Assessing Hydrology, Carbon Flux, and Soil Spatial Variability Within Vernal Pool Wetlands

Author

Bianca N. Ross, University of Rhode IslandFollow

Date of Award

2017

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Department

Interdepartmental Program

First Advisor

Mark Stolt

Abstract

Vernal pools are small isolated wetlands that are covered by shallow water for variable periods in the winter and spring but may be completely dry for most of the summer and fall. Despite their small size, vernal pools are a dominant wetland type throughout New England. These wetlands are hotspots of floral and faunal biodiversity, as their unique hydrology and landscape characteristics allow them to serve as a home and breeding ground for many distinct plant and animal species. Because of their abundance in New England, vernal pools may also be an important long-term regional storehouse for organic carbon. Despite the functional and ecological values of vernal pools, few studies have investigated how variations in hydrology, pool size, geomorphic setting, and surrounding landscape attributes affect soil carbon in these systems and the associated hydric soils that sequester the carbon. Therefore, the primary objectives of this thesis were to i) assess the effect of landscape characteristics on hydrologic and edaphic conditions; ii) investigate the need for additional hydric soil indicators for vernal pool soils; iii) quantify the relationship between vernal pool hydrology and greenhouse gas fluxes; and iv) evaluate processes of carbon cycling throughout vernal pools.

Twenty-one vernal pools in southern Rhode Island were selected and their landscape attributes were characterized using spatial tools in GIS. Sixteen of the 21 pools formed in glaciofluvial deposits on outwash plains, kame terraces, and moraines. The rest formed in till or alluvial landscapes. Vernal pool basin areas ranged from 6 to 381 m² and mean slopes of the adjacent landscape ranged from 3 to 20%. Slope class was not significantly correlated to basin area (R²= 0.03). Four vernal pools were selected for detailed studies of hydrology, soils, and vegetation in the basin, transition, and upland zones. Water table levels were monitored in each hydrologic zone from June 2015 to October 2016. Median water table levels increased in depth from the soil surface with increased distance from the vernal pool basin. Basin zones were consecutively inundated for the longest period of time, followed by transitional zones; upland zones were never inundated. Water table gradients indicated discharge into pool basins for the majority of the year. Vernal pools with steep slopes showed recharge gradients during periods of significant inundation suggesting a relationship between slope class and hydrology. Vernal pool soils classified as Spodosols, Inceptisols, and Histosols. Although all of the basin and transitional zones met the saturation requirements for hydric soils, 25% of the soils did not meet a hydric soil indicator. Both of these soils were Spodosols, suggesting the need for continued evaluation of hydric soils with spodic morphologies. Twenty-one plant species were identified across all study sites. Specifically, basin zones were dominated by obligate wetland plants, while transitional and upland zones consisted primarily of facultative and facultative wetland plants. Analysis allowed for the identification of plant species that accurately reflected the hydrologic nature of each zone, which affirmed the relationship between vegetation and hydrology.

Carbon pools and the contributions to the wetland soil carbon cycle, including leaf litter additions, decomposition of coarse woody debris and leaves, and CO₂ from respiration, were monitored during the majority of two growing seasons. On average, basin and transitional zone soils possessed the largest soil organic carbon (SOC) pools (11 kg m^-2), while SOC pools in upland zones were substantially less (8 kg m^-2). Leaf litter additions ranged from 40 to 149 g C m^-2 depending on the site and hydrologic zone. Leaf litter bags and wooden dowel rods (representing coarse woody debris) were placed at the surface of each zone in order to investigate above-ground decomposition. Dowel rods were also inserted vertically into the soil to 25 cm in 2015 to investigate below-ground decomposition. On average, basin zones experienced the highest leaf litter loss (12 g C m^-2; 18%), while upland zones experienced the lowest loss (6 g C m^-2; 10%). Basin zones also exhibited the highest mean surface dowel loss in 2015 (4%). Losses from 2016 followed similar trends, but lower precipitation experienced by the pools resulted in significantly lower losses. On average, the highest below-ground decomposition occurred in the upland zone (16%). Analysis comparing vernal pool hydrology to organic matter decomposition yielded results that trended toward significance in 2015 for leaf litter (p = 0.06) and ground dowels losses (p = 0.07). Above-ground decomposition decreased as the water table receded further from the soil surface; conversely, below-ground decomposition increased as the water table receded further from the surface. Carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes were measured between the months of August and November in 2015, and between May and August in 2016. CO2 flux was significantly correlated with soil temperature (p < 0.001) and hydrologic zone across all study sites (p = 0.03). Flux increased as soil temperature increased and as the depth between the soil surface and the water table increased. Although the partitioning of heterotrophic and autotrophic respiration yielded weak correlations (R² < 0.5), analysis suggested that root respiration accounted for < 40% of the total CO2 flux. Mean monthly CO₂ fluxes were highest in the transitional zones, ranging from 5 to 22 μmole m^-2 min^-1 across all zones. CH₄ flux was significantly correlated with vernal pool hydrology in all study sites (p < 0.003). Positive emissions ranged from 0.02 to 0.03 μmole m^-2 min^-1 and only occurred in the basin zones during the months of May and June, when the basins were inundated with surface water. Transitional and upland zones exhibited net CH4 absorption, as did the basin zones during months other than May and June. Nitrous oxide fluxes ranged from -0.009 to 0.008 μmole m^-2 min^-1 and did not vary significantly with hydrologic zone. Despite their small size and ephemeral nature, their significant contribution to greenhouse gas efflux and removal from the atmosphere warrant future investigations and conservation of vernal pools.

Recommended Citation

Ross, Bianca N., "Assessing Hydrology, Carbon Flux, and Soil Spatial Variability Within Vernal Pool Wetlands" (2017). Open Access Master's Theses. Paper 1014.
https://digitalcommons.uri.edu/theses/1014