A Spatial Study of Soil Organic Matter in Two Central Pennsylvania Created Wetlands
Open Access
- Author:
- Gordon, Katherine Dorothy
- Graduate Program:
- Forest Resources
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 29, 2009
- Committee Members:
- Wayne Lawrence Myers, Thesis Advisor/Co-Advisor
Charles Andrew Cole, Thesis Advisor/Co-Advisor
Wayne Lawrence Myers, Thesis Advisor/Co-Advisor - Keywords:
- soil organic matter
created wetlands
carbon cycle - Abstract:
- Wetlands provide society with a variety of valuable ecosystem goods and services. Section 404 of the Clean Water Act regulates wetlands and gives the USACE the authority to issue permits that allow dredging or filling wetlands. These section 404 permits often require the creation or restoration of wetlands elsewhere on the landscape to mitigate for the loss of the area and function of the impacted wetlands. While wetland mitigation is widely practiced, there is increasing evidence that many of these created wetlands do not replace the functions of the impacted sites. SOM has been proposed as a good indicator of soil quality and is related to a number of wetland functions, including water storage and the retention or transformation of nutrients and pollutants. Many studies have found that created wetlands contain significantly lower concentrations of SOM than natural wetlands and some studies suggest that SOM does not increase with wetland age. Limited pools of SOM may impact the ability of created wetlands to function similarly to natural wetlands. More information about the patterns of SOM concentration in created wetlands is necessary to understand the functioning of these wetlands and to inform the design of future created wetlands. The objectives of this study were to examine patterns of processes potentially related to the accumulation of SOM in two central Pennsylvania created wetlands and to develop a carbon budget for one of the created wetland study sites. The hypotheses tested include 1) the concentration of SOM is greater in areas that are more frequently inundated or saturated, with high biomass, low litter quality and low rates of soil respiration and 2) the exports of carbon via respiration and DOC export are limiting the accumulation of SOM in a created wetland. The study sites, Stewart 1 and Stewart 2, are located in central PA and were constructed in 2002/2003 as mitigation for impacts to wetlands during the construction of I-99. A previous study included these sites in a comparison of the functional capacity of created and reference natural wetlands and found that the created sites functioned differently than the natural wetlands and contained significantly lower levels of SOM. Since SOM accumulates when inputs of organic material (e.g., primary productivity or hydrologic import) are greater than outputs (e.g., decomposition or hydrologic export) some of these controlling processes were assessed. The vegetation communities at each site were mapped and biomass (a proxy for net primary productivity), biomass C:N ratio (a proxy for decomposition) and soil respiration (a proxy for decomposition) were sampled within each community. Wetland hydrology was also assessed through the use of automatic water level recorders coupled with detailed elevation information obtained from lidar and topographic surveys. Soil samples were collected from each wetland site on a 20 m grid and were analyzed for SOM using a Loss on Ignition protocol. The relationships between the measured variables and SOM (concentration and density) were assessed using a spatial regression analysis. A strong negative correlation between bulk density and SOM concentration was able to explain 69-74 percent of the variance in SOM concentration at Stewart 1 and about 45- 47 percent of the variance in SOM concentration at Stewart 2. SOM densities were significantly spatially autocorrelated at both sites. At Stewart 2, the negative correlation between percent time inundated or saturated and SOM density was able to explain 24-27 percent of the variance in SOM density. The hypothesis that the concentration of SOM is greater in areas that are more frequently inundated, with low litter quality and low rates of soil respiration was not supported by the results of this study. Biomass was insufficiently sampled to draw any definitive conclusions about the relationship between productivity and SOM. It is likely that the relationship between hydrology and SOM is largely due to the effect of hydrology on productivity, with low levels of SOM found in the open water areas where NPP is low and the litter quality of the aquatic vegetation is high. Additional research is needed to determine whether similar relationships are present in other created wetlands and to further investigate the links between SOM accumulation and NPP, litter quality, soil respiration, hydrology and topography. Studies should examine whether alternate wetland designs can promote the accumulation of SOM. Depending on the assumptions made when developing the carbon budget for Stewart 2, that site could either be a source or a sink for carbon. The hypothesis that the loss of carbon via respiration and DOC export are limiting the accumulation of SOM in Stewart 2 could not be confirmed or rejected. Further investigations are necessary to better quantify the carbon budget of created freshwater wetlands.