Agricultural drainage of organic soils has resulted in vast soil subsidence and contributed to increased atmospheric carbon dioxide (CO₂) concentrations. The Sacramento‐San Joaquin Delta in ...California was drained over a century ago for agriculture and human settlement and has since experienced subsidence rates that are among the highest in the world. It is recognized that drained agriculture in the Delta is unsustainable in the long‐term, and to help reverse subsidence and capture carbon (C) there is an interest in restoring drained agricultural land‐use types to flooded conditions. However, flooding may increase methane (CH₄) emissions. We conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land‐use types (a rice paddy and two restored wetlands) to assess the impact of drained to flooded land‐use change on CO₂and CH₄fluxes in the Delta. We found that the drained sites were net C and greenhouse gas (GHG) sources, releasing up to 341 g C m⁻² yr⁻¹as CO₂and 11.4 g C m⁻² yr⁻¹as CH₄. Conversely, the restored wetlands were net sinks of atmospheric CO₂, sequestering up to 397 g C m⁻² yr⁻¹. However, they were large sources of CH₄, with emissions ranging from 39 to 53 g C m⁻² yr⁻¹. In terms of the full GHG budget, the restored wetlands could be either GHG sources or sinks. Although the rice paddy was a small atmospheric CO₂sink, when considering harvest and CH₄emissions, it acted as both a C and GHG source. Annual photosynthesis was similar between sites, but flooding at the restored sites inhibited ecosystem respiration, making them net CO₂sinks. This study suggests that converting drained agricultural peat soils to flooded land‐use types can help reduce or reverse soil subsidence and reduce GHG emissions.
•Spurious correlation theory for canopy photosynthesis and respiration was derived.•Theory was tested with FLUXNET data and alfalfa.•Spurious correlation was lowest for alfalfa and deciduous ...forests.•Spurious correlation was greatest for boreal conifer forests.
It is necessary to partition eddy covariance measurements of carbon dioxide exchange into its offsetting gross fluxes, canopy photosynthesis, and ecosystem respiration, to understand the biophysical controls on the net fluxes. And independent estimates of canopy photosynthesis (G) and ecosystem respiration (R) are needed to validate and parametrize carbon cycle models that are coupled with climate and ecosystem dynamics models. Yet there is a concern that carbon flux partitioning methods may suffer from spurious correlation because derived values of canopy photosynthesis and ecosystem respiration both contain common information on net carbon fluxes at annual time scales.
We hypothesized that spurious correlation among canopy photosynthesis and ecosystem respiration can be minimized using day–night conditional sampling of CO2 exchange; daytime fluxes are dominated by photosynthesis and nighttime fluxes are dominated by respiration. To test this hypothesis, we derived explicit equations that quantify the degree of spurious correlation between photosynthesis and respiration. Theoretically, day and night samples of net carbon exchange share a different common variable, daytime ecosystem respiration, and the degree of spurious correlation depends upon the variance of this shared variable.
We then applied this theory to ideal measurements of carbon exchange of over a vigorous, irrigated, and frequently harvested alfalfa field in the sunny and windy region of California, the Sacramento-San Joaquin Delta, where soil CO2 efflux is strong. In this case, we found that the correlation coefficient between canopy photosynthesis and ecosystem respiration was −0.79. This relatively high correlation between canopy photosynthesis and respiration was mostly real as the degree of spurious correlation was only −0.32.
We then expanded this analysis to the FLUXNET database that spans a spectrum of climate and plant functional types. We found, on average, that the correlation between gross photosynthesis and ecosystem respiration, using day–night sampling, was close to minus one (−0.828±0.130). For perspective, a large fraction of this correlation was real, as the degree of spurious correlation (Eq. (22)) was −0.526. We conclude that the potential for spurious correlation between canopy photosynthesis and ecosystem respiration across the FLUXNET database was moderate. Looking across the database, we found that the least negative spurious correlations coefficients (>−0.3) were associated with seasonal deciduous forests. The most negative spurious correlations coefficients (<−0.7) were associated with evergreen forests found in boreal climates.
Regional quantification of arctic CO2 and CH4 fluxes remains difficult due to high landscape heterogeneity coupled with a sparse measurement network. Most of the arctic coastal tundra near Barrow, ...Alaska is part of the thaw lake cycle, which includes current thaw lakes and a 5500‐year chronosequence of vegetated thaw lake basins. However, spatial variability in carbon fluxes from these features remains grossly understudied. Here, we present an analysis of whole‐ecosystem CO2 and CH4 fluxes from 20 thaw lake cycle features during the 2011 growing season. We found that the thaw lake cycle was largely responsible for spatial variation in CO2 flux, mostly due to its control on gross primary productivity (GPP). Current lakes were significant CO2 sources that varied little. Vegetated basins showed declining GPP and CO2 sink with age (R2 = 67% and 57%, respectively). CH4 fluxes measured from a subset of 12 vegetated basins showed no relationship with age or CO2 flux components. Instead, higher CH4 fluxes were related to greater landscape wetness (R2 = 57%) and thaw depth (additional R2 = 28%). Spatial variation in CO2 and CH4 fluxes had good satellite remote sensing indicators, and we estimated the region to be a small CO2 sink of −4.9 ± 2.4 (SE) g C m−2 between 11 June and 25 August, which was countered by a CH4 source of 2.1 ± 0.2 (SE) g C m−2. Results from our scaling exercise showed that developing or validating regional estimates based on single tower sites can result in significant bias, on average by a factor 4 for CO2 flux and 30% for CH4 flux. Although our results are specific to the Arctic Coastal Plain of Alaska, the degree of landscape‐scale variability, large‐scale controls on carbon exchange, and implications for regional estimation seen here likely have wide relevance to other arctic landscapes.
Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Few studies have quantified the full GHG budget of wetlands ...due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO2, CH4, and N2O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO2 and CH4 exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO2, CH4, and N2O emissions were 915 ± 95 g C‐CO2 m−2 yr−1, 2.9 ± 0.5 g C‐CH4 m−2 yr−1, and 62 ± 17 mg N‐N2O m−2 yr−1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO2 and N2O emissions, and ~71% of CH4 emissions. Seasonality was minor for CO2 emissions, whereas CH4 and N2O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO2‐eq m−2 yr−1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO2‐eq m−2 yr−1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO2 (−1.4 ± 0.6 kt CO2‐eq yr−1) did not offset CH4 emission (3.7 ± 0.03 kt CO2‐eq yr−1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO2‐eq yr−1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.
Wetlands are landscape hotspots for greenhouse gas exchange, but their overall climate effects are uncertain due to simultaneous uptake and release of different greenhouse gases and the complexity of regulating processes. We estimated an annual greenhouse gas budget for a restored wetland in the Sacramento Delta California using a suite of complementary techniques to overcome the challenges of wetland complexity. We found that greenhouse gas fluxes produced an overall warming climate effect and were sensitive to aboveground cover type, gas transport pathway, and wetland seasonality.
Wetlands are the largest source of methane (CH4) globally, yet our understanding of how process‐level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable ...soil properties influence ecosystem CH4 emissions on annual time scales. We measured ecosystem carbon dioxide (CO2) and CH4 fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento–San Joaquin Delta of California. Annual CH4 fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH4 fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH4 production in the rhizosphere. Soil carbon content and CO2 uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH4 flux differences. Differences in wetland CH4 fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH4 emissions with time could not be explained by an empirical model based on dominant CH4 flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre‐restoration parent soils, suggesting that CH4 emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem‐scale wetland CH4 flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.
Legacy soil properties may influence current greenhouse gas fluxes from restored wetlands, but soil interactions with ecosystem‐scale methane emissions are often uncertain. Using an eddy covariance tower network, we observed substantially reduced methane emissions from wetlands restored on high iron, alluvium soils compared with nearby sites restored on peat soils; however, these differences disappeared multiple years postrestoration as new wetland soils were accreted. Our results demonstrate how soil properties can regulate methane fluxes from restored wetlands, though these effects are short in duration compared with the lifetime of wetland restoration projects.
We present 6.5 years of eddy covariance measurements of fluxes of methane (FCH4) and carbon dioxide (FCO2) from a flooded rice paddy in Northern California, USA. A pronounced warming trend throughout ...the study associated with drought and record high temperatures strongly influenced carbon (C) budgets and provided insights into biophysical controls of FCO2 and FCH4. Wavelet analysis indicated that photosynthesis (gross ecosystem production, GEP) induced the diel pattern in FCH4, but soil temperature (Ts) modulated its amplitude. Forward stepwise linear models and neural networking modeling were used to assess the variables regulating seasonal FCH4. As expected due to their competence in modeling nonlinear relationships, neural network models explained considerably more of the variance in daily average FCH4 than linear models. During the growing season, GEP and water levels typically explained most of the variance in daily average FCH4. However, Ts explained much of the interannual variability in annual and growing season CH4 sums. Higher Ts also increased the annual and growing season ratio of FCH4 to GEP. The observation that the FCH4 to GEP ratio scales predictably with Ts may help improve global estimates of FCH4 from rice agriculture. Additionally, Ts strongly influenced ecosystem respiration, resulting in large interannual variability in the net C budget at the paddy, emphasizing the need for long‐term measurements particularly under changing climatic conditions.
Key Points
Long‐term year‐round measurements of CH4 and CO2 fluxes from rice agriculture
Interannual variability in CH4 and CO2 fluxes was largely driven by temperature
Temperature also influenced the ratio of CH4 to CO2 emissions to across diel to interannual scales
Methane (CH4) exchange in wetlands is complex, involving nonlinear asynchronous processes across diverse time scales. These processes and time scales are poorly characterized at the whole‐ecosystem ...level, yet are crucial for accurate representation of CH4 exchange in process models. We used a combination of wavelet analysis and information theory to analyze interactions between whole‐ecosystem CH4 flux and biophysical drivers in two restored wetlands of Northern California from hourly to seasonal time scales, explicitly questioning assumptions of linear, synchronous, single‐scale analysis. Although seasonal variability in CH4 exchange was dominantly and synchronously controlled by soil temperature, water table fluctuations, and plant activity were important synchronous and asynchronous controls at shorter time scales that propagated to the seasonal scale. Intermittent, subsurface water table decline promoted short‐term pulses of methane emission but ultimately decreased seasonal CH4 emission through subsequent inhibition after rewetting. Methane efflux also shared information with evapotranspiration from hourly to multiday scales and the strength and timing of hourly and diel interactions suggested the strong importance of internal gas transport in regulating short‐term emission. Traditional linear correlation analysis was generally capable of capturing the major diel and seasonal relationships, but mesoscale, asynchronous interactions and nonlinear, cross‐scale effects were unresolved yet important for a deeper understanding of methane flux dynamics. We encourage wider use of these methods to aid interpretation and modeling of long‐term continuous measurements of trace gas and energy exchange.
Key Points
Multiscale, nonlinear, asynchronous dynamics of methane flux are analyzed
Traditional linear correlation misses important lagged, cross‐scale dynamics
Short‐term water table and plant dynamics influence seasonal methane flux
Temperate freshwater wetlands are among the most productive terrestrial ecosystems, stimulating interest in using restored wetlands as biological carbon sequestration projects for greenhouse gas ...reduction programs. In this study, we used the eddy covariance technique to measure surface energy carbon fluxes from a constructed, impounded freshwater wetland during two annual periods that were 8 years apart: 2002–2003 and 2010–2011. During 2010–2011, we measured methane (CH4) fluxes to quantify the annual atmospheric carbon mass balance and its concomitant influence on global warming potential (GWP). Peak growing season fluxes of latent heat and carbon dioxide (CO2) were greater in 2002–2003 compared to 2010–2011. In 2002, the daily net ecosystem exchange reached as low as −10.6 g C m−2 d−1, which was greater than 3 times the magnitude observed in 2010 (−2.9 g C m−2 d−1). CH4 fluxes during 2010–2011 were positive throughout the year and followed a strong seasonal pattern, ranging from 38.1 mg C m−2 d−1 in the winter to 375.9 mg C m−2 d−1 during the summer. The results of this study suggest that the wetland had reduced gross ecosystem productivity in 2010–2011, likely due to the increase in dead plant biomass (standing litter) that inhibited the generation of new vegetation growth. In 2010–2011, there was a net positive GWP (675.3 g C m−2 yr−1), and when these values are evaluated as a sustained flux, the wetland will not reach radiative balance even after 500 years.
Key Points
Wetland energy and carbon fluxes varied seasonally and between study periods
Standing dead plant material may have reduced overall wetland productivity
Large measured methane emissions offset the radiative cooling from photosynthetic carbon uptake
•We conducted eddy covariance measurements over 3 wetland and 3 agricultural sites.•Wetlands tended to have higher evapotranspiration than drained agricultural sites.•Evapotranspiration from wetlands ...with dense vegetation was similar to drained sites.•Nighttime evapotranspiration was markedly higher for wetlands with more open water.•Average nighttime evapotranspiration was strongly related to water temperature.
Water is a limited and valuable resource in California. A large proportion of the fresh water for southern California is supplied by the Sacramento and San Joaquin rivers. With recent efforts to restore large areas of land in the Sacramento–San Joaquin Delta region from farmland to managed wetlands, it is important to investigate the effect of wetland restoration on the local water cycle. We measured evapotranspiration using the eddy covariance method over six different sites in the Sacramento–San Joaquin Delta from 2013 to 2016. The three restored wetlands sites differed in time since restoration and wetland structure, i.e. the area of open water compared to closed vegetation. The three agricultural sites were a flooded rice field, an alfalfa field, and a grazed cattle pasture. In most years, annual evapotranspiration was significantly lower at the drained agricultural sites, with 652 ± 131 mm (2 year average ± standard deviation, SD) for the pasture and 901 ± 24 mm (3 year average ± SD) for the alfalfa field, compared to the two open water wetlands, with 1091 ± 144 mm (4 year average ± SD) for the Mayberry wetland and 1140 ± 67 mm (3 year average ± SD) at the East End wetland. Annual evapotranspiration at the flooded rice site (975 ± 50 mm, 4 year average ± SD) or the closed vegetation wetland (West Pond wetland, 996 ± 63 mm, 4 year average ± SD) was not significantly different from the drained alfalfa site across individual years. Our analysis showed that the structural difference between the wetlands, specifically the fraction of open water compared to closed vegetation, has a large impact on evapotranspiration dynamics. Through analysis of normalized equilibrium evaporation and nighttime evapotranspiration measurements we deduced that evapotranspiration at the wetland with a low fraction of open water surfaces was almost entirely dominated by plant transpiration with very little contribution from evaporation, despite the fact that the site was flooded and water was readily available. Both evaporation and transpiration contributed substantially to evapotranspiration at the two wetlands with larger fraction of open water surfaces. At the closed canopy site evaporation from subcanopy water seemed to be inhibited through two mechanisms: first, the closed canopy prevented heating of the water column and led to significantly cooler water temperatures, which reduced surface vapor pressure. Second, the closed canopy decoupled the water surface from the atmosphere and inhibited turbulent transport of water vapor away from the water surface. Our results provide valuable insights into the water use in California wetlands and can inform decisions on how to maximize water conservation during wetland restoration.
•Digital cameras as a simple, cost-effective means of estimating wetland productivity.•Landsat data can also be used to model wetland productivity for regional upscaling.•Model is relevant to carbon ...market-funded wetland conservation and restoration.
Wetlands have the ability to accumulate large amounts of carbon (C), and therefore wetland restoration has been proposed as a means of sequestering atmospheric carbon dioxide (CO2) to help mitigate climate change. There is a growing interest in using the C services of wetlands to help reduce habitat loss and finance restoration projects. However, including wetlands in C markets worldwide requires a better mechanistic understanding of CO2 and methane exchange and instruments and models that can accurately and inexpensively monitor and predict these fluxes across global wetlands. Remote sensing technology, including near-surface and satellite instruments/approaches, is an effective tool for modeling C fluxes including gross primary productivity (GPP) from the site to global scale. In this study, we evaluate the potential of using digital cameras as a simple, cost-effective means of estimating GPP in restored wetlands, and assess the suitability of using Landsat data to model GPP in these environments for regional upscaling. Our research focused on restored temperate freshwater marshes due to their high C sequestration potential.
As observed in other ecosystems, daily GPP was strongly correlated with site greenness derived from camera imagery (GCCcam). Based on this, we show the potential of using GCCcam and eddy covariance data to adapt and parameterize a light use efficiency (LUE) model to predict daily GPP. The LUE model combining GCCcam and meteorological data was able to explain up to 91% of the variation in daily GPP at the restored marshes, and predict annual GPP budgets within 0% to 20% of observed budgets. However, model performance decreased with increasing site complexity, highlighting the need to explicitly consider spatial heterogeneity in LUE models. We also tested a similar model using Landsat-derived indices, and found that although model performance was high at a homogeneous wetland dominated by emergent vegetation, data-model agreement decreased at a site comprised of a mixture of open water and vegetation, reflecting limitations of Landsat data. Nonetheless, we show that digital camera and Landsat imagery can be used to model photosynthesis in restored wetlands, providing low-cost methods for monitoring C capture that can be used in C market-funded wetland conservation and restoration.
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