The recovery of denitrification potential after thin layer sediment placement: Does sediment composition influence functional recovery?
Date
2024
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Publisher
University of Delaware
Abstract
Sediment thin layer placement (TLP) applications are used to mitigate losses in coastal wetland cover caused by anthropogenic and climate-related pressures. Although biophysical characteristics such as vegetation and hydrology can recover quickly after sediment placements, biogeochemical processes like denitrification may take much longer to reach equivalence with natural marsh systems. As a result, the use of biochar, a C-rich material formed from the pyrolysis of organic matter, as a sediment amendment to improve functional recovery within placed sediments has gained recent interest. To understand how sediment N-removal processes recover after sediment placement restorations, I conducted a 1.5-year manipulative field study which implemented small scale TLPs within a Delaware tidal marsh. Two sediment treatments were applied within 1m2 wooden frames overtop of the marsh surface: (1) unamended sediments collected from a nearby tidal creek and (2) biochar amended sediments which consisted of 10% (v/v) P. australis biochar and creek sediments. These sediment treatments were applied to a native S. alterniflora zone and adjacent invasive P. australis zone and subsequently sampled seasonally beginning 1 year after sediment placement. Sediment cores and porewater samples were taken across the growing season to measure denitrification potentials, sediment and porewater nutrients, and functional gene abundances (nirS and nirK). ☐ The recovery of sediment denitrification potential was influenced primarily by vegetation recovery after sediment placement rather than by the presence or absence of biochar in the sediment applied. Overall, denitrification potentials were greatest in the S. alterniflora zone compared to the adjacent P. australis zone. Within each zone, denitrification potentials were equivalent between biochar amended and unamended sediment treatments. When compared against denitrification potentials from nearby undisturbed reference sediments, sediment treatments within the S. alterniflora zone possessed comparable denitrification potentials by the end of the experiment. In contrast, denitrification potentials within the P. australis sediment treatments lagged significantly behind those in reference sediments. Differences in vegetation recovery between the two vegetation zones after sediment placement likely explain the lagging denitrification potentials in the P. australis plots as S. alterniflora reestablished much quicker (after 16 months) than P. australis, highlighting the importance of plant-soil interactions which strongly influence sediment N removal processes. ☐ Biochar amendments did not improve denitrification potentials in either vegetation zone possibly because the influence of biochar was masked by the greater influence of vegetation establishment. The P. australis biochar used in this study may not have possessed suitable physicochemical properties capable of enhancing denitrification as seen in other studies. It is possible that P. australis biochar (or a biochar of a different feedstock) produced under optimal conditions may have provided the enhanced N removal ecosystem service, however, in this study, both biochar amended and unamended sediments possessed equivalent denitrification potentials to the reference S. alterniflora marsh. ☐ This thesis demonstrates that vegetation establishment after TLP is a significant factor controlling the return of marsh ecosystem functional capacity. While biochar has been utilized in a variety of environments to improve N-removal processes, its application in our TLP experiments was unproductive. Additional studies are needed to confirm biochar’s influence on sediment biogeochemical processes in tidal marshes. For example, biochar production conditions and parent feedstock material have a significant influence on the resulting structure and thus function of biochar in the environment. The influence of these parameters should be evaluated in future laboratory and field studies to identify what biochar properties are most beneficial for promoting denitrification. Spatial variability must also be constrained as the results of this study are most applicable for the Mid-Atlantic region and for biochar amendments within thin (~5cm) sediment placements. Optimal dosages for biochar amendments must also be developed as only one dose (10% v/v) was assessed in the present study. While this thesis provides tidal marsh managers with information that they may leverage to restore N-removal services within their marsh systems, much work is still needed to understand if and how biochar can be utilized effectively within tidal marsh restorations.
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Keywords
Denitrification, Restorations, Salt Marsh, Thin layer placement, Belowground biomass