers and thermal stability indices, compared to the marsh. Below pond surface horizons, soil properties and organic matter composition were similar to the marsh, indicating that ponding effects are shallow. Soil bulk density, elemental content, and accretion rates were similar between marsh sites but different from ponds, suggesting that lateral effects are spatially confined. Consequently, ponds negatively impact ecosystem carbon storage but at current densities are not causing pervasive degradation of marshes in this system. 

Abstract. Environmental gradients can affect organic matter decay within and across wetlands and contribute to spatial heterogeneity in soil carbon stocks. We tested the sensitivity of decay rates to tidal flooding and soil depth in a minerogenic salt marsh using the tea bag index (TBI). Tea bags were buried at 10- and 50- cm along transects sited at lower, middle, and higher elevations that paralleled a headward eroding tidal creek. Plant and animal communities and soil properties were characterized once while replicate tea bags and porewaters were collected several times over one year. TBI decay rates were faster than prior litterbag studies in the same marsh, largely due to rapid green tea loss. Rooibos decay rates were comparable to natural marsh litter, potentially suggesting that is more useful as a standardized organic matter proxy than green tea. Decay was slowest at higher marsh elevations and not consistently related to other biotic (e.g., plants, crab burrows) and abiotic factors (e.g., porewater chemistry), indicating that local hydrology strongly affects organic matter loss rates. Tea BI rates were 32–118 % faster in the 10 cm horizon compared to 50 cm. Rates were fastest in the first three months and slowed 54–60 % at both depths between 3- and 6- months. Rates slowed further between 6- and 12- months but this was less dramatic at 10 cm (17 %) compared to 50 cm (50 %). Slower rates at depth and with time were unlikely due to the TBI stabilization factor, which was similar across depths and decreased from 6 to 12 months. Slower decay at 50 cm demonstrates that rates were constrained by the environmental conditions of this deeper horizon rather than the molecular composition of litter. Overall, these patterns suggest that hydrologic setting, which affects oxidant introduction and reactant removal and is often overlooked in marsh decomposition studies, may be a particularly important control on organic matter decay in the short term (3–12 months). transects sited at lower, middle, and higher elevations that paralleled a headward eroding tidal creek. 

Environmental gradients can affect organic matter decay within and across wetlands and contribute to spatial heterogeneity in soil carbon stocks. We tested the sensitivity of decay rates to tidal flooding and soil depth in a minerogenic salt marsh using the tea bag index (TBI). Tea bags were buried at 10- and 50- cm along transects sited at lower, middle, and higher elevations that paralleled a headward eroding tidal creek. Plant and animal communities and soil properties were characterized once while replicate tea bags and porewaters were collected 3 and 4 times respectively over one year. 

Roots of salt marsh grasses contribute to soil building but also affect decomposition by releasing bioavailable carbon exudates and oxygen. Disentangling exudate and oxygen effects on decomposition is difficult in the field but essential for marsh carbon models and predicting the impacts of global change disturbances. We tested how pulsed, simulated exudates affect soil metabolism under oxic and anoxic conditions, and whether carbon and oxygen availability facilitate mineralization of existing organic matter (i.e., priming). We conducted a laboratory experiment in flow-through reactors, adding carbon pulses weekly for 84 days and then following starvation under low carbon conditions. Oxygen consumption and sulfide production were inhibited under anoxic and oxic conditions and slowed by 21±10% and 55±8%, respectively, between 1- and 5- days following exudate pulses. Respiration rates immediately following and between pulses increased over time, suggesting that microbes capitalize on and may acclimate to patchy resources. Starvation caused oxygen consumption and sulfide production to fall 28% and 78% in oxic and anoxic treatments. Smaller decreases in oxygen consumption following pulses could suggest greater access to secondary carbon sources and that sulfate reducers were more reliant on exudates. Soil organic carbon was not the likely secondary source because porewater dissolved inorganic carbon 13C values did not change during transit through the reactors, despite a ~26‰ difference between the supplied seawater and marsh soil. Interpretation of oxygen consumption rates is complicated by non-respiratory oxidation of reduced inorganic compounds and possibly significant lithoautotrophy. Exudate pulses elicited rapid and ephemeral respiratory responses, particularly under anoxia, but non-respiratory oxidation of reduced compounds obscured the impact of oxygen availability in our experimental system. Despite this, greater aerobic respiration rates suggest that oxygen availability has more potential to regulate carbon mineralization in coastal wetlands than root exudates. 

Salt marshes sit at the terrestrial–aquatic interface of oceans around the world. Unique features of salt marshes that differentiate them from their upland or offshore counterparts include high rates of primary production from vascular plants and saturated saline soils that lead to sharp redox gradients and a diversity of electron acceptors and donors. Moreover, the dynamic nature of root oxygen loss and tidal forcing leads to unique biogeochemical conditions that promote nitrogen cycling. Here, we highlight recent advances in our understanding of key nitrogen cycling processes in salt marshes and discuss areas where additional research is needed to better predict how salt marsh N cycling will respond to future environmental change.