Search for: All records

Abstract Runnels, a climate adaptation technique that drains surface water to restore marsh vegetation and habitat, are increasingly being used to prevent the formation of shallow water impoundments or pannes in salt marshes that result in the loss of important ecosystem services. However, we know little about the effect of runnels on salt marsh biogeochemistry. This study measured how sediment characteristics and rates of nitrogen cycle processes were altered by impounded water and vegetation loss, and whether runnels can restore these marsh attributes to reference conditions. Impounded areas were 52 ± 4% less vegetated than nearby intact marsh, with 11 ± 2% less organic matter and 24 ± 5% higher bulk density. Additionally, impoundments removed 32 ± 32 µmol N m⁻²d⁻¹less than reference marsh areas via denitrification. At six of the 11 runneled sites, vegetation percent cover increased by 40 ± 5%, accompanied by a 7 ± 3% recovery of organic matter and a 9 ± 6% reduction of bulk density. At sites where vegetation recovered to within 70% of reference plots at a site, runneled plots removed 97 ± 31 µmol more N m⁻²d⁻¹than impoundments, which was also 82 ± 31 µmol more N m⁻²d⁻¹than reference areas. The driver of recovery is related to initial site conditions, including higher redox potentials and lower porewater salinities, compared with sites where revegetation was unsuccessful. The extent of runnel effectiveness and the recovery of vegetation, sediment characteristics, and nitrogen cycle processes was variable among runneled marshes, and the effectiveness of runnels may depend on initial site-specific characteristics and degree of initial degradation. 

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.