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

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

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

CO2 Uptake Offsets Other Greenhouse Gas Emissions from Salt Marshes with Chronic Nitrogen Loading

Coastal wetlands are known for exceptional productivity, but they also receive intense land-based nitrogen (N) loading. In Narragansett Bay, RI (USA), coastal ecosystems have received anthropogenic N inputs from wastewater for more than two centuries. Greenhouse gas fluxes were studied throughout a growing season (2016) in three coastal wetlands with contrasting histories of nitrogen loading. The wetland with the highest historic N load (Mary’s Creek, Warwick, RI) had significantly greater nitrous oxide (N2O) and methane (CH4) emissions than the other two sites. However, the two marshes with historic N loads (Mary’s Creek and Mary Donovan, Little Compton, RI) also had greater rates of CO2 uptake than the reference site (Nag Marsh, Prudence Island, RI). Their CO2 uptake rates far outpaced their other greenhouse gas emissions. Mary’s Creek had the greatest above- and below-ground plant biomass, vertical accretion rates, and carbon content of soils. Spartina alterniflora height was greatest at Mary’s Creek and Mary Donovan marsh. The following growing season (2017), greenhouse gases were compared across four plant-defined ecological zones in Mary’s Creek. Higher rates of CO2 uptake and CH4 emissions were found in the S. alterniflora-vegetated creekbank compared to high marsh zones or bare mudflats. Potential denitrifying enzyme activity did not significantly differ across the four zones nor between Mary’s Creek and Nag Marsh, suggesting a consistently high capacity to completely reduce N loads. These results support efforts to protect and restore these coastal ecosystems for their carbon sequestration function even despite prevalence of anthropogenic N loading

Comparison of N2O emissions and gene abundances between wastewater nitrogen removal systems

Biological nitrogen removal (BNR) systems are increasingly used in the United States in both centralized wastewater treatment plants (WWTPs) and decentralized advanced onsite wastewater treatment systems (OWTS) to reduce N discharged in wastewater effluent. However, the potential for BNR systems to be sources of nitrous oxide (N2O), a potent greenhouse gas, needs to be evaluated to assess their environmental impact. We quantified and compared N2O emissions from BNR systems at a WWTP (Field\u27s Point, Providence, RI) and three types of advanced OWTS (Orenco Advantex AX 20, SeptiTech Series D, and Bio-Microbics MicroFAST) in nine Rhode Island residences (n = 3 per type) using cavity ring-down spectroscopy. We also used quantitative polymerase chain reaction to determine the abundance of genes from nitrifying (amoA) and denitrifying (nosZ) microorganisms that may be producing N2O in these systems. Nitrous oxide fluxes ranged from -4 × 10-3 to 3 × 10-1 mmol N2O m-2 s-1 and in general followed the order: centralized WWTP \u3e Advantex \u3e SeptiTech \u3e FAST. In contrast, when N2O emissions were normalized by population served and area of treatment tanks, all systems had overlapping ranges. In general, the emissions of N2O accounted for a small fraction (\u3c 1%) of N removed. There was no significant relationship between the abundance of nosZ or amoA genes and N2O emissions. This preliminary analysis highlights the need to evaluate N2O emissions from wastewater systems as a wider range of technologies are adopted. A better understanding of the mechanisms of N2O emissions will also allow us to better manage systems to minimize emissions

Plant manipulations and diel cycle measurements test drivers of carbon dioxide and methane fluxes in a Phragmites australis-invaded coastal marsh

Invasion of coastal marshes by Phragmites australis may alter carbon cycling, including fluxes of the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4). Understanding patterns and drivers of these GHG fluxes in P. australis-invaded coastal marshes is critical to predicting how this widespread biological invasion may impact carbon (C) sequestration in coastal marshes. The objectives of this study were (1) to test effects of P. australis aboveground vegetation removal on GHG fluxes over short timescales (up to 4 months) and (2) to contrast diel patterns of GHG fluxes in P. australis-vegetated and cleared plots. First, effects of mechanical aboveground P. australis biomass removal on GHG fluxes and soil variables were tested over a series of short-term durations (from min to months). Next, on 3 dates, GHG fluxes were measured every 3 h over complete diel cycles. Net daytime CO2 uptake (−60 to −100 mmol m−2 s−1) was observed where P. australis was left intact. All durations of vegetation removal produced similar CO2 emissions to those measured from intact P. australis plots during evening hours. CH4 fluxes did not differ where P. australis was removed or left intact. Greater daytime CH4 emissions (75–100 μmol m−2 h−1) were found than at night (20–40 μmol m−2 h−1) from both cleared and vegetated plots. Results of this study suggest that CO2 fluxes in this system vary primarily due to substantial photosynthetic uptake by P. australis, and that CH4 emissions are likely driven by abiotic factors, such as temperature, that vary on diel cycles. Calculation of net GHG fluxes in this P. australis-invaded coastal marsh indicates that it is a GHG sink during the growing season