Soil indicators of hydrologic health and resilience in cypress domes of West-Central Florida

Highlights

• Accumulation of organic matter in cypress wetlands retains soil moisture.

• Extensive soil saturation is needed to preserve wetland resilience.

• Altered water balances that shift plant communities reinforce wetland degradation.

• Soil moisture sampling may determine groundwater withdrawal impacts to wetlands.

• Soil carbon isotopes indicate wetland plant community shifts to drier conditions.

Abstract

Groundwater pumping in west-central Florida has caused water table declines that disrupt hydroperiods and negatively impact already threatened, geographically isolated freshwater wetlands in this region. Currently, aboveground biological indicators are used to determine the health status of wetlands impacted by groundwater withdrawals, however some soil properties, such as organic matter content, are partly a function of the duration of saturation and could be used as additional indicators. Therefore, this study evaluated soil organic carbon and related soil properties for comparison with the biological indicators of wetland health status. Soil samples were taken from the top 30 cm of the land surface (i.e., bulk density, soil water content, nitrogen content, and carbon and nitrogen isotopic composition) within pond cypress (Taxodium ascendens) domes previously categorized by the local water management district using wetland health classifications of “healthy”, “significantly changed”, or “severely changed”. Mean soil water content (w) was significantly higher in “healthy” wetlands as compared to “significantly changed” and “severely changed” sites, and “healthy” sites also tended to have lower bulk density and higher soil carbon and soil nitrogen content. This trend was most pronounced toward the center and lowest elevation of the wetland, presumably where soils remain saturated longest. Stable carbon isotope values (δ¹³C) of soil organic matter showed that “severely changed” sites exhibited more ¹³C-enriched values that arise from long term contributions of warm-season (C₄) grasses, consistent with the observed upland grasses encroaching into the most impacted wetlands. Notably, soil characteristics may be crucial for determining the resilience of wetlands, as one sampled “healthy” site contained comparably less soil organic matter, reducing the water holding capacity and leaving it more vulnerable to water table declines. These results suggest that extensive annual saturation of soils is required to maintain the cypress dome ecological community structure and preserve wetland health and resilience. Frequent monitoring of soil water may prove a convenient proxy for soil water retention properties and ecosystem health of wetlands in similar settings.

Introduction

Wetlands are some of the most important habitats worldwide due to their high biodiversity and crucial ecosystem services, yet they are increasingly threatened by human activities and urbanization that divert water, reduce wetland area, and modify ecosystem processes (Gibbs, 2000, Zedler and Kercher, 2005, McCauley et al., 2013, Creed et al., 2017, Golden et al., 2017). One such threat, groundwater pumping, causes water-table declines that can negatively affect wetlands in the surrounding regions. While damage to individual wetlands is a great concern, the overall impact to hydrology at the landscape level warrants further attention as well (Rains et al., 2016). With urban demand for freshwater resources increasing (McDonald et al., 2011), ecosystem stresses from groundwater pumping are not likely to subside. In Florida, wetland area has already decreased approximately 50%, from an estimated 82,000 km² in 1845 down to 46,000 km² in 1996 (Dahl, 2005). Further, wetlands in west-central Florida now only cover approximately 15% of the total land area in the Tampa Bay Watershed (Landry, 2010). Wetlands such as cypress domes are particularly vulnerable to lowering water tables because they are geographically isolated wetlands – wetlands surrounded by uplands (Tiner, 2003, Leibowitz, 2015, Marton et al., 2015, Mushet et al., 2015).

The Southwest Florida Water Management District (SWFWMD) is responsible for permitting groundwater withdrawals while also monitoring wetland hydrologic and ecological status for indications of harm from those withdrawals. Since instrumenting and monitoring all wetlands in this region on a long-term basis is not feasible, scientist from SWFWMD developed a cypress dome assessment method based on several biological indicators (i.e., elevation of Lyonia lucida root crown bases) that approximate the long-term hydrology (Carr et al., 2006, Hull et al., 1989).

Wetland vegetation is partly controlled by the quality of the soil and, conversely, changes in the aboveground vegetation are chronicled by changes in the quantity and quality of organic inputs to the soil. This can create a positive feedback between the belowground and aboveground components of the habitat, strengthening some soil characteristics. Furthermore, the current composition and texture of the soil is, in part, a function of the duration of saturation, which is directly related to the accumulation of organic matter. Thus, soil characteristics could also provide a long record of ecological and hydrological status.

Soil organic matter accumulates in the upper horizons of hydric soils in wetlands where saturated conditions lead to slow, anaerobic biomass decomposition (Ewing and Vepraskas, 2006, Tiner, 2003). While these rates are controlled by climatic, edaphic, and biotic processes (Schimel et al., 1994), studies of soil catena from uplands to wetlands provide a means to isolate edaphic and biotic processes, such as those deriving from changes in water table position in impacted wetlands (Harden et al., 2002, Wynn et al., 2006, Yoo et al., 2006). While soil temperature has a strong influence on the rate of decomposition, increasing soil water content (w) lowers soil oxygen content which eventually leads to a slower, anaerobic decomposition rate.

In wetlands that remain saturated for much of the growing season, the rate of organic inputs to the soil pool from litter and roots exceeds the rate of organic matter loss from decomposition, resulting in an increase in the pool of soil organic matter, and thus soil organic carbon (SOC) content (Jastrow et al., 2006, Schlesinger and Andrews, 2000). Sustained periods of such conditions lead to long-term accumulation of SOC (Amundson, 2001, Chun-Yan et al., 2006). However, groundwater pumping causes water table declines in overlying wetlands, reducing the extent and duration of inundation, which leads to more energetic aerobic decomposition of the accumulated pool of soil organic matter (Caldwell et al., 2007, McPherson et al., 1976, Vepraskas and Craft, 2016). If this increased rate of decomposition is not matched by higher rates of organic inputs, sustained water table declines lead to a long-term reduction of soil organic matter and SOC stocks (Amundson, 2001, Bridgham et al., 2006, Maltby and Immirzi, 1993). Similarly, changes in soil nitrogen content is positively correlated with changes soil organic carbon, and further, the ratio of carbon to nitrogen can indicate if mineralization or immobilization is the dominant decomposition process.

Analysis of stable isotopic ratios of soil organic carbon can reveal shifts in wetland plant communities as hydrology changes. All plants are depleted in the heavy, stable isotope of carbon (¹³C) compared to the atmosphere due to physical and enzymatic processes during photosynthesis (Brugnoli and Farquhar, 2000), with warm-season or C₄ grasses less depleted in ¹³C than C₃ vegetation which consists of most other non-succulent plants (trees, shrubs, lianas, most forbs, cool-season grasses, etc). This isotopic difference enables researchers to estimate the relative contribution of original plant communities with different photosynthetic pathways to soil organic matter. In and around the wetlands studied, C₃ vegetation is generally found in wetland and marsh habitats, whereas the limited C₄ vegetation, consisting of warm-season grasses better adapted to low soil water content, is more likely to occupy drier or upland areas. For example, in a study of coastal marshes along northwest Florida, (Choi et al., 2001) found that soil organic matter in deeper soils was less depleted in ¹³C than at the surface, which they interpreted to mean that the landscape was dominated by C₄ vegetation before sea level rise and the landward expansion of the current C₃-dominated marsh.

In west-central Florida, the organic-rich O and A horizons found in isolated wetland soils are typically absent from adjacent upland soils (NRCS, 2008). Given these observations and the basis of carbon dynamics in wetland soils described above, we hypothesized that SOC content should measurably differ along a continuum of wetlands variously impacted by water-table declines caused by groundwater pumping or other types of drainage. In addition, soil properties related to SOC content (i.e., soil water content, bulk density, nitrogen content, and carbon and nitrogen isotopes) may covary with SOC in wetlands of differing impacts.

The aim of this study was to answer several research questions. First, is the current status classification system for the condition of these wetlands sufficient for assessing ecological and hydrological resilience? Second, are there measurable soil characteristics that could aid in identifying wetlands that have sustained impacts from water table withdrawals? Finally, could soil properties related to cypress dome health in this region be applied widely to other geographically isolated wetlands? Therefore, the goals of this research were to test whether (1) soil organic content statistically correlates with the biological indicators of wetland status and (2) physical and biochemical soil properties can serve as proxies for soil organic matter, and therefore the health of wetlands in response to groundwater withdrawals.