Session 3A: Soils in Transition
June 12
10:15 - 11:45 am
Choptank Ballroom
Hydrologic and chemical context of salinization alters greenhouse gas emissions from coastal wetland soils
Ashley Helton
University of Connecticut
Changes in sea-level rise and precipitation regimes are altering patterns of coastal wetland inundation and salinization. At the same time, upstream urban and agricultural land practices increase nutrient and contaminant loads to coastal wetland ecosystems. We conducted series of laboratory and field experiments designed to disentangle various aspects of seawater intrusion (salinity and sulfate), pollution loading (copper and nitrogen), and hydrology (permanent and intermittent flooding) on greenhouse gas (GHG; CO 2 , CH 4 , and N 2 O) emissions from freshwater coastal wetland soils. In response to common urban contaminants, we found elevated copper reduced CH 4 emissions while elevated nitrate increased CH 4 emissions, but neither affected CO 2 or N 2 O emissions. In response to elevated marine salts, we found that the magnitude, and even the direction, of GHG responses depended on the hydrologic context in which marine salt exposure occurred. Under permanent flooding, sulfate and salt ions suppressed carbon emissions across experiments. Under intermittent flooding, elevated salinity reduced carbon mineralization and CO 2 fluxes, but enhanced CH 4 emissions relative to both controls and treatments with elevated sulfate. Nitrous oxide (N 2 O) fluxes had contrasting responses in lab and field experiments. In the lab, N 2 O emissions increased with elevated marine salts, and were directly related to porewater ammonium concentrations, which increased in salinity treatments via cation exchange. In intermittently flooded field conditions, elevated salinity strongly suppressed N 2 O fluxes because ammonium did not accumulate in porewater; it was likely lost through advection, dispersion, or plant uptake. Understanding dynamic hydrologic, chemical, and vegetation patterns across wetland landscapes will be critical for predicting both the magnitude and direction of wetland soil carbon GHG responses to increasing seawater intrusion across broad spatial scales.
Effects of saltwater intrusion on tidal marsh soil metabolism: Comparing short-term vs. decadal-scale responses
Scott Neubauer
Virginia Commonwealth University
Saltwater intrusion into tidal freshwater wetlands alters the soil physicochemical environment, impacts microbial and plant communities, and causes cascading effects on important ecosystem processes including primary production and carbon sequestration. Here, I describe tidal marsh responses to saltwater intrusion in the Pamunkey River, Virginia, USA over multiple time scales using data from an in situ salinity manipulation experiment (responses over months to years) and space-for-time sampling along the estuarine salinity gradient (responses over decades and longer). In each approach, rates of soil respiration and methanogenesis (anaerobic CO 2 and CH 4 production, respectively) were measured in the root zone and in deeper soils. The impacts of salinization on methane production were consistent across both sampling approaches: even modest amounts of salinization reduced rates of methanogenesis by an order of magnitude. In contrast, soil respiration was not significantly affected by salinization during any of the seven sampling events that spanned the first three years of the in situ experiment. Although rates of soil respiration along the estuarine gradient were generally greatest at the high salinity site, rates did not vary systematically with salinity, suggesting that metabolic rates at the different marsh sites were influenced by differences in soil properties and plant community composition as well as differing porewater chemistries. These results suggest that a persistent environmental stressor like saltwater intrusion can initiate long-term ecosystem transitions in biotic communities and physicochemical properties, with significant effects on soil processes (e.g., respiration) that are not necessarily seen over short time scales.
Carbon (de)stabilization in tidal salt marshes and nearby uplands prone to climate change
Angelia Seyfferth
University of Delaware
Tidal salt marshes are considered important reservoirs of soil carbon, storing more carbon per land area than terrestrial environments. This increased rate of storage is due to the slow oxidation of carbon compounds in wetland environments; however, less is known about the stability of this carbon pool. Tidal salt marshes vary in space and time as a result of linked hydrologic and biogeochemical drivers, which influences carbon stability. Whereas some marsh areas that are nearly always saturated may lead to more carbon storage, other areas of the marsh that experience extreme tidal oscillations such as near-channel soils may be prone to more carbon destabilization as a result of mineral control on C storage. Moreover, these hydrologic drivers affect the dominant vegetation across the marsh platform with smaller plants in saturated areas and larger plants in tidally dynamic areas, with consequent differences in root exudation and thus carbon supply. Finally, sea-level rise will likely alter the controlling processes on C storage in tidal salt marsh soils in the future. We will discuss the dynamic controls on carbon cycling in salt marsh soils using data obtained from combined laboratory and field investigations of a Mid-Atlantic mesohaline marsh. We conducted laboratory incubations of marsh soils subjected to tidal oscillations (control) and simulated sea-level rise (treatment) and monitored the fate of carbon and mineral control on C release. At the field scale, we investigated the spatial and temporal gradients of C concentrations, fluxes, and production of greenhouse gases across the marsh platform. We found that soil carbon and fluxes of greenhouse gases near tidal channels are dynamic in space and time. In these soils, sea-level rise simulations and flood tides destabilize C-Fe oxide assemblages. Moreover, phenophases contribute to the dynamic nature of C cycling in these soils. In contrast, soil C is more stable in the marsh interior due to the slow oxidation of soil C under more reducing conditions. Moreover, we observed high levels of methane production that coincided with sulfate reduction, which calls into question the current paradigm of sulfate reduction inhibiting methane production in these important ecosystems. In addition, marsh migration into upland areas is causing these biogeochemical changes to occur upland, and will be discussed.
Restoration species and soil dynamics on salt-intruded lands
Alison Schulenburg & Patricia R. de Barros
University of Maryland
As saltwater intrusion (SWI) advances across the landscape, it will alter soil salinity and phosphorus (P) concentrations, thus altering biogeochemical cycling. Across two studies, we were able to better understand the trajectories of different land management practices for farmers experiencing SWI. In our first study, we examined the ability of three land management practices (e.g., remediate, restore, abandon) to reduce P levels in soils. We determined that agricultural fields with high P levels should utilize a remediation approach (e.g., plant and harvest Panicum virgatum) to remove P from the system, and fields with high salinity should choose a restoration approach (e.g., plant Spartina patens) to transition their field to marsh. In our second study, we explored more species in the restoration approach (e.g., Paspalum floridanum, Panicum amarum, Spartina pectinata, and Tripsacum dachtylodies). This study focused on the ion concentration dynamics in soil and plant leaf tissue (e.g. potassium [K], sodium [Na], and sulfur [S]). We found that electrical conductivity (EC), Na, and S increased with depth in the soil profile as well as over time, implying that SWI is advancing through the groundwater below. In Spartina species leaf tissue, Na was highest compared to other species, while K was lowest, despite the increased Na in the soil over time. We believe this is due to the species ability to co-regulate cations. In sum, restoring farm fields to native marshes could benefit nearby waterbodies by reducing soil P levels and communities by protecting against coastal flooding.