Metal accumulation in salt marsh soils along the East Coast of the United States.

Coastal salt marshes are depositional environments that can accumulate pollutants introduced to the environment from human activities. Metals are a contaminant of concern in coastal environments due to their longevity and toxicity. We assessed metal concentrations and accumulation rates in nine salt marsh sites along the U.S. East Coast from Maine to Georgia. Following a metal mobility assay in organic-rich and mineral dominated salt marsh soils under aerobic/anaerobic and freshwater/saltwater conditions, we focused on profiles of chromium, nickel, copper, zinc, cadmium, lead, and uranium in two soil cores from each of the nine marshes that had previously been dated using lead-210 radioisotope techniques. We examined how land cover and the spatial distribution of land cover, marsh vertical accretion, and other watershed characteristics correlated with metal concentrations and depth/time-integrated accumulation of metals. We found statistically significant differences in metal concentrations and/or inventories between sites, with accumulation of metals positively correlated with both developed land cover in the watershed and rates of vertical accretion in the tidal marsh. The accumulation of chromium, cadmium, and lead were significantly correlated with developed land cover while the accumulation of chromium, nickel, copper, zinc, and lead were correlated with factors that determine sediment delivery from the landscape (e.g., riverine suspended sediment, soil erodibility in the watershed, and agricultural land cover skewed towards the coast) and measured wetland accretion rates. We observed declines in the concentration of many metals since 1925 at sites along the U.S. East Coast, indicating pollution mitigation strategies have succeeded in reducing metal pollution and delivery to the coastal zone. However, increasing rates of salt marsh vertical accretion over recent decades largely offset reductions in metal concentrations, resulting in rates of metal accumulation in coastal salt marsh soils that have not changed or, in some instances, increased over time.

Urban-Soil Pedogenesis Drives Contrasting Legacies of Lead from Paint and Gasoline in City Soil.

This study analyzed the impact of urban-soil pedogenesis on soil lead (Pb) contamination from paint and gasoline in the historic core of Durham, North Carolina. Total soil Pb in 1000 samples from streetsides, residential properties, and residual upland and floodplains ranged from 6 to 8825 mg/kg (mean = 211 mg/kg), with 50% of samples between 50 and 200 mg/kg soil Pb. The highest Pb concentrations were within 1 m of pre-1978 residential foundations, with concentrations inversely correlated with house age. Streetside soil Pb concentrations were elevated over the geologic background of <30 mg/kg and correlated with traffic flow. Streetside soil Pb concentrations were lower than Durham streetside soils collected in the 1970s, which was attributed to urban pedogenesis, the complex of natural and human processes that change soils over time. Accelerated erosion redistributes legacy Pb and floodplain sampling indicates sedimentation rates of up to 4 mm/year. Mixing and burial of soil with elevated Pb are also lowering soil Pb concentrations over time. These mechanisms are likely of greater significance on streetsides than near foundation soils. The development of an urban-pedogenesis framework can help guide public health approaches to Pb exposure by incorporating pedogenic processes that reduce and dissipate soil Pb contamination.

Effects of 10 yr of nitrogen and phosphorus fertilization on carbon and nutrient cycling in a tidal freshwater marsh.

Tidal freshwater marshes can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads, but recent studies in salt marshes suggest nutrient loading compromises their structural and functional integrity. Here, we present data on changes in plant biomass, microbial biomass and activity, and soil chemistry from plots in a tidal freshwater marsh on the Altamaha River (GA) fertilized for 10 yr with nitrogen (+N), phosphorus (+P), or nitrogen and phosphorus (+NP). Nitrogen alone doubled aboveground biomass and enhanced microbial activity, specifically rates of potential nitrification, denitrification, and methane production measured in laboratory incubations. Phosphorus alone increased soil P and doubled microbial biomass but did not affect microbial processes. Nitrogen or P alone decreased belowground biomass and soil carbon (C) whereas +NP increased aboveground biomass, microbial biomass and N cycling, and N, P, and C assimilation and burial more than either nutrient alone. Our findings suggest differential nutrient limitation of tidal freshwater macrophytes by N and microbes by P, similar to what has been observed in salt marshes. Macrophytes outcompete microbes for P in response to long-term N and P additions, leading to increased soil C storage through increased inputs of belowground biomass relative to N and P added singly. The susceptibility of tidal freshwater marshes to long-term nutrient enrichment and, hence their ability to mitigate eutrophication will depend on the quantity and relative proportion of N vs. P entering estuaries and tidal wetlands.

Declines in plant productivity drive loss of soil elevation in a tidal freshwater marsh exposed to saltwater intrusion.

We experimentally increased salinities in a tidal freshwater marsh on the Altamaha River (Georgia, USA) by exposing the organic rich soils to 3.5 yr of continuous (press) and episodic (pulse) treatments with dilute seawater to simulate the effects of climate change such as sea level rise (press) and drought (pulse). We quantified changes in root production and decomposition, soil elevation, and soil C stocks in replicated (n = 6) 2.5 × 2.5 m field plots. Elevated salinity had no effect on root decomposition, but it caused a significant reduction in root production and belowground biomass that is needed to build and maintain soil elevation capital. The lack of carbon inputs from root production resulted in reduced belowground biomass of 1631 ± 308 vs. 2964 ± 204 g/m2 in control plots and an overall 2.8 ± 0.9 cm decline in soil surface elevation in the press plots in the first 3.5 yr, whereas the control (no brackish water additions) and the fresh (river water only) treatments gained 1.2 ± 0.4 and 1.7 ± 0.3 cm, respectively, in a 3.5-yr period. There was no change in elevation of pulse plots after 3.5 yr. Based on measurements of bulk density and soil C, the decline of 2.8 cm of surface elevation resulted in a loss of 0.77 ± 0.5 kg C/m2 in press plots. In contrast, the control and the fresh treatment plots gained 0.25 ± 0.04 and 0.36 ± 0.03 kg C/m2, respectively, which represents a net change in C storage of more than 1 kg C/m2. We conclude that, when continuously exposed to saltwater intrusion, the tidal freshwater marsh's net primary productivity, especially root production, and not decomposition, are the main drivers of soil organic matter (SOM) accumulation. Reduced productivity leads to loss of soil elevation and soil C, which has important implications for tidal freshwater marsh persistence in the face of rising sea level.

Chronic but not acute saltwater intrusion leads to large release of inorganic N in a tidal freshwater marsh.

Sea level rise is expected to increase inundation and saltwater intrusion into many tidal freshwater marshes and forests. Saltwater intrusion may be long-term, as with rising seas, or episodic, as with low river flow or storm surge. We applied continuous (press) and episodic (pulse) treatments of dilute seawater to replicate 2.5 × 2.5 m field plots for three years and measured soil attributes, including soil porewater, oxidation-reduction potential, soil carbon (C), and nitrogen (N) to investigate the effects of continuous and episodic saltwater intrusion and increased inundation on tidal freshwater marsh elemental cycling and soil processes. Continuous additions of dilute seawater resulted in increased porewater chloride, sulfate, sulfide, ammonium, and nitrate concentrations. Plots that received press additions also had lower soil oxidation-reduction potentials beginning in the second year. Episodic additions of dilute seawater during typical low flow conditions (Sept.-Oct.) resulted in transient increases in porewater chloride and sulfate that returned to baseline conditions once dosing ceased. Freshwater additions did not affect porewater inorganic N or soil C or N. Persistent saltwater intrusion in freshwater marshes alters the N cycle by releasing ammonium-N from sorption sites, increasing nitrification and severely reducing N storage in macrophyte biomass. Chronic saltwater intrusion, as is expected with rising seas, is likely to shift tidal freshwater marshes from a sink to a source of N whereas intermittent intrusion from drought may have no long term effect on N cycling.

Differential effects of chronic and acute simulated seawater intrusion on tidal freshwater marsh carbon cycling.

Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO4 2-, Cl-, organic C, inorganic nitrogen and phosphorus), ecosystem CO2 and CH4 exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2-4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO2 and CH4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects.

Functional Differences between Natural and Restored Wetlands in the Glaciated Interior Plains.

We measured soil properties, carbon and nutrient (nitrogen, phosphorus) pools, ambient and potential denitrification, and phosphorus sorption index (PSI) in natural depressional wetlands and depressional wetlands restored through the U.S. Department of Agriculture (USDA) Wetland Reserve Program. We measured the same suite of variables in natural and USDA Conservation Reserve Program-restored riparian buffers and in agricultural fields adjacent to both systems to determine the degree to which ecosystem services are being provided through restoration in different hydrogeomorphic settings. Organic carbon and nutrient pools, PSI, and denitrification were greater in natural than in 5- to 10-yr-old restored depressional wetlands. In riparian soils, carbon and nutrient pools, PSI, and denitrification were comparable between restored and natural systems, suggesting that these services develop quickly after restoration. Restored depressional wetlands had lower soil organic C, N, and P relative to agricultural soils, whereas the opposite trend was observed in restored riparian soils. Four-year-old restored riparian buffers achieved equivalence to natural riparian buffers within 4 yr, whereas restored depressional wetlands took longer to provide these ecosystem services (i.e., PSI, denitrification, C storage) at levels comparable to natural wetlands. Restored depressional wetlands and riparian buffers provide ecosystem services lost through previous conversion to agriculture throughout the Midwest; however, the development of these services depends on hydrodynamics (pulsed versus nonpulsed), parent material, soil texture (sand, clay), and disturbance regime (prescribed fire) of the site. As restoration continues throughout the region, C sequestration and nutrient removal in these systems is expected to increase water quality at the local and regional levels.

Effects of sea-level rise and anthropogenic development on priority bird species habitats in coastal Georgia, USA.

We modeled changes in area of five habitats, tidal-freshwater forest, salt marsh, maritime shrub-scrub (shrub), maritime broadleaf forest (oak) and maritime narrowleaf (pine) forest, in coastal Georgia, USA, to evaluate how simultaneous habitat loss due to predicted changes in sea level rise (SLR) and urban development will affect priority bird species of the south Atlantic coastal plain by 2100. Development rates, based on regional growth plans, were modeled at 1% and 2.5% annual urban growth, while SLR rates, based on the Intergovernmental Panel on Climate Change's A1B mean and maximum scenarios, were modeled at 52 cm and 82 cm, respectively. SLR most greatly affected the shrub habitat with predicted losses of 35-43%. Salt marsh and tidal forest also were predicted to lose considerable area to SLR (20-45 and 23-35%, respectively), whereas oak and pine forests had lesser impact from SLR, 18-22% and 11-15%, respectively. Urban development resulted in losses of considerable pine (48-49%) and oak (53-55%) habitat with lesser loss of shrub habitat (21-24%). Under maximum SLR and urban growth, shrub habitat may lose up to 59-64% compared to as much as 62-65% pine forest and 74-75% oak forest. Conservation efforts should focus on protection of shrub habitat because of its small area relative to other terrestrial habitats and use by Painted Buntings (Passerina ciris), a Partners In Flight (PIF) extremely high priority species. Tidal forests also deserve protection because they are a likely refuge for forest species, such as Northern Parula and Acadian Flycatcher, with the decline of oak and pine forests due to urban development.

Effects of bridge shading on estuarine marsh benthic invertebrate community structure and function.

The effect of bridge shading on estuarine marsh food webs was assessed by comparing benthic invertebrate communities beneath seven highway bridges with marshes outside of bridge-affected areas (reference marshes). We used light attenuation and height-width ratio (HW ratio), which takes into account the two main bridge characteristics that determine the degree of shading, to quantify the impact of shading on invertebrate communities. Low bridges, with HW ratio <0.7 and light attenuation greater than 85-90%, had benthic invertebrate densities and diversity that were significantly lower than reference marshes. Density of benthic invertebrates at low bridges was 25-52% (29,685-72,920 organisms/m(2)) of densities measured in adjacent reference marshes (119,329-173,351 organisms/m(2)). Likewise, there were fewer taxa under low bridges (5.8/11.35 cm(2) core) as compared to the reference marshes (9.0/11.35 cm(2) core). Density of numerically dominant taxa (e.g., oligochaetes and nematodes) as well as surface- and subsurface deposit feeders also were reduced under low bridges. Decreased invertebrate density, diversity, dominant taxa, and alterations of trophic feeding groups beneath low bridges was correlated with diminished above- and below-ground macrophyte biomass that presumably resulted in fewer food resources and available refuges from predators. With a greater knowledge of bridge shading effects, bridge construction and design may be improved to reduce the impacts on estuarine benthic invertebrate communities and overall ecosystem structure and function.