Long-term assimilation wetlands in coastal Louisiana: Review of monitoring data and management

Emerging Wetlands From River Diversions Can Sustain High Denitrification Rates in a Coastal Delta

“…Previous studies have attempted to establish a direct relationship between denitrification and wetland vegetation plant assemblages and/or individual plant species traits (e.g., Alldred & Baines, 2016) given the wetland environment characterized by low redox and lack of electron acceptors (Pan et al., 2019). Since denitrification is a complex biogeochemical transformation, there is a direct benefit in determining if specific plant assemblages can maximize denitrification, particularly when wetlands are either constructed (“treatment wetlands”; Liu et al., 2009) for secondary treatment of excess N or when used naturally as “assimilation” wetlands” (i.e., non‐constructed; Day et al., 2019). For example, a meta‐analysis study of 419 published denitrification rates in several wetland communities, including treatment wetlands, showed that denitrification rates in wetlands with plants of different species reported denitrification rates that on average was 55% higher (Alldred & Baines, 2016).…”

“…This peat collapse mechanism initiated by high denitrification respiration rates might theoretically occur in the Louisiana delta plain or other coastal regions, yet there is no direct experimental evidence. For instance, initial studies underscoring this process of OM consumption via denitrification have been based on the use of indirect techniques (e.g., soil slurries and DEA; Deegan et al., 2012) or stoichiometric calculations (Day et al., 2019; VanZomeren et al., 2012). In fact, recently published stoichiometric calculations to theoretically estimate how much soil OM decomposition could be accounted for by denitrification respiration suggest that only under extremely high N loading rates (i.e., 100 g m 2 yr −1 ) could denitrification account for significant soil OM decomposition (Day et al., 2018).…”

“…The apparent dichotomy about the positive impact of sediments for land building versus denitrification as an “ecosystem service” (N removal; Burgin, Hamilton, et al., 2013; Burgin, Lazar, et al., 2013; Sousa et al., 2012) versus a “ecosystem disservice” (Turner, 2011) (i.e., peat collapse; production of N 2 O as greenhouse gas) needs continuous assessment of the net water/sediment discharge from freshwater diversions under different environmental conditions and management scenarios, including the location of these diversions across the landscape (Day et al., 2019; Rivera‐Monroy et al., 2019; White et al., 2019). The high denitrification rates in coastal Louisiana deltaic lobes—especially in habitats when soil OM content is low (aggrading)—supports the recommendation of using denitrification respiration to ameliorate the increasing N loading rate into the adjacent coastal ocean that currently fuels the largest hypoxia zone in the Gulf of Mexico and without negatively affecting wetland sustainability in the long term.…”

“…Indeed, further studies are needed to evaluate the effect of the soil labile C concentrations in the observed linear relationship between denitrification and temperature; for example, by spiking different concentrations of labile C in sediments cores from benthic habitats characterized by the lowest OM value. Additionally, stoichiometric properties of both soil and organic material (Pina‐Ochoa & Alvarez‐Cobelas, 2006) and NO 3 − residence time associated with river discharge and tidal exchange (Day et al., 2019; Hiatt & Passalacqua, 2015; Rivera‐Monroy et al., 2010) should be closely considered when spatially scaling up the consequences of enhanced denitrification microbial respiration. Especially, when there are other dissimilatory pathways influencing N removal (e.g., anammox, sulfur‐driven nitrate reduction, iron‐driven denitrification) (Burgin & Hamilton, 2007; Pina‐Ochoa & Alvarez‐Cobelas, 2006).…”

“…Therefore, to implement and expand freshwater diversions as a wetland restoration strategy, it is critical to advance our understanding of how all environmental drivers (e.g., water level, elevation, temperature, vegetation presence/biomass) interact to influence nitrogen cycling processes collectively across different spatial scales. Most of the studies, for instance, assessing the N removal efficiency in wetlands in general (Rosenzweig et al., 2018), and in coastal Louisiana in particular (Day et al., 2019), show that even after long term operation (26–70 years), treatment wetlands continue to be nutrient sinks (Day et al., 2018). The direct attribution to denitrification is variable since most studies assessing wetland treatment efficiencies consider several processes (e.g., plant uptake, soil N accumulation), yet denitrification is one critical process that can explain significant N removal (Alldred & Baines, 2016).…”