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

Co-Regulations of Spartina alterniflora Invasion and Exogenous Nitrogen Loading on Soil N2O Efflux in Subtropical Mangrove Mesocosms

We thank Zhonglei Wang, Cunxin Ning, Hui Chen, Qian Huang, Fang Liu and Jian Zhou for their assistance with the greenhouse experiments and gas sampling. We are also grateful to Weimin Song, Rashid Rafique, Junyi Liang, Zheng Shi and Jianyang Xia for editing the manuscript.Both plant invasion and nitrogen (N) enrichment should have significant impact on mangrove ecosystems in coastal regions around the world. However, how N2O efflux in mangrove wetlands responds to these environmental changes has not been well studied. Here, we conducted a mesocosm experiment with native mangrove species Kandelia obovata, invasive salt marsh species Spartina alterniflora, and their mixture in a simulated tide rotation system with or without nitrogen addition. In the treatments without N addition, the N2O effluxes were relatively low and there were no significant variations among the three vegetation types. A pulse loading of exogenous ammonium nitrogen increased N2O effluxes from soils but the stimulatory effect gradually diminished over time, suggesting that frequent measurements are necessary to accurately understand the behavior of N-induced response of N2O emissions. With the N addition, the N2O effluxes from the invasive S. alterniflora were lower than that from native K. obovata mesocosms. This result may be attributed to higher growth of S. alterniflora consuming most of the available nitrogen in soils, and thus inhibiting N2O production. We concluded that N loading significantly increased N2O effluxes, while the invasion of S. alterniflora reduced N2O effluxes response to N loading in this simulated mangrove ecosystem. Thus, both plant invasion and excessive N loading can co-regulate soil N2O emissions from mangrove wetlands, which should be considered when projecting future N2O effluxes from this type of coastal wetland.Yeshttp://www.plosone.org/static/editorial#pee

Potential nitrous oxide production by marine shellfish in response to warming and nutrient enrichment

Bivalves facilitate microbial nitrogen cycling, which can produce nitrous oxide (N2O), a potent greenhouse gas. Potential N2O production by three marine bivalves (Mytilus edulis, Mercenaria mercenaria and Crassostrea virginica) was measured in the laboratory including responses to nitrogen (N) loading and/or warming over short-terms (up to 14 or 28 days). N additions (targeting 100 μM-N ammonium nitrate) or warming (22 °C) individually and in combination were applied with experimental controls (20 μM-N, 19 °C). N2O production rates were higher with N additions for all species, but warming lacked significant direct effects. Ammonium and nitrate concentrations varied but were consistent with nitrification as a potential N2O source for all bivalves. Highest N2O emissions (7.5 nmol N2O g−1 h−1) were from M. edulis under hypoxic conditions coincident with a drop in pH. Macro-epifauna on M. edulis did not significantly alter N2O production. Thus, under short-term hypoxic conditions, micro-organisms in M. edulis guts may be a particularly significant source of N2O

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

Appendix B. Independent and overall effects of nitrogen, sediment, and their interaction as well as plot (location) on nitrogen fixation rates and diazotroph richness in surface sediments (0–1 cm) in Experiment 1, based on multiple-factor ANOVA tests.

Independent and overall effects of nitrogen, sediment, and their interaction as well as plot (location) on nitrogen fixation rates and diazotroph richness in surface sediments (0–1 cm) in Experiment 1, based on multiple-factor ANOVA tests

Appendix A. Lack of significant effect of experimental treatment or day of experiment initiation on nitrogen fixation rates in Experiment 1 and 2 determined via two-factor repeated measures ANOVA tests.

Lack of significant effect of experimental treatment or day of experiment initiation on nitrogen fixation rates in Experiment 1 and 2 determined via two-factor repeated measures ANOVA tests