Spatial and temporal variability in methane emissions from lowland pastures

Background/Question/Methods

Pastures are globally widespread and play an important role in greenhouse gas exchange and terrestrial carbon (C) storage. C uptake by pasture grasslands may offset methane (CH₄) emissions from grazing cattle, however pastures also emit CH₄ from seasonally flooded soils. Soil CH₄ fluxes are poorly quantified, vary widely over spatial and temporal scales, and are easily underestimated if emission hotspots or episodic fluxes are overlooked. Quantifying these fluxes can be challenging and requires a multifaceted approach to observe variable CH₄ emissions. We used three methods to quantify spatially and temporally variable CH₄ emissions from a south Florida pasture. We used standard static chamber and eddy covariance techniques to quantify spatially and temporally variable CH₄ fluxes from pastures and the major landforms within the pasture landscape (canals, ditches, wetlands). We also interpolated CH₄ emission maps for the pasture (dry and wet season) using high-resolution CH₄ concentration data collected with a mobile cavity-ringdown spectrometer and GPS. Our work is the first to use mobile spectrometers to map biogenic CH₄ emissions at the landscape scale. Combining these approaches allows for a more comprehensive understanding of variable CH₄fluxes.

Results/Conclusions

Methane fluxes varied significantly between landforms and seasons. During the dry season, the main drainage canals emitted CH₄ (~11.8 mg CH₄ m^-2 hr^-1), while no emissions were observed from pasture, ditches, and wetlands. All landforms emitted CH₄ in the wet season, and canal and wetland fluxes (~12.6-32.8 mg CH₄ m^-2 hr^-1) were significantly higher than pasture and ditch fluxes (~5.5-6.0 mg CH₄ m^-2 hr^-1). Eddy covariance measured CH₄ fluxes varied seasonally, and sustained CH₄ emissions were observed during the wet season and positively correlate to soil volumetric water content. Mobile CH₄ concentration surveys revealed that wetlands are the dominant wet season emission source, and that the entire pasture landscape emitted CH₄ during the wet season and exhibited average near-surface CH₄ concentrations ~33% above background atmospheric concentrations. Negligible CH₄ uptake was observed in both seasons, indicating pasture soils are not a significant atmospheric CH₄ sink. Our results demonstrate that pasture CH₄ emissions exhibit high spatial and temporal variability, and high magnitude emissions are driven by wet season rainfall that flood low-lying landforms.