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Salt marshes are hot spots for nitrogen (N) cycling due to complex and dynamic redox gradients. Soils of salt marshes contain a wide diversity of microbes capable of carrying out steps in the N cycle. Recent discoveries demonstrate that chemolithotrophic (e.g., sulfide oxidation coupled with N 2 fixation), as well as heterotrophic metabolisms, are important for N cycling in salt marshes. N removal via denitrification is an important ecosystem service provided by salt marshes. N fixation and N retention due to dissimilatory nitrate reduction to ammonium (DNRA) are more important than previously thought and may contribute to salt marsh productivity. 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. [ABSTRACT FROM AUTHOR]

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 δ13C 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. • Respiration rates immediately following and between carbon pulses increased over time. • Respiratory responses to carbon pulses and starvation were greater under anoxia. • Non-respiratory oxidation of reduced compounds obscured oxygen availability effects. • Carbon mineralization may be more sensitive to oxygen than root exudates in wetlands. [ABSTRACT FROM AUTHOR]

Salt marshes, due to their capability to bury soil carbon (C), are potentially important regional C sinks. Efforts to restore tidal flow to former salt marshes have increased in recent decades in New England (USA), as well as in some other parts of the world. In this study, we investigated plant biomass and carbon dioxide (CO 2) fluxes at four sites where restoration of tidal flow occurred five to ten years prior to the study. Site elevation, aboveground biomass, CO 2 flux, and porewater chemistry were measured in 2015 and 2016 in both restored marshes and adjacent marshes where tidal flow had never been restricted. We found that the elevation in restored marsh sites was 2–16 cm lower than their natural references. Restored marshes where porewater chemistry was similar to the natural reference had greater plant aboveground biomass, gross ecosystem production, ecosystem respiration, as well as net ecosystem CO 2 exchange (NEE) than the natural reference, even though they had the same plant species. We also compared respiration rates in aboveground biomass (AR) and soil (BR) during July 2016, and found that restored marshes had higher AR and BR fluxes than natural references. Our findings indicated that well-restored salt marshes can result in greater plant biomass and NEP, which has the potential to enhance rates of C sequestration at 10-yrs post restoration. Those differences were likely due to lower elevation and greater flooding frequency in the recently restored marshes than the natural marsh. The inverse relationship between elevation and productivity further suggests that, where sea-level rise rate does not surpass the threshold of plant survival, the restoration of these salt marshes may lead to enhanced organic and mineral sedimentation, extending marsh survival under increased sea level, and recouping carbon stocks that were lost during decades of tidal restriction. The photos showing the tidal channel before (A) and after restoration (B) at Bass creek, Barnstable, MA, USA. Our study indicated that successful restoration of salt marshes leads to greater rates of C sequestration for a decade, at minimum. Moreover, the negative relationship between elevation and plant productivity suggested that sea level rise may lead to enhanced sedimentation, extending marsh survival under the increased sea level, and recouping carbon stocks that were lost during tidal restriction periods. Unlabelled Image • Coastal wetlands are important regional C sinks. • The plant biomass and CO 2 fluxes was compared in restored marsh and reference. • The recently restored salt marsh had higher plant aboveground biomass and NEE than the natural marsh. • Negative feedbacks between plant productivity and sea levels rise help salt marsh survival. [ABSTRACT FROM AUTHOR]