Evaluation of laser-based spectrometers for greenhouse gas flux measurements in coastal marshes

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography: Methods 14 (2016): 466–476, doi:10.1002/lom3.10105.Precise and rapid analyses of greenhouse gases (GHGs) will advance understanding of the net climatic forcing of coastal marsh ecosystems. We examined the ability of a cavity ring down spectroscopy (CRDS) analyzer (Model G2508, Picarro) to measure carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes in real-time from coastal marshes through comparisons with a Shimadzu GC-2014 (GC) in a marsh mesocosm experiment and with a similar laser-based N2O analyzer (Model N2O/CO, Los Gatos Research) in both mesocosm and field experiments. Minimum (analytical) detectable fluxes for all gases were more than one order of magnitude lower for the Picarro than the GC. In mesocosms, the Picarro analyzer detected several CO2, CH4, and N2O fluxes that the GC could not, but larger N2O fluxes (218–409 μmol m−2 h−1) were similar between analyzers. Minimum detectable fluxes for the Picarro were 1 order of magnitude higher than the Los Gatos analyzer for N2O. The Picarro and Los Gatos N2O fluxes (3–132 μmol m−2 h−1) differed in two mesocosm nitrogen addition experiments, but were similar in a mesocosm with larger N2O fluxes (326–491 μmol m−2 h−1). In a field comparison, Picarro and Los Gatos N2O fluxes (13 ± 2 μmol m−2 h−1) differed in plots receiving low nitrogen loads but were similar in plots with higher nitrogen loads and fluxes roughly double in magnitude. Both the Picarro and Los Gatos analyzers offer efficient and precise alternatives to GC-based methods, but the former uniquely enables simultaneous measurements of three major GHGs in coastal marshes.This study was funded by the USDA National Institute of Food and Agriculture (Hatch project # 229286, grant to Moseman-Valtierra) and a Woods Hole Sea Grant award to Moseman-Valtierra and Tang

Nitrifying and Denitrifying Microbial Communities in Centralized and Decentralized Biological Nitrogen Removing Wastewater Treatment Systems

Biological nitrogen removal (BNR) in centralized and decentralized wastewater treatment systems is assumed to be driven by the same microbial processes and to have communities with a similar composition and structure. There is, however, little information to support these assumptions, which may impact the effectiveness of decentralized systems. We used high-throughput sequencing to compare the structure and composition of the nitrifying and denitrifying bacterial communities of nine onsite wastewater treatment systems (OWTS) and one wastewater treatment plant (WTP) by targeting the genes coding for ammonia monooxygenase (amoA) and nitrous oxide reductase (nosZ). The amoA diversity was similar between the WTP and OWTS, but nosZ diversity was generally higher for the WTP. Beta diversity analyses showed the WTP and OWTS promoted distinct amoA and nosZ communities, although there is a core group of N-transforming bacteria common across scales of BNR treatment. Our results suggest that advanced N-removal OWTS have microbial communities that are sufficiently distinct from those of WTP with BNR, which may warrant different management approaches

Environmental controls, emergent scaling, and predictions of greenhouse gas (GHG) fluxes in coastal salt marshes

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 123 (2018): 2234-2256, doi:10.1029/2018JG004556.Coastal salt marshes play an important role in mitigating global warming by removing atmospheric carbon at a high rate. We investigated the environmental controls and emergent scaling of major greenhouse gas (GHG) fluxes such as carbon dioxide (CO2) and methane (CH4) in coastal salt marshes by conducting data analytics and empirical modeling. The underlying hypothesis is that the salt marsh GHG fluxes follow emergent scaling relationships with their environmental drivers, leading to parsimonious predictive models. CO2 and CH4 fluxes, photosynthetically active radiation (PAR), air and soil temperatures, well water level, soil moisture, and porewater pH and salinity were measured during May–October 2013 from four marshes in Waquoit Bay and adjacent estuaries, MA, USA. The salt marshes exhibited high CO2 uptake and low CH4 emission, which did not significantly vary with the nitrogen loading gradient (5–126 kg · ha−1 · year−1) among the salt marshes. Soil temperature was the strongest driver of both fluxes, representing 2 and 4–5 times higher influence than PAR and salinity, respectively. Well water level, soil moisture, and pH did not have a predictive control on the GHG fluxes, although both fluxes were significantly higher during high tides than low tides. The results were leveraged to develop emergent power law‐based parsimonious scaling models to accurately predict the salt marsh GHG fluxes from PAR, soil temperature, and salinity (Nash‐Sutcliffe Efficiency = 0.80–0.91). The scaling models are available as a user‐friendly Excel spreadsheet named Coastal Wetland GHG Model to explore scenarios of GHG fluxes in tidal marshes under a changing climate and environment.National Oceanic and Atmospheric Administration Grant Numbers: NA09NOS4190153, NA14NOS4190145; National Science Foundation (NSF) Grant Numbers: 1705941, 1561941/1336911; USGS LandCarbon Program; NOAA National Estuarine Research Reserve Science Collaborative Grant Number: NA09NOS4190153 and NA14NOS41901452019-01-2

Carbon Dioxide Fluxes Reflect Plant Zonation and Belowground Biomass in a Coastal Marsh

Coastal wetlands are major global carbon sinks; however, they are heterogeneous and dynamic ecosystems. To characterize spatial and temporal variability in a New England salt marsh, greenhouse gas (GHG) fluxes were compared among major plant-defined zones during growing seasons. Carbon dioxide (CO2) and methane (CH4) fluxes were compared in two mensurative experiments during summer months (2012–2014) that included low marsh (Spartina alterniflora), high marsh (Distichlis spicata and Juncus gerardiidominated), invasive Phragmites australis zones, and unvegetated ponds. Day- and nighttime fluxes were also contrasted in the native marsh zones. N2O fluxes were measured in parallel with CO2 and CH4 fluxes, but were not found to be significant. To test the relationships of CO2 and CH4 fluxes with several native plant metrics, a multivariate nonlinear model was used. Invasive P. australis zones (−7 to −15 μmol CO2·m−2·s−1) and S. alterniflora low marsh zones (up to −14 μmol CO2·m−2·s−1) displayed highest average CO2 uptake rates, while those in the native high marsh zone (less than −2 μmol CO2·m−2·s−1) were much lower. Unvegetated ponds were typically small sources of CO2 to the atmosphere (\u3c0.5 μmol CO2·m−2·s−1). Nighttime emissions of CO2 averaged only 35% of daytime uptake in the low marsh zone, but they exceeded daytime CO2 uptake by up to threefold in the native high marsh zone. Based on modeling, belowground biomass was the plant metric most strongly correlated with CO2 fluxes in native marsh zones, while none of the plant variables correlated significantly with CH4 fluxes. Methane fluxes did not vary between day and night and did not significantly offset CO2 uptake in any vegetated marsh zones based on sustained global warming potential calculations. These findings suggest that attention to spatial zonation as well as expanded measurements and modeling of GHG emissions across greater temporal scales will help to improve accuracy of carbon accounting in coastal marshe

Carbon Dioxide Fluxes Reflect Plant Zonation and Belowground Biomass in a Coastal Marsh

Coastal wetlands are major global carbon sinks; however, they are heterogeneous and dynamic ecosystems. To characterize spatial and temporal variability in a New England salt marsh, greenhouse gas (GHG) fluxes were compared among major plant-defined zones during growing seasons. Carbon dioxide (CO2) and methane (CH4) fluxes were compared in two mensurative experiments during summer months (2012–2014) that included low marsh (Spartina alterniflora), high marsh (Distichlis spicata and Juncus gerardiidominated), invasive Phragmites australis zones, and unvegetated ponds. Day- and nighttime fluxes were also contrasted in the native marsh zones. N2O fluxes were measured in parallel with CO2 and CH4 fluxes, but were not found to be significant. To test the relationships of CO2 and CH4 fluxes with several native plant metrics, a multivariate nonlinear model was used. Invasive P. australis zones (−7 to −15 μmol CO2·m−2·s−1) and S. alterniflora low marsh zones (up to −14 μmol CO2·m−2·s−1) displayed highest average CO2 uptake rates, while those in the native high marsh zone (less than −2 μmol CO2·m−2·s−1) were much lower. Unvegetated ponds were typically small sources of CO2 to the atmosphere (\u3c0.5 μmol CO2·m−2·s−1). Nighttime emissions of CO2 averaged only 35% of daytime uptake in the low marsh zone, but they exceeded daytime CO2 uptake by up to threefold in the native high marsh zone. Based on modeling, belowground biomass was the plant metric most strongly correlated with CO2 fluxes in native marsh zones, while none of the plant variables correlated significantly with CH4 fluxes. Methane fluxes did not vary between day and night and did not significantly offset CO2 uptake in any vegetated marsh zones based on sustained global warming potential calculations. These findings suggest that attention to spatial zonation as well as expanded measurements and modeling of GHG emissions across greater temporal scales will help to improve accuracy of carbon accounting in coastal marshe

Greenhouse Gas Fluxes Vary Between Phragmites Australis and Native Vegetation Zones in Coastal Wetlands Along a Salinity Gradient

The replacement of native species by invasive Phragmites australis in coastal wetlands may impact ecosystem processes including fluxes of the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4). To investigate differences in daytime CH4 and CO2 fluxes as well as vegetation properties between Phragmites and native vegetation zones along a salinity gradient, fluxes were measured via cavity ringdown spectroscopy in 3 New England coastal marshes, ranging from oligohaline to polyhaline. While daytime CH4 emissions decreased predictably with increasing soil salinity, those from Phragmites zones were larger (15 to 1254 μmol m−2 h−1) than those from native vegetation (4–484 μmol m−2 h−1) across the salinity gradient. Phragmites zones displayed greater daytime CO2 uptake than native vegetation zones (−7 to −15 μmol m−2 s−1 vs. -2 to 0.9 μmol m−2 s−1) at mesohaline-polyhaline, but not oligohaline, sites. Results suggest that vegetation zone and salinity both impact net emission or uptake of daytime CO2 and CH4 (respectively). Future research is warranted to demonstrate Phragmites-mediated impacts on GHG fluxes, and additional measurements across seasonal and diel cycles will enable a more complete understanding of Phragmites\u27 net impact on marsh radiative forcing

Different short-term responses of greenhouse gas fluxes from salt marsh mesocosms to simulated global change drivers

Salt marshes are valued as important greenhouse gas (GHG) sinks, but global changes in climate, nitrogen (N) pollution, and exotic species invasion may alter this marsh function. With the goal of better understanding the potential responses of coastal marsh GHG fluxes to interacting global changes, a multifactorial experiment was conducted. Two climate treatments (present-day and end-of-century temperatures and carbon dioxide (CO2) concentrations) and two N treatments (non-enriched and simulated eutrophic estuary conditions) were applied to mesocosms containing either invasive Phragmites australis (Cav. Trin. Ex Steud.) or native Spartina patens (Aiton) Muhl. vegetated soil cores. Fluxes of CO2, methane (CH4), and nitrous oxide (N2O) were measured in each mesocosm before and after treatment using cavity ring-down spectrometry, along with vegetation growth, edaphic conditions, and pore water chemistry. Methane emissions increased in P. australis but not in S. patens mesocosms under climate change conditions, while CO2 fluxes were similar between vegetation types and treatments. Nitrous oxide emissions increased with N loading from both S. patens and P. australis mesocosms, but were decreased in N-enriched S. patens mesocosms under climate change conditions. These findings demonstrate complex GHG flux responses to global change and suggest the potential for vegetation community-specific responses, though further research is needed to test mechanisms underlying observed GHG flux patterns

Greenhouse gas response and Spartina alterniflora resilience to macroalgal exposure

Climate change can alter salt marsh plant communities and their associated carbon flux dynamics via several mechanisms. Due to warming waters and sea level rise, macroalgal wrack accumulation rates in salt marshes are expected to increase. The smothering and shading effects of macroalgal wrack can have adverse effects on salt marsh vegetation. Most studies have focused on the impacts of high-density accumulation or future accumulation projections, but the impacts of current accumulations of macroalgae on Spartina alterniflora (S. alterniflora) are unclear. We investigated how current, relatively low-density ephemeral (Ulva spp.) and perennial (Fucus spp.) algal wrack coverage impacts S. alterniflora survival and associated greenhouse gas fluxes. We created mesocosms with S. alterniflora-vegetated soil cores using a 2 × 2 factorial design with Fucus and Ulva present or absent. After one month, S. alterniflora cores with Ulva (but no Fucus) had a significantly lower stem density, but they later recovered, attesting to S. alterniflora\u27s resilience to low-density macroalgal coverage. Our results also suggest that Fucus can alter the interaction between Ulva and S. alterniflora by potentially mitigating Ulva\u27s impacts. Macroalgal presence did not significantly influence greenhouse gas fluxes, which suggests the low-density exposure is not altering the salt marsh carbon flux dynamics

Observations of greenhouse gases and nitrate concentrations in a maine river and fringing wetland

In the Sheepscot River, ME, we measured percent saturations of dissolved methane (CH4) and carbon dioxide (CO2) and concentrations of nitrate (NO3-) four times in the main stem and once in the West Branch. River water was super-saturated with CH4 at all sites throughout the study, and measurements were generally higher at lower-gradient sites (1000-5000% saturation) than higher-gradient sites (generally \u3c1000%). Percent saturations of CO2 in the main stem varied in both time and space and were under-saturated at some sites. CO2 percent saturations and NO3- concentrations in the more-developed West Branch were significantly higher than the main stem, likely because of the position of mainstem sites downstream of Sheepscot Pond where primary production and degassing could occur. We also measured CH4 and CO2 fluxes from wetland soil adjacent to the main stem, which averaged 710 (± 59) molCH4/m2/h and-51 (± 6.4) mmolCO2/m2/h. Our findings suggest that rivers and fringing wetlands in the formerly glaciated northeastern US contribute to the production of greenhouse gasses, and that dissolved methane shows spatial variations with channel morphology

Anthropogenic impacts on nitrogen fixation rates between restored and natural Mediterranean salt marshes

To test the effects of site and successional stage on nitrogen fixation rates in salt marshes of the Venice Lagoon, Italy, acetylene reduction assays were performed with Salicornia veneta- and Spartina townsendii-vegetated sediments from three restored (6-14 years) and two natural marshes. Average nitrogen fixation (acetylene reduction) rates ranged from 31 to 343 μmol C2H4·m-2·h-1 among all marshes, with the greatest average rates being from one natural marsh (Tezze Fonde). These high rates are up to six times greater than those reported from Southern California Spartina marshes of similar Mediterranean climate, but substantially lower than those found in moister climates of the Atlantic US coast. Nitrogen fixation rates did not consistently vary between natural and restored marshes within a site (Fossei Est, Tezze Fonde, Cenesa) but were negatively related to assayed plant biomass within the acetylene reduction samples collected among all marshes. Highest nitrogen fixation rates were found at Tezze Fonde, the location closest to the city of Venice, in both natural and restored marshes, suggesting possible site-specific impacts of anthropogenic stress on marsh succession