Investigating carbon flux variability in subtropical peat soils of the Everglades using hydrogeophysical methods

Abstract

The spatial and temporal variability in accumulation and release of greenhouse gases (mainly methane and carbon dioxide) to the atmosphere from peat soils remains very uncertain. The use of near-surface geophysical methods such as ground penetrating radar (GPR) has proven useful during the last decade to expand scales of measurement as related to in situ gas distribution and dynamics beyond traditional methods (i.e., gas chambers). However, this approach has focused exclusively on boreal peatlands, while no studies in subtropical systems like the Everglades using these techniques exist. In this paper GPR is combined with gas traps, time-lapse cameras, gas chromatography, and surface deformation measurements to explore biogenic gas dynamics (mainly gas buildup and release) in two locations in the Everglades. Similar to previous studies in northern peatlands, our data in the Everglades show a statistically significant correlation between the following: (1) GPR-estimated gas content and gas fluxes, (2) GPR-estimated gas content and surface deformation, and (3) atmospheric pressure and both GPR-estimated gas content and gas flux. From these results several gas-releasing events ranging between 33.8 and 718.8 mg CH 4 m À2 d À1 were detected as identified by the following: (1) decreases in GPR-estimated gas content within the peat matrix, (2) increases in gas fluxes captured by gas traps and time-lapse cameras, and (3) decreases in surface deformation. Furthermore, gas-releasing events corresponded to periods of high atmospheric pressure. Changes in gas accumulation and release were attributed to differences in seasonality and peat soil type between sites. These results suggest that biogenic gas releases in the Everglades are spatially and temporarily variable. For example, flux events measured at hourly scales were up to threefold larger when compared to daily fluxes, therefore suggesting that flux measurements decline when averaged over longer time spans. This research therefore questions what the appropriate spatial and temporal scale of measurement is necessary to properly capture the dynamics of biogenic gas release in subtropical peat soils.

Ebullition of methane from peatlands: Does peat act as a signal shredder?

“…They recorded 339 pressure periods (periods during which atmospheric pressure was consistently increasing or decreasing), but only in 28% of these periods did episodes of ebullition occur. In addition, and contrary to Tokida et al [] who found that falling atmospheric pressure was a trigger, increases in pressure can also cause ebullition [ Comas and Wright , , ; Klapstein et al , ]. It is also possible that ebullition from peat may be similar to ebullition from lakes [ Wik et al , ], where changes in atmospheric pressure trigger large degassing events that exhaust the available store of gas.…”

Gaseous Carbon Emissions (Methane and Carbon Dioxide) from Wetland Soils in a Re-created Everglades Landscape

“…Their results show that the production and efflux of CH4 from these areas is very highly variable, both spatially and temporally (Comas and Wright, 2014). The nature of these ebullition events is such that they accumulate under flooded soils, until they have enough mass to be able to bubble up through the soil.…”

“…First, the timing and the amount of ebullition events are highly variable and unpredictable (Comas et al, 2014). The gas traps used in this experiment at LILA were relatively small compared to those used in prior studies, but they were intended to be minimally invasive to the surrounding soils.…”

“…Using larger vials could provide a larger window in which to collect appropriate volumes for samples, rather than trying to time it so that gas volumes were emitted between a 2-20 ml range. Comas and Wright (2014) have also recently shown a high degree of variability in results from studies on ebullition in nearby Everglades areas.…”

“…Previous work at LILA has utilized this method to examine the relationship between hydrology and CO2 efflux (Schroeder, 2012), finding that rates of CO2 efflux generally decreased as flooding increased ( Figure 7). In deeper areas where soils remain permanently flooded, various methods have attempted to quantify ebullition by collecting bubbling biogenic gases in "traps" (Comas et al, 2014). This gas trap method can be as simple as attaching vials to inverted funnels in submerged areas.…”