| 1. |
- Goodrich, Jordan P., et al.
(author)
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High-frequency measurements of methane ebullition over a growing season at a temperate peatland site
- 2011
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 38, s. L07404-
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Journal article (peer-reviewed)abstract
- Bubbles can contribute a significant fraction of methane emissions from wetlands; however the range of reported fractions is very large and accurate characterization of this pathway has proven difficult. Here we show that continuous automated flux chambers combined with an integrated cavity output spectroscopy (ICOS) instrument allow us to quantify both CH(4) ebullition rate and magnitude. For a temperate poor fen in 2009, ebullition rate varied on hourly to seasonal time scales. A diel pattern in ebullition was identified with peak release occurring between 20:00 and 06:00 local time, though steady fluxes (i.e., those with a linear increase in chamber headspace CH(4) concentration) did not exhibit diel variability. Seasonal mean ebullition rates peaked at 843.5 +/- 384.2 events m(-2) d(-1) during the summer, with a mean magnitude of 0.19 mg CH(4) released in each event.
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| 2. |
- Santoni, Gregory W., et al.
(author)
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Mass fluxes and isofluxes of methane (ch4) at a new hampshire fen measured by a continuous wave quantum cascade laser spectrometer
- 2012
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. D10301-
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Journal article (peer-reviewed)abstract
- We have developed a mid-infrared continuous-wave quantum cascade laser direct-absorption spectrometer (QCLS) capable of high frequency (>= 1 Hz) measurements of (CH4)-C-12 and (CH4)-C-13 isotopologues of methane (CH4) with in situ 1-s RMS delta C-13(CH4) precision of 1.5 parts per thousand and Allan-minimum precision of 0.2 parts per thousand. We deployed this QCLS in a well-studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 +/- 0.7 mg CH4 m(-2) hr(-1) over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 +/- 0.8 mgCH(4) m(-2) hr(-1)) over the same period. Mean delta C-13(CH4) composition of emitted CH4 calculated using EC isoflux methods was -71 +/- 8 parts per thousand (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of -64.5 +/- 0.8 parts per thousand Ebullitive fluxes, representing similar to 10% of total CH4 fluxes at this site, were on average 1.2 parts per thousand enriched in C-13 compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process-based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.
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| 3. |
- Yuan, Kunxiaojia, et al.
(author)
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Causality guided machine learning model on wetland CH4 emissions across global wetlands
- 2022
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In: Agricultural and Forest Meteorology. - : Elsevier. - 0168-1923 .- 1873-2240. ; 324
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Journal article (peer-reviewed)abstract
- Wetland CH4 emissions are among the most uncertain components of the global CH4 budget. The complex nature of wetland CH4 processes makes it challenging to identify causal relationships for improving our understanding and predictability of CH4 emissions. In this study, we used the flux measurements of CH4 from eddy covariance towers (30 sites from 4 wetlands types: bog, fen, marsh, and wet tundra) to construct a causality-constrained machine learning (ML) framework to explain the regulative factors and to capture CH4 emissions at sub -seasonal scale. We found that soil temperature is the dominant factor for CH4 emissions in all studied wetland types. Ecosystem respiration (CO2) and gross primary productivity exert controls at bog, fen, and marsh sites with lagged responses of days to weeks. Integrating these asynchronous environmental and biological causal relationships in predictive models significantly improved model performance. More importantly, modeled CH4 emissions differed by up to a factor of 4 under a +1C warming scenario when causality constraints were considered. These results highlight the significant role of causality in modeling wetland CH(4 )emissions especially under future warming conditions, while traditional data-driven ML models may reproduce observations for the wrong reasons. Our proposed causality-guided model could benefit predictive modeling, large-scale upscaling, data gap-filling, and surrogate modeling of wetland CH4 emissions within earth system land models.
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| 4. |
- Zona, Donatella, et al.
(author)
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Earlier snowmelt may lead to late season declines in plant productivity and carbon sequestration in Arctic tundra ecosystems
- 2022
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 12:1
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Journal article (peer-reviewed)abstract
- Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.
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