The effects of inorganic nitrogen and phosphorus enrichment on herbaceous species growth of the Kimages Creek wetland (VA)

Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) infiltrate waterways through fertilizer application, urban stormwater runoff, and sewer infrastructure leaks. As surrounding waterbodies experience increased DIN and DIP inputs, wetlands can experience corresponding nutrient enrichment. Vegetation uses DIN and DIP for structural growth, color, and seed production. Changes in DIN and DIP availability can influence species distribution due to differences in photosynthetic rates, root morphology and structure, and tissue type. DIP and DIN inputs are projected to increase 15-30% and 30-60% in the next fifty years¹. It is of interest to examine plant growth characteristics within this nutrient enrichment projection as well as nutrient enrichment from a potential 100-year projection to analyze future species composition responses within a freshwater tidal marsh

On the challenges of modeling the net radiative forcing of wetlands: reconsidering Mitsch et al. 2013

Wetlands play a role in regulating global climate by removing carbon dioxide (CO2) from the atmosphere and sequestering it as soil carbon, and by emitting methane (CH4) and nitrous oxide (N2O) to the atmosphere. In a recent article in this journal (Mitsch et al. Landscape Ecol 28:583–597, 2013), CO2 sequestration and CH4 emissions were modeled for several freshwater wetlands that vary in vegetation type, climate, and hydrology. The authors of that study made significant errors that caused them to underestimate the importance of wetland CH4 emissions on climate dynamics. Here, I reanalyze the Mitsch et al. dataset and show that all of their wetlands had an initial warming effect but eventually caused negative net radiative forcing within ~60–14,000 years, depending on the ratio of CO2 sequestration to CH4 emissions. The addition of a N2O component to the model suggested that typical wetland N2O emission rates would contribute only a minor burden to wetland radiative forcing, although specific application of this three-gas model is limited by the paucity of sites where CO2 sequestration, CH4 emission, and N2O exchange rates have all been measured. Across the landscape, many natural wetlands may already cause negative net radiative forcing when integrated over their lifetime. However, caution should be applied when using carbon sequestration as a rationale for designing wetland construction and restoration projects since freshwater wetlands may have a net positive (warming) effect on climate for decades to centuries or longer

Carbon dynamics in a tidal freshwater marsh

The sources and fates of carbon in a tidal freshwater marsh (Sweet Hall marsh; Pamunkey River, Virginia) were determined to understand the role that these marshes play with respect to estuarine carbon cycling. A carbon gas flux model, based on measured carbon dioxide and methane fluxes, was developed to calculate annual rates of macrophyte and microalgal photosynthesis and community and belowground respiration. Because carbon fluxes out of marsh sediments may underestimate true belowground respiration if sediment-produced gases are transported through plant tissues, gross nitrogen mineralization was used as a proxy for belowground carbon respiration. Annual community respiration exceeded gross photosynthesis, suggesting an allochthonous input of organic carbon to the marsh. Sediment deposition during tidal flooding was measured as a potential exogenous carbon source. Short term deposition rates (biweekly to monthly) were spatially and temporally variable, with highest rates measured near a tidal creek during summer. Annual deposition on the marsh was sufficient to balance relative sea level rise and measured respiration rates. Sediment inventories of 7Be indicated that spatial patterns of sedimentation were not due to sediment redistribution within the marsh. Accretion rates calculated from 137Cs (decadal scale) and 14C (centuries to millennia) were substantially less than annual deposition rates. The concentration and isotopic composition of dissolved and particulate inorganic and organic carbon (DIC, DOC, POC) were measured in a marsh creek which drained the study site. Seasonal isotopic variations in DIC were explained by marsh porewater drainage and decomposition of marsh-derived carbon. A model linking DIC concentrations and water transport showed that DIC export from tidal marshes could explain a significant portion of excess DIC production in the adjacent estuary. Isotopic mixing models indicated seasonal variability in the importance of phytoplankton as a source of DOC and POC although there was no evidence for a net flux of these materials between the marsh and estuary. Annually, the marsh carbon budget was closely balanced, with sources exceeding sinks by approximately 5 percent. This similarity suggests that those processes which were not quantified (e.g. consumption by marsh and riverine fauna) were quantitatively unimportant with respect to the entire marsh carbon budget

Biophysical drivers of carbon dioxide and methane fluxes in a restored tidal freshwater wetland

Wetlands store large amounts of carbon (C) in biomass and soils, playing a crucial role in offsetting greenhouse gas (GHG) emissions; however, they also account for 30% of global yearly CH4 emissions. Anthropogenic disturbance has led to the decline of natural wetlands throughout the United States, with a corresponding increase in created and restored wetlands. Studies characterizing biogeochemical processes in restored forested wetlands, particularly those that are both tidal and freshwater, are lacking but essential for informing science- based carbon management

Assessing how disruption of methanogenic communities and their syntrophic relationships in tidal freshwater marshes via saltwater intrusion may affect CH4 emissions

Tidal freshwater wetlands (TFW), which lie at the interface of saltwater and freshwater ecosystems, are predicted to experience moderate salinity increases due to sea level rise. Increases in salinity generally suppress CH4 production, but it is uncertain to what extent elevated salinity will affect CH4 cycling in TFW. It is also unknown whether CH4 production will resume when freshwater conditions return. The ability to produce CH4 is limited to a monophyletic group of the Euryarchaeota phylum called methanogens (MG), who are limited to a small number of substrates (e.g., acetate, H2, and formate) produced from the breakdown of fermentation products. In freshwater anaerobic soils, the degradation of certain fermentation products (e.g., butyrate, propionate) is only energetically favorable when their catabolic byproduct, H2 or formate, is consumed to low concentrations by MGs. This is considered a form of obligate syntrophy. Sulfate reducing bacteria (SRB) are capable of utilizing a larger variety of substrates than MG, including substrates degraded by methanogenic syntrophy (e.g., butyrate, propionate). The introduction of sulfate (SO4 -2) into TFW via saltwater intrusion events may allow SRB to disrupt syntrophic relationships between hydrogenotrophic MG and syntrophic fermenters. This may select for MG taxa that differ in their rate of CH4 production. The objectives of this study were to determine the effect of oligohaline SO4 -2 concentrations on MG community functions (i.e., CH4 production and syntrophic butyrate degradation); and, to assess whether these functions recover after competition with SRB has been removed

Tower-based greenhouse gas fluxes in a restored tidal freshwater wetland: A shared resource for research and teaching.

The goals of this study are: 1) to use an eddy-covariance system to continuously measure wetland-atmosphere CO2 and CH4 exchange in a restored forested wetland, 2) to quantity C sequestration in plant biomass and soils in restored (Kimages Creek watershed) and old-growth (Harris Creek watershed) forested wetlands, and 3) to establish a shared long-term, shared research and teaching platform centered on eddy-covariance tower measurements. Since the old-growth forest wetland has had longer to accumulate C, the current C stocks are likely much larger than those of the restored wetland; however, the rate of C accumulation (i.e., C sequestration or net ecosystem production) may be higher in young ecosystems (De Simon et al. 2 | Goodrich-Stuart (Stuart-Haëntjens) 2012). While natural wetlands generally offset the warming effect of CH4 emissions by also sequestering large amounts of CO2, but it has been suggested that, in the short-term, this may not hold true for restored wetlands (Petrescu et al. 2015). Very few restored wetlands have studied, however, so knowledge is lacking in this area

Exchanges of carbon and nitrogen between tidal freshwater wetlands and adjacent tributaries : a final report

Tidal freshwater marshes are hypothesized to export materials and energy that support primary and secondary production in estuaries, yet there are few data available to test this hypothesis. A major objective of our study was to measure net exchange of carbon between marsh and atmosphere to determine whether biogenic carbon inputs are in excess of those required to produce observed biomass, satisfy the measured accretion rate, and keep pace with the historical rate of sea level rise. To determine whether the marsh exports materials and energy we measured exchanges of nutrients between marsh sediments and overlying water and of nutrients, total suspended solids, and chlorophyll a between the adjacent tidal creek and river. Studies were performed in Sweet Hall Marsh, a National Estuarine Research Reserve, located on the Pamunkey River in Virginia. A gaseous carbon flux model was developed to calculate annual net CO2 and CI4 fluxes between the atmosphere and marsh. In addition, we performed seasonal measurements of macrophyte diversity and biomass, sediment microalgal biomass, standing stocks of porewater nutrients, %C and %N in sediments and macrophytes, and sediment gross mineralization and nitrification. Based upon two years of measurements of net ecosystem metabolism, the marsh is net heterotrophic. Estimates of sediment respiration based on net sediment metabolism greatly underestimated the true respiration rate. When gross N-mineralization, expressed in units of carbon, was used as a surrogate for sediment respiration, net autotrophic fixation accounted for estimated biomass production. A process-based carbon mass balance model for Sweet Hall Marsh was constructed to determine whether calculations of carbon exchange using the gaseous carbon flux model and results of exchange studies were reasonable and to guide future research at Sweet Hall Marsh. Results of mass balance analysis showed that inputs and exports of carbon to or from the marsh are reasonably in balance. While additional information on sediment and chlorophyll exchanges would strengthen our model, it appears that on an annual basis Sweet Hall Marsh imports sediments and exports chlorophyll. In addition, the marsh is a sink for N03- throughout the year. N}4+ produced by organic matter mineralization appears to be removed by coupled nitrification - denitrification so that there is little, if any, export of dissolved inorganic nitrogen from the marsh. These conclusions indicate that tidal freshwater marshes may export materials (chlorophyll) to adjacent waters, but the ultimate fate of these materials and their effects on estuarine primary and secondary production are still unknown

The Inception of a Long-Term Study of Elevation Change and Sediment Accretion in Three Forested Tidal Freshwater Wetlands and in the Restored Freshwater Marsh at Kimages Creek

Sediment accretion and the corresponding ability to keep pace with sea level rise in both mature forested tidal freshwater wetlands and restored wetland sites represent significant data gaps in the current body of literature pertaining to wetland sustainability. In order to address these data gaps, Surface Elevation Tables (SETs) were installed along with feldspar marker horizons to measure contemporary sediment accretion rates in three mature forested tidal freshwater wetlands, as well as accretion within a tidal marsh currently undergoing restoration. These are the first SETs installed in tidal forests in the James River watershed, and establish VCU Rice Rivers Center as a contributing partner in the NOAA Chesapeake Bay Sentinel Sites Cooperative

CO2 and CH4 fluxes across a Nuphar lutea (L.) Sm. stand

Floating-leaved rhizophytes can significantly alter net carbon dioxide (CO 2) and methane (CH 4) exchanges with the atmosphere in freshwater shallow environments. In particular, CH 4 efflux can be enhanced by the aerenchyma-mediated mass flow, while CO 2 release from supersaturated waters can be reversed by the plant uptake. Additionally, the floating leaves bed can hamper light penetration and oxygen (O 2) diffusion from the atmosphere, thus altering the dissolved gas dynamics in the water column. In this study, net fluxes of CO 2 and CH 4 were measured seasonally across vegetated [Nuphar lutea (L.) Sm.] and free water surfaces in the Busatello wetland (Northern Italy). Concomitantly, dissolved gas concentrations were monitored in the water column and N. lutea leaf production was estimated by means of biomass harvesting. During the vegetative period (May-August), the yellow waterlily stand resulted a net sink for atmospheric carbon (from 97.5 to 110.6 g C-CO 2 m -2), while the free water surface was a net carbon source (166.3 g C-CO 2 m -2). Both vegetated and plant-free areas acted as CH 4 sources, with an overall carbon release comprised between 71.6 and 113.3 g C-CH 4 m -2. On the whole, water column chemistry was not affected by the presence of the floating leaves; moreover, no significant differences in CH4 efflux were evidenced between the vegetated and plant-free areas. In general, this study indicates that the colonization of shallow aquatic ecosystems by N. lutea might not have the same drastic effect reported for free-floating macrophytes

Contributions of organic and inorganic matter to sediment volume and accretion in tidal wetlands at steady state

A mixing model derived from first principles describes the bulk density (BD) of intertidal wetland sediments as a function of loss on ignition (LOI). The model assumes that the bulk volume of sediment equates to the sum of self-packing volumes of organic and mineral components or BD = 1/[LOI/k1 + (1-LOI)/k2], where k1 and k2 are the self-packing densities of the pure organic and inorganic components, respectively. The model explained 78% of the variability in total BD when fitted to 5075 measurements drawn from 33 wetlands distributed around the conterminous United States. The values of k1 and k2 were estimated to be 0.085 ± 0.0007 g cm−3 and 1.99 ± 0.028 g cm−3, respectively. Based on the fitted organic density (k1) and constrained by primary production, the model suggests that the maximum steady state accretion arising from the sequestration of refractory organic matter is ≤ 0.3 cm yr−1. Thus, tidal peatlands are unlikely to indefinitely survive a higher rate of sea-level rise in the absence of a significant source of mineral sediment. Application of k2 to a mineral sediment load typical of East and eastern Gulf Coast estuaries gives a vertical accretion rate from inorganic sediment of 0.2 cm yr−1. Total steady state accretion is the sum of the parts and therefore should not be greater than 0.5 cm yr−1 under the assumptions of the model. Accretion rates could deviate from this value depending on variation in plant productivity, root:shoot ratio, suspended sediment concentration, sediment-capture efficiency, and episodic events