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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]

Salt marsh survival with sea‐level rise (SLR) increasingly relies on soil organic carbon (SOC) accumulation and preservation. Using a novel combination of geochemical approaches, we characterized fine SOC (≤1 mm) supporting marsh elevation maintenance. Overlaying thermal reactivity, source (δ13C), and age (F14C) information demonstrates several processes contributing to soil development: marsh grass production, redeposition of eroded material, and microbial reworking. Redeposition of old carbon, likely from creekbanks, represented ∼9%–17% of shallow SOC (≤26 cm). Soils stored marsh grass‐derived compounds with a range of reactivities that were reworked over centuries‐to‐millennia. Decomposition decreases SOC thermal reactivity throughout the soil column while the decades‐long disturbance of ponding accelerated this shift in surface horizons. Empirically derived estimates of SOC turnover based on geochemical composition spanned a wide range (640–9,951 years) and have the potential to inform predictions of marsh ecosystem evolution. Plain Language Summary: Salt marsh survival with rising sea levels increasingly depends on the accumulation and preservation of buried organic carbon. Marsh soil organic carbon development reflects at least three processes: burial of differently reactive compounds that derive from local grasses, redeposition of old carbon (9%–17%), and microbial reworking. Decomposition results in a progressive decrease in the thermal reactivity of soil organic carbon, and disturbances such as ponding can accelerate this shift. Modeled rates of geochemically defined soil organic carbon pools turnover on the order of centuries‐to‐millennia and can refine predictions of salt marsh sustainability. Key Points: Salt marsh soils preserved compounds with a range of reactivities that were derived from local grasses9%–17% of surface soil carbon was old, likely reflecting redeposition of eroded creekbank materialDecomposition decreases soil organic carbon thermal reactivity, and disturbances accelerate this shift, particularly in surface horizons [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]