Nitrate reduction capacity is limited by belowground plant recovery in a 32‐year‐old created salt marsh

Abstract

Human activities have decreased global salt marsh surface area with a subsequent loss in the ecosystem functions they provide. The creation of marshes in terrestrial systems has been used to mitigate this loss in marsh cover. Although these constructed marshes may rapidly recover ecosystem structure, biogeochemical processes may be slow to recover. We compared denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates between a 32‐year‐old excavation‐created salt marsh (CON‐2) and a nearby natural reference salt marsh (NAT) to assess the recovery of ecosystem function. These process rates were measured at 5 cm increments to a depth of 25 cm to assess how plant rooting depth and organic matter accumulation impact N‐cycling. We found that, for both marshes, denitrification and DNRA declined with depth with the highest rates occurring in the top 10 cm. In both systems, N‐retention by DNRA accounted for upwards of 75% of nitrate reduction, but denitrification and DNRA rates were nearly 2× and 3× higher in NAT than CON‐2, respectively. Organic matter was 6× lower in CON‐2, likely due to limited plant belowground biomass production. However, there was no response to glucose additions, suggesting that the microbial functional community, not substrate limitation, limited nitrate reduction recovery. Response ratios showed that denitrification in CON‐2 recovered in surficial sediments where belowground biomass was highest, even though biomass recovery was minimal. This indicates that although recovery of ecosystem function was constrained, it occurred on a faster trajectory than that of ecosystem structure.

Ecosystem carbon exchange and nitrogen removal rates in two 33‐year‐old constructed salt marshes are similar to those in a nearby natural marsh

“…Although we did not directly link plant activity to N cycling, previous studies have shown tight coupling between rhizosphere activity and biogeochemical processes (Lynch & Whipps 1990; Philippot et al 2009; Koop‐Jakobsen & Giblin 2010), supporting the idea that the recovery of N‐cycling in constructed marshes may be heavily dependent on primary productivity. Further, while this work and that of Tatariw et al (2021) support the notion of structure‐function coupling (Lovell 2005; Alldred & Baines 2016) in these ecosystems, functional recovery at the ecosystem scale is dependent on widespread structural recovery.…”

“…Our study shows that where ecosystem structure (i.e., plant biomass) has recovered in constructed marshes, N-removal and CO 2 exchange are comparable to natural marshes. Our results also suggest that biogeochemical functions are tightly coupled to the recovery of plant biomass, which has not occurred consistently across the constructed marshes (Tatariw et al 2021). Despite this spatial variability in structural and functional recovery, this study shows that constructed marshes may have comparable biogeochemical function to natural marshes where vegetation has recovered only decades after construction.…”

“…CO 2 fluxes and biomass) was equivalent between constructed and natural marshes, and not %OM, suggests that plant activity plays a more significant role in supporting N-cycling at this stage of recovery. This is further supported by a concomitant study conducted in CON-2, which found that NO 3 À reduction processes had not recovered in areas where belowground biomass had not reached equivalency with the natural marsh (Tatariw et al 2021). Although we did not directly link plant activity to N cycling, previous studies have shown tight coupling between rhizosphere Average annual ecosystem respiration (ER CO2 ) versus average annual gross ecosystem production (GEP) from various marshes around Mobile Bay.…”

Enhanced susceptibility to oiling may limit denitrification recovery in marshes subjected to woody encroachment

“…Triplicate samples were collected from each plot for nitrate reduction (i.e., denitrification, anammox, and DNRA) potential assays as described in Tatariw et al (2021). Two cores (0-5 cm, i.d.…”

“…Following the 2010 Deepwater Horizon (DWH) oil spill, ecosystem recovery in mixed mangrove-marsh systems was dominated by marsh vegetation (Lin and Mendelssohn, 2012;Shapiro et al, 2016), suggesting enhanced oil sensitivity in mangroves. Although oiling has a neutral (Kleinhuizen et al, 2017) or even positive (Levine et al, 2017a) effect on N removal in marsh sediments, it can lead to long-term losses in marsh N removal capacity (Hinshaw et al, 2017;Tatariw et al, 2021), highlighting a need to understand potential differences in oiling response in marshes and mangroves.…”