Changes in Physical Properties of Everglades Peat Soils Induced by Increased Salinity at the Laboratory Scale: Implications for Changes in Biogenic Gas Dynamics

Abstract

Saltwater intrusion due to sea level rise is a major concern for the Florida Everglades because it may induce shifts in ecosystem productivity and physical soil properties. However, the effects of saline water intrusion into the current carbon gas dynamics of the Everglades (particularly in terms of biogenic gas production and emissions, i.e., CH4 and CO2) are still uncertain. In this work, we present a laboratory‐based study to simulate how sea level rise may alter the physical properties (i.e., hydraulic conductivity) of peat soils from the Everglades and consequently affect the accumulation and release of biogenic gases within the peat matrix. Peat monoliths collected from the Everglades were subjected to progressive increases in salinity from a NaCl solution, and changes to the biogenic gas dynamics regime were simultaneously monitored using a combination of time‐lapse ground‐penetrating radar measurements, manometers, time‐lapse photography, and gas traps. Physical changes to the peat matrix at each salinity interval were assessed using constant head permeameter tests. Consistent with previous research, results show that a progressive increase in salinity (from fresh to saltwater) results in (1) a progressive increase in hydraulic conductivity and (2) a progressive decrease in gas content within the peat matrix (i.e., production) and gas releases. This work has implications for better understanding the potential effects of saltwater intrusion into freshwater peatland systems in the Everglades, particularly in terms of carbon gas dynamics.

Freeze-thaw cycles alter soil hydro-physical properties and dissolved organic carbon release from peat

“…frontiersin.org 2019), microhabitat (Morris et al, 2015), and soil pore water chemistry (Kettridge and Binley, 2010;McCarter et al, 2019McCarter et al, , 2020Sirianni and Comas, 2020). Branham and Strack (2014) observed that the K s of Sphagnum peat (pristine peat) was strongly positively correlated with the macropore occurrence (pore diameter of 2000 μm; Spearman correlation > 0.8).…”

Understanding Peat Soil Deformation and Mechanisms of Peat Collapse Across a Salinity Gradient in the Southwestern Everglades

“…This difference suggests that freshwater peat soils have an increased degree of pore dilation (approximately three times on average) when exposed to salt compared to peat soils previously exposed to elevated salinity where pore dilation has already occurred (i.e., Figure 7a). Previous work investigating the pore dilation effect in Everglades peat soils (i.e., Sirianni & Comas, 2020) found that a threshold may exist at approximately 0.25 S m −1 where the peat matrix loses some of its ability to dilate further despite further increases in salinity level and may represent the initial stages of peat collapse. Considering that average field salinity conditions at Site 1 and Site 2 are markedly larger than this threshold (i.e., 3.9 and 0.95 S m −1 , respectively), it is reasonable to expect that peat soils at Site 1 and Site 2 would not dilate to the same degree as peat soils at Site 3 as they are in a more permanently deformed state due to previous cycles of pore dilation and subsequent pore compression (i.e., during periods of no inundation).…”

“…Pore dilation in peat soils has been observed in several previous laboratory and field experiments in relation to changes in peat pore water salinity (e.g., Dijk et al, 2016;Kettridge & Binley, 2010;Ours et al, 1997;Sirianni & Comas, 2020). The pore dilation effect is explained by the relation between ionic strength and electrical double layer charge density in humic substances.…”

“…Gas content distribution within the soil was measured non-invasively using a set of two 1.2 GHz GPR antennas in transmission mode to collect ZOP profiles across each sample. See Sirianni & Comas (2020) for further details on a similar experimental setup for a different study. Error in gas content estimates from the CRIM were based on propagation of measurement errors (Bevington, 1969) following Comas et al (2011) and accounted for a total of <±1.0%.…”

“…In southwestern Florida, mangrove expansion into marsh areas has tracked this trend and the inland migration of marine conditions has already raised salinity levels within previously brackish marsh environments, stressing marsh vegetation and compromising their adaptive capacity (Andres et al., 2019; Krauss et al., 2011). Recent research has shown how saltwater intrusion may affect wetland biogeochemistry (see Herbert et al., 2015 for review) and hydrological properties (e.g., Comas & Slater, 2004; Sirianni & Comas, 2020; van Dijk et al., 2016) and suggests that coastal wetlands may experience accelerated soil decomposition, altered carbon storage and biogeochemical cycling, and changes in the physical properties of the peat matrix in response to sea‐level rise. Conservatively, and figuring a linear sea‐level rise of 2.24 mm yr −1 , over the next 100 years we can expect sea levels in southwestern Florida to rise by approximately 0.2 m. Considering that the greater Everglades (i.e., from Lake Okeechobee to Florida Bay) has a slope of approximately 0.045 m per 1,000 m (Fling et al., 2004), a sea‐level rise of 0.2 m over the next century would shift the coastline and coastal habitats inland a distance of 4,444 m. This equates to approximately 44 m of new area each year being exposed to marine conditions conducive to peat collapse.…”