Biogeochemistry of blue carbon in coastal wetlands under rising seas: combined laboratory and field experiments
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Blue carbon (C) stored in coastal wetland soils has recently been considered a natural climate change solution due to the high rate of soil C storage inherent in these ecosystems. However, coastal wetlands continue to be lost at a global rate of 1-2% each year and the specific lateral and vertical C fluxes in these ecosystems remain highly uncertain. In addition, sea level rise (SLR) adds another layer of complexity; SLR is expected to drastically alter soil chemistry and coastal ecosystem functions while also causing soil salinization and migration of marshes into upland forests and agricultural fields. To decrease uncertainty in salt marsh blue C dynamics under current and future climate scenarios, this dissertation investigated the spatial and temporal variability of lateral and vertical C flux, examined how SLR will alter these fluxes in tidal salt marshes, and further investigated how SLR induced marsh migration will alter the soil biogeochemistry of upland forests and farms. These studies comprised a variety of laboratory and field experiments to achieve several main objectives including to (1) determine the temporal and spatial variability of salt marsh soil C concentration and investigate soil biogeochemical mechanisms that explain this variability; (2) understand how SLR may alter C cycling and the lateral and vertical C fluxes; and (3) investigate how marsh migration will alter belowground biogeochemistry in forests and farms experiencing salinization due to SLR. Taken together, the results gathered throughout this dissertation improve our understanding of blue C cycling. The results were obtained from four separate experiments, organized here into four main chapters. ☐ The first experiment (Chapter 2) of this dissertation was designed to investigate the uncertainty surrounding salt marsh soil C concentration which causes wide ranges in soil storage rates (i.e., 18-1700 g C m-2 year-1). While the range in storage values for these ecosystems is large, the reasons for such large uncertainty remain largely unknown, particularly at the marsh scale. We investigated soil C concentration across temporal and spatial scales to better understand variables controlling soil C concentration across the marsh platform. We hypothesized that soil C would change spatially between different vegetative zones across the marsh, as well as temporally between plant phenology phases. We further hypothesized that soil biogeochemical variables would explain some of the variance in soil C concentration. We found significant (p<0.05) spatial and temporal differences in soil C concentration and found increasing soil C with increasing soil sulfur and sulfide. These results indicate the importance of considering several site level factors that cause uncertainty in blue C estimates. ☐ The second experiment (Chapter 3 of this dissertation) was designed to investigate the uncertainty in the salt marsh lateral C flux which is defined as the C being exported and imported via tidal channel surface water. The current estimated lateral flux in North America is 16±16 Tg C year-1, indicating an uncertainty level of 100% of the estimated value. This again calls for additional research to determine the reasons causing such a large range in flux values. Rather than focusing on the lateral movement of C between tidal channels and adjacent estuaries like most studies have done, we took a different approach and examined the smaller scale horizonal exchange of C between the marsh platform and the tidal creek. We hypothesized that Fe oxides stabilize dissolved organic carbon (DOC) during ebb tide when soils are oxic, but release DOC into the porewater during flood tide when the soils are sub to anoxic. We further hypothesized that the hydraulic gradient between the marsh platform groundwater and the tidal creek surface water drives changes in the concentration and source of C being exported or imported out of/ into the marsh. Our final hypothesis was that soil trace gases are physically pushed up and out of the creek bank soil with rising tides. We found evidence for a mechanism in which carbon-bearing Fe oxides reductively dissolve during flood tide and precipitate as soils oxidize during ebb tide, causing DOC mobility to change based on tidal cycle. We also found that the hydraulic gradient alters both the concentration and source of C in the tidal creek. Finally, we found that trace gas fluxes move horizontally out of the creek bank as the tide rises. The results indicate several physiochemical mechanisms at the tidal creek-marsh platform interface that may be causing uncertainty in the lateral C flux. ☐ The third experiment (Chapter 4 of this dissertation) was designed to investigate how sea level rise (SLR) will alter the dynamics and fluxes discussed in the first two chapters. This was achieved by conducting a laboratory-controlled mesocosm experiment. We hypothesized that Fe oxides would increasingly dissolve under SLR conditions, causing DOC to be more mobile and be more readily fluxed laterally. We further hypothesized that mineral-associated DOC released under SLR conditions would be more easily accessed by soil microbes, thereby increasing respiration and the soil gas flux (vertical flux) from the soils. We found that Fe oxides increasingly dissolved under SLR conditions, releasing DOC into the porewater, and increasing the lateral C flux. We however did not see a significant increase in soil gas flux under SLR conditions as we hypothesized. We observed the opposite; a decreased soil gas flux under SLR conditions, likely due to decreased oxygen levels under flooded conditions and decreased respiration rates. These results indicate that SLR may decrease the stability of mineral-associated C, thereby increasing the lateral flux, but decrease the soil respiration rate, thereby decreasing the vertical flux of soil greenhouse gases. ☐ The fourth study (Chapter 5 of this dissertation) investigated how the soil biogeochemistry changes along transects from coastal marshes to upland farms or forests in the Delmarva peninsula, which is at the forefront of SLR. We hypothesized that carbon concentrations would increase while soil redox potential would decrease following the salinity gradient from upland sites to the marshes. There was more soil C found in the marsh than all upland sites, supporting our hypothesis. In addition, we observed a biogeochemical redox gradient that followed changes in salinity from upland to lowland, where sulfate was being reduced in the marsh, Fe was being reduced in the transitional areas, and oxygen was likely being reduced in the upland areas. These results highlight biogeochemical gradients over short distances from marshes to upland endmembers, some of which are already experiencing salinization and more reducing conditions with SLR, with consequent impacts on ecosystem function. ☐ Taken together, our results help improve our understanding of coastal wetland blue C cycling under both current and future climate scenarios. These findings will be used to improve ecosystem vertical and lateral C flux estimates, better understand the underlying mechanisms controlling variability in C flux and C cycling and improve our understanding of how these ecosystems will function under increased inundation with SLR. These results can be further incorporated into Earth System Models (ESMs) to increase the predictive capacity of C feedback cycles and may also be used to improve policy aimed at protecting and restoring these critical natural ecosystems.
Description
Keywords
Natural ecosystems, Blue carbon, Iron oxide, Salt marsh, Soil carbon, Biogeochemical mechanisms