High-resolution stable isotope measurements elucidate vegetation and climate drivers of CH4 metabolism in a temperate wetland
Abstract
Methane flux from wetlands is both a critical component of the global CH4 budget, and highly sensitive to environmental drivers. Shifts in CH4 dynamics can arise from alterations in temperature and precipitation patterns as well as shifts in plant community composition such as shrub encroachment. Gaps in our understanding of the mechanisms driving CH4 production and consumption dynamics, however, limit our ability to predict the magnitude of changes under future conditions. To address these gaps, we quantified the isotopic composition of CH4 fluxes from a temperate wetland undergoing shrub encroachment (Sallie's Fen, NH). We used quantum cascade laser technology, linked to automated chambers and with a custom sample injection manifold, to quantify δ13C-CH4 of porewater and emissions across multiple years. We used this data to identify shifts in methanogenesis (including both acetoclastic, and hydrogenotrophic pathways) and methanotrophy (which consumes CH4 primarily via aerobic metabolism) across plant communities and under a range of normal to above and below average temperature and precipitation years. Emitted δ13C-CH4 varied considerably across years, largely driven by water table dynamics, with summertime averages ranging from -62.5 to -72.6 ‰ across years, with lighter signatures in periods with low water table. Shrub encroachment that replaces sedge species reduced CH4 emissions and had a small impact on the isotopic signature of emitted CH4; during low water table years 13C-CH4 signatures of emissions were 3-5 ‰ higher in shrub sites. This suggests that CH4 transport by sedge's is reduced following shrub encroachment, leading to higher rates of CH4 oxidation. Across all years, porewater CH4 was heavier near the peat surface and increased with depth, consistent with higher rates of CH4 oxidation in surface peat. Porewater CH4 was consistently heavier than emitted CH4, with deviations as high as 20 ‰. This emphasizes the need for flux isotope measurements when projecting the impact of environmental change on atmospheric δ13 C-CH4. Together these results provide insights into the role of plant communities and variable environmental conditions in shaping CH4 production and consumption patterns in wetland ecosystems and how these changes will impact the isotopic composition of the atmosphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13J2416M
- Keywords:
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- 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0490 Trace gases;
- BIOGEOSCIENCES;
- 0497 Wetlands;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE