To understand patterns in CO
2
partial pressure (P
CO2
) over time in wetlands’ surface water and porewater, we examined the relationship between P
CO2
and land–atmosphere flux of CO
2
at the ...ecosystem scale at 22 Northern Hemisphere wetland sites synthesized through an open call. Sites spanned 6 major wetland types (tidal, alpine, fen, bog, marsh, and prairie pothole/karst), 7 Köppen climates, and 16 different years. Ecosystem respiration (R
eco
) and gross primary production (GPP), components of vertical CO
2
flux, were compared to P
CO2
, a component of lateral CO
2
flux, to determine if photosynthetic rates and soil respiration consistently influence wetland surface and porewater CO
2
concentrations across wetlands. Similar to drivers of primary productivity at the ecosystem scale, P
CO2
was strongly positively correlated with air temperature (T
air
) at most sites. Monthly average P
CO2
tended to peak towards the middle of the year and was more strongly related to R
eco
than GPP. Our results suggest R
eco
may be related to biologically driven P
CO2
in wetlands, but the relationship is site-specific and could be an artifact of differently timed seasonal cycles or other factors. Higher levels of discharge do not consistently alter the relationship between R
eco
and temperature normalized P
CO2
. This work synthesizes relevant data and identifies key knowledge gaps in drivers of wetland respiration.
Tidal wetlands play an important role in global carbon cycling by storing carbon in sediment at millennial time scales, transporting dissolved carbon into coastal waters, and contributing ...significantly to global CH4 budgets. However, these ecosystems' greenhouse gas monitoring and predictions are challenging due to spatial heterogeneity and tidal flooding. We utilized eddy covariance and chamber measurements to quantify fluxes of CO2 and CH4 at a restored tidal saltmarsh across spatial and temporal scales. Eddy covariance data revealed that the site was a strong net sink for CO2 (−387 g C‐CO2 m−2 yr−1, SD = 46) and a small net source of CH4 (0.7 g C‐CH4 m−2 yr−1, SD = 0.4). After partitioning net ecosystem exchange of CO2 into gross primary production and ecosystem respiration, we found that high net uptake of CO2 was due to low respiration emissions rather than high photosynthetic rates. We also found that respiration rates varied between land covers with increased respiration in mudflats compared to vegetated areas. Daytime soil chamber measurements revealed that the greatest CO2 emission was from higher elevation mudflat soils (0.5 μmol m−2s−1, SE = 1.3) and CH4 emission was greatest from lower elevation Spartina foliosa soils (1.6 nmol m−2s−1, SD = 8.2). Overall, these results highlight the importance of the relationships between wetland plant community and elevation, and inundation for CO2 and CH4 fluxes. Future research should include the use of high‐resolution imagery, automated chambers, and a focus on quantifying carbon exported in tidal waters.
Plain Language Summary
At the ecosystem level, a restored tidal salt marsh in the South San Francisco Bay California took in more carbon dioxide (CO2) from the atmosphere through photosynthetic activity than it emitted through respiration, and it emitted very small amounts of methane (CH4). This site appears to be a stronger sink for CO2 compared to other tidal marsh sites due to the very low rate of CO2 being lost through respiration to the atmosphere, rather than strong photosynthetic rates. We also found that ecosystem level CO2 emissions and the responses to temperature and light varied based on land cover type. By measuring soil surface emissions from each of the main land cover types of pickleweed, cordgrass, and mudflats we found that on average soils with lower elevation where cordgrass grows were stronger sources of CH4 while mudflat soils with greater elevation were stronger sources of CO2.
Key Points
Soil chamber measurements were able to detect significant differences in CO2 and CH4 fluxes between land cover types
Vegetation and microtopography are drivers of the spatially heterogeneous CO2 and CH4 emissions within the wetland
At the ecosystem level, high net uptake of CO2 was the result of low respiration emissions, suggesting lateral transport of dissolved CO2
Abstract
Tidal wetlands play an important role in global carbon cycling by storing carbon in sediment at millennial time scales, transporting dissolved carbon into coastal waters, and contributing ...significantly to global CH
4
budgets. However, these ecosystems' greenhouse gas monitoring and predictions are challenging due to spatial heterogeneity and tidal flooding. We utilized eddy covariance and chamber measurements to quantify fluxes of CO
2
and CH
4
at a restored tidal saltmarsh across spatial and temporal scales. Eddy covariance data revealed that the site was a strong net sink for CO
2
(−387 g C‐CO
2
m
−2
yr
−1
, SD = 46) and a small net source of CH
4
(0.7 g C‐CH
4
m
−2
yr
−1
, SD = 0.4). After partitioning net ecosystem exchange of CO
2
into gross primary production and ecosystem respiration, we found that high net uptake of CO
2
was due to low respiration emissions rather than high photosynthetic rates. We also found that respiration rates varied between land covers with increased respiration in mudflats compared to vegetated areas. Daytime soil chamber measurements revealed that the greatest CO
2
emission was from higher elevation mudflat soils (0.5 μmol m
−2
s
−1
, SE = 1.3) and CH
4
emission was greatest from lower elevation
Spartina foliosa
soils (1.6 nmol m
−2
s
−1
, SD = 8.2). Overall, these results highlight the importance of the relationships between wetland plant community and elevation, and inundation for CO
2
and CH
4
fluxes. Future research should include the use of high‐resolution imagery, automated chambers, and a focus on quantifying carbon exported in tidal waters.
Plain Language Summary
At the ecosystem level, a restored tidal salt marsh in the South San Francisco Bay California took in more carbon dioxide (CO
2
) from the atmosphere through photosynthetic activity than it emitted through respiration, and it emitted very small amounts of methane (CH
4
). This site appears to be a stronger sink for CO
2
compared to other tidal marsh sites due to the very low rate of CO
2
being lost through respiration to the atmosphere, rather than strong photosynthetic rates. We also found that ecosystem level CO
2
emissions and the responses to temperature and light varied based on land cover type. By measuring soil surface emissions from each of the main land cover types of pickleweed, cordgrass, and mudflats we found that on average soils with lower elevation where cordgrass grows were stronger sources of CH
4
while mudflat soils with greater elevation were stronger sources of CO
2
.
Key Points
Soil chamber measurements were able to detect significant differences in CO
2
and CH
4
fluxes between land cover types
Vegetation and microtopography are drivers of the spatially heterogeneous CO
2
and CH
4
emissions within the wetland
At the ecosystem level, high net uptake of CO
2
was the result of low respiration emissions, suggesting lateral transport of dissolved CO
2
