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Browsing College of Science by Author "Ahn, Changwoo"
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Item A Study of Forested Wetland Soil Color and Biogeochemistry in the Region of Northern Virginia: Implications for Wetland Ecology and Management(2022) Schmidt, Stephanie A; Ahn, ChangwooSoil color patterns are essential to understand hydrologic regime and biogeochemical processes in wetland ecosystems. The Munsell Soil Color Chart (MSCC) has been traditionally and predominantly used to identify and quantify hydric soil field indicators that are based in soil color and redoximorphic features (RMFs) using Munsell hue (H), value (V), and chroma (CM). In the first part of this study, hydro-physicochemical (HP) settings and soil color attributes including redoximorphic features (RMFs) were assessed at four forested wetlands in Northern Virginia, USA, to identify whether four simply measurable HP attributes—inundation/saturation frequency, bulk density, soil moisture, and percent sand—can provide an explanatory framework for characterizing and classifying soil color attributes related to hydric soil field indicators. Study plots (n = 16) were grouped by site for initial characterizations and comparisons of HP (n = 4) and color attributes (n = 11); each attribute was additionally characterized and compared between three HP-based clusters formulated through k-means clustering analysis. Whereas only one HP attribute (inundation/saturation frequency) significantly differed between sites, all HP attributes but percent sand differed between HP-based clusters (p < 0.05), with PCA Dimensions 1 and 2 explaining over 80% of variability in plot HP attributes. Moreover, more sets of color attributes were significantly different when plots were grouped by HP-based cluster (n = 5: frequency of concentrations, non-matrix color count, hue, chroma, and depth to concentrations) compared to by site (n = 3: value, frequency of depleted matrices, depth to depletions) (p < 0.10). Simply measurable HP attributes are thus closely associated with certain soil RMF and color characteristics beyond site identity, potentially serving as a suite of measurements that can be adopted to assess and monitor RMFs indicative of wetland soils. As an alternative to the MSCC, several simple, low-cost alternatives have become recently available and may be able to complement the MSCC in soil color assessment. An intensive literature review on studies utilizing different methods was conducted to identify and quantify hydric soil colors and associated patterns; these include 1) the MSCC, 2) the Nix Color Sensor (Nix), 3) mobile phone camera (MPC) and medium-end digital camera photography, and 4) colorimetry and spectrometry. A review of these methods elucidates their respective strengths and weaknesses and highlights the importance of considering study-specific attributes in determining which method to choose for field studies of hydric soil colors. RMFs require methods capable of capturing small and heterogeneous soil surfaces and features such that the MSCC and digital photography are the most appropriate methods; on the other hand, the Nix provides rapid assessment of soil color that does not necessitate rigorous training to overcome biases that might come about in more subjective methods such as the MSCC. Overall, all alternative methods reviewed have their own merits and capacity to complement measurements made by the MSCC. While the MSCC is the most frequently used, well-established field method for reading soil color, the Nix is an inexpensive, app-based alternative that can complement or potentially substitute for the MSCC. Using the Nix and the MSCC, soil colors were measured from each forested site in Northern Virginia (n = 4). For each observed color, 3 MSCC variables and 15 Nix variables were collected in the field; a methodology was established to use these measured (M) variables to derive 9 Nix calculated (C) variables. A stepwise correlation identified Nix variables most suitable for relating the Nix to each of the MSCC attributes; ultimately, Munsell H, V, and CM were deemed to be best represented by HRGB calculated from the RGB color space (ρ = 0.56), L from the CIE–Lab color space (ρ = 0.73), and ẑ = Z/(X+Y+Z) from the XYZ color space (ρ = ˗0.80), respectively (p < 0.001). The corresponding explanatory powers of final Nix variables (i.e., HRGB, L, and ẑ) for H, V, and CM were 26%, 54%, and 62%, respectively (p < 0.01). Significant differences in ẑ between soils identified as hydric and nonhydric, but lack of nonoverlapping ranges, indicate a potential for the Nix to complement the MSCC in assessing wetland soil color in an accessible and reproducible manner, including hydric soil identifications for wetland delineation practices. Further study with more data over various types of soils is necessary to establish stronger relationships between the Nix and MSCC. Nonetheless, the method of characterizing soil color variables from the two field methods presented in the study can serve as a template for future applications including use as indicators of soil biogeochemistry or environmental education programs. Using the same collected color measurements from the Nix in four forested wetland soils within the Piedmont and Coastal Plain physiographic provinces of Northern Virginia (NOVA), the utility of the Nix for predicting carbon contents (TC) and stocks (TC stocks) from on-site color measurements was investigated. Both the Nix color variables (n = 15) and carbon contents significantly differed between sites, with redder soils (higher a and h) at Piedmont sites, and higher TC at sites with darker soils (lower values of L, or lightness; p < 0.05). Nix–carbon correlation analysis revealed strong relationships between L (lightness), X (a virtual spectral variable), R (additive red), and KK (black) and log-transformed TC (Ln[TC]; |r| = 0.70; p < 0.01 for all). Simple linear regressions were conducted to identify how well these four final Nix variables could predict soil carbon. Using all color measurements, about 50% of Ln(TC) variability could be explained by L, X, R, or KK (p < 0.01), yet with higher predictive power obtained for Coastal Plain soils (0.55 < R2 < 0.65; p < 0.01). Regression model strength was maximized between Ln(TC) and the four final Nix variables using simple linear regressions when color measurements observed at a specific depth were first averaged (0.66 < R2 < 0.70; p < 0.01). While further study is warranted to investigate Nix applicability within various soil settings, these results demonstrate potential for the Nix and its soil color measurements to assist with rapid field-based assessments of soil carbon in forested wetlands. Finally, a case study is presented and discussed that highlights the applications of the Nix in monitoring and assessing soil colors for wetland ecology and land management. Finally, within an Ecological Sustainability undergraduate class at George Mason University, a class project was designed and executed in which students investigated soil colors across campus green sites using the Nix. Students were given direction on measurement steps and techniques, including at which depths to collect colors, through a Standard Operating Procedure (SOP) made to be adaptable to various locations and/or soil types. Not only were students able to collect, store, and share soil color data for various locations across campus more rapidly than possible using the MSCC, but they also gained an understanding and appreciation for soil ecology and the importance of color as an indicator. With continued refinement and adaptation to intended use, the SOP herein presented has the potential to aid land/watershed planning by providing data on soil colors that can be tracked over time and may identify wetland areas, while also encouraging citizen science endeavors in soil ecology that can engage and connect communities to their belowground soilscape.Item A Study of the Development of Plant Community and Soil Properties in Mitigation Wetlands Created in the Virginia Piedmont, USA(2011-09-22) Dee, Suzanne M.; Dee, Suzanne M.; Ahn, ChangwooInvestigating the progress of created mitigation wetlands can provide useful information on current and future wetland design and management efforts, including monitoring activities legally mandated to ensure ecosystem development to properly mitigate the loss of natural wetlands. The study investigated structural vegetative and soil properties along with functional vegetative measures in four non-tidal freshwater wetlands created in the Piedmont region of Virginia. During the 2009 growing season, vegetation and soil samples were collected from wetlands ranging in age from 3 to 10 years. Vegetation attributes included percent cover (i.e., total, seeded, volunteer and non-native), richness (S), diversity (H'), floristic quality assessment index (FQAI), prevalence index(PI) and productivity (i.e., peak above and below-ground biomass). Soil condition attributes included soil organic matter (SOM), total organic carbon (TOC), total nitrogen(TN), C:N ration, gravimetric soil moisture (GSM), pH and bulk density(Db). There were no significant differences in vegetation percent cover, S, H' and FQAI by site. The lack of significant vegetation differences between sites was attributed to the abundance of a few common species. with the common soft rush, Juncus effusus, L., being the most dominant. However, significant site-based differences were detected for soil condition attributes (p<0.001), thus soil attributes were further analyzed using clustering statistics (60% dissimilarity applied), which resulted in four soil condition (SC) groups across the study sites. Vegetation data was then analyzed based on the SC groups. SC groups with greater SOM, lower Db, more circumneutral pH, and higher GSM, all indicative of maturity in wetland ecosystem development. were associated with higher H' and FQAI, and total and volunteer percent cover, and lower AGB, PI and seeded percent cover. A significant predictive relationship was found between peak AGB and other attributes of vegetation and soils (Standardized: AGB = 0.41H'+0.37PI-0.29SOM-0.24pH,R2=0.47, p<0.001), which can be of use in assessment of the functional trajectory of the wetlands. The outcomes of the study suggest that the inclusion of soil attributes can significantly enhance understanding and prediction of plant community development in created mitigation wetlands.Item Bacterial Community Composition and Physicochemical Properties of Soils in Created and Natural Wetlands in Virginia(2011-05-31) Peralta, Rita M; Peralta, Rita M; Ahn, ChangwooSoil properties are often studied along with vegetation to examine the status of ecosystem development in created wetlands. Soil bacterial communities are essential to the biogeochemical processes in wetlands, yet are rarely examined when assessing ecosystem development in created wetlands. We used two molecular methods, amplicon length heterogeneity polymerase chain reaction (LH-PCR) and multi-tag pyrosequencing (MTPS), of 16S ribosomal DNA to characterize the bacterial communities of soils collected from two created (i.e., LC and BR) and two natural wetlands (i.e., BN and BP) during the growing and non-growing season in the Piedmont physiographic province of Virginia, USA. Soil physicochemical attributes [i.e., percent moisture, pH, soil organic matter (SOM) (%), total organic carbon (TOC) (%), total nitrogen (TN) (%), and C:N ratio were also investigated. The measures of both bacterial communities and physicochemistry of soils were tested for any association or relationship, and examined within and between these wetlands, in terms of age and hydrologic connectivity to a surrounding fluvial system. Soil moisture was significantly higher during the growing season compared to non-growing season in the wetlands, primarily due to the natural wetlands (i.e., BN and BP), and an older and hydrologically better connected created wetland (i.e., BR) holding consistently higher soil moisture over two seasons. Soil pH of the wetlands ranged between 4.2 and 5.8, typical of the acidic soils of the Piedmont. Soil organic matter content were approximately 3 to 6 % with one of the LC wetland sites (LC1) and BN being consistently higher than in the other wetland sites. TOC and TN content followed the same pattern as SOM with no major seasonal differences. Soil bacterial community patterns were more clearly distinguished with MTPS (Ribosomal Database Project level 6: genus) method compared to LH-PCR, showing the merits of the new sequencing method. Based on the MTPS results there were many significant differences found in bacterial community patterns between the wetland sites (ANOSIM R > 0.5, p < 0.05), but with no differences between created and natural wetlands. However, one natural wetland (i.e., BP) with a lower soil pH seemed to be associated with least diverse bacterial communities. Bray Curtis dissimilarities of bacterial communities from each wetland site were correlated with C:N ratio (ρ = 0.43, p < 0.01) during the non-growing season, and with pH and SOM (ρ = 0.40, p < 0.01) during the growing season. In addition, relative abundance and distribution of major bacterial taxa varied both between the wetlands and between the seasons with certain taxa more sensitively responding than the others. The results also show phyla level correlations between certain major bacterial groups with Acidobacteria negatively correlated with Bacteroides (r = -0.70, Bootstrap adjusted p < 0.05), Firmicutes (r = -0.78, Bootstrap adjusted p < 0.05) and Actinobacteria (r = -0.48, Bootstrap adjusted p<0.05). When all plots of the wetlands studied were classified into three hydrologic connectivity (HC) settings the group with the higher HC (i.e., BR3) seemed to support higher bacterial community diversity. However, BN, the natural wetland with low HC, showed as high a bacterial community diversity measure as that of BR3. This might be attributed to the comparable soil moisture content in BR3 and BN plots, which were higher than the rest of the wetland sites/plots. Further investigation is needed on the relationship between bacterial community measures and physicochemical attributes in wetland soils, which may help us develop an useful microbial community indicator that can be used to track the functional progress of wetland soils in created wetlands.Item Multiple controls of carbon and nitrogen processes in freshwater wetlands of the mid-Atlantic United States with implications for ecosystem restoration and disturbance(2017) Korol, Alicia R.; Korol, Alicia R.; Ahn, ChangwooABSTRACTItem Recovery of Structure and Function of the Vegetation Community After a Disturbance as Affected by Initial Planting Richness in Created WetlandsMeans, Mary; Means, Mary; Ahn, ChangwooCreating and restoring wetlands is commonplace because many wetlands have been threatened or destroyed by urban expansion. Structural and functional aspects of created wetland ecosystems, however, remain poorly understood. One of the challenges facing created wetlands is ensuring that the ecosystem is able to recover after a disturbance, such as a mass herbivory eat-out or a large storm event. In this study, we use a controlled environment to examine how original planting diversity affects the ability of a created plant community to recover structurally (vegetation morphology and soil characteristics) and functionally (acquiring above ground biomass, potential denitrification and microbial respiration) after a disturbance. We used four macrophyte species, planted along a gradient of functional richness (FG 1 – FG 4) to assess community resilience based on morphometric measurements and biomass estimates. The vegetative results were compared to the two growing seasons prior to the disturbance. Soil biogeochemical characteristics were examined both during the growing season and the non-growing season. After the disturbance (i.e., aboveground harvesting and extensive soil coring), total mesocosm percent vegetative cover increased as planting richness increased, indicating that higher richness positively impacts the recovery of the overall plant community. The facultative annual and the reed both produced fewer and shorter stems than prior to the disturbance. The sedge had a shorter maximum canopy height. The facultative annual and the sedge were unsuccessful in monoculture with 50% failure of monocultures for both species. All four species produced less biomass one year after the disturbance as compared to the two years prior. The obligate annual was dominant where it was planted, contributing positively to total mesocosm cover and aboveground biomass, and providing support for the growth of other species. No seasonal differences were observed with any of the soil characteristics measured, although unplanted mesocosms (FG 0) were consistently different than those of higher planting richness, with lower bulk density, soil moisture, and C:N. Potential denitrification increased with increasing planting richness. Microbial respiration was initially much higher in unplanted mesocosms, however over the 324-day soil incubation, all functional groups converged at a moderate respiration rate. Our results indicate the importance of having a speciesspecific planting regime when creating/restoring wetlands to ensure the development of functional resilience.Item Sustainable Stormwater Management Using a Floating Wetland—a System ApproachMcAndrew, Brendan; McAndrew, Brendan; Ahn, ChangwooNitrogen is widely recognized as a chronic urban stormwater pollutant. In the United States, wet retention ponds have become widely used to treat urban runoff for quantity and quality. While wet ponds typically function well for the removal of sediments, nitrogen removal performance can be inconsistent due to poor design and/or lack of maintenance. Renovating ponds to improve their nitrogen capture performance, however, is typically expensive. A relatively untested technology called floating wetlands (FWs) has been proposed as a sustainable means of improving the nitrogen capture performance of stormwater wet ponds. The FWs are comprised of an artificial floating island that supports the hydroponic growth of plants on a pond, lake, or canal. As the plants grow on the floating island, their roots remove nitrogen directly from the water column and may trap waterborne sediments. Few studies have been performed on the effectiveness realworld stormwater systems, however. In this study, the nitrogen and sediment capture performance of a 50 m2 floating wetland deployed for 137 days on Mason Pond was investigated. A total of 2684 g of biomass was produced, 3100 g of sediment captured, and 191 g of nitrogen removed from the pond. Although biomass production was relatively low (53 g/m2), nitrogen uptake rate by the plants (0.009 g/m2/day) was comparable to contemporary FW studies. A system model was then developed from the collected data to simulate nitrogen removal performance of the FW on Mason Pond. The model was then used to test the nitrogen removal efficiency of the FW over longer deployment periods and with greater surface area coverage. While the literature suggests that FWs must cover at least 10-15% of the pond to significantly aid nitrogen removal, the model suggests only modest nitrogen removal efficiency (~6%) by an FW covering 25% of the surface of Mason Pond. These results may inform municipalities or developers that are considering the use of FWs on stormwater ponds.Item The Effects of Hydrologic Connectivity, Age-Relate Soil Properties, and Microtopography on Nitrogen Dynamics in Created Wetlands of the Virginia Piedmont(2011-05-12) Wolf, Kristin; Wolf, Kristin; Ahn, Changwoo; Noe, Gregory B.Wetlands are unique ecosystems that provide the essential ecosystem service of excess nutrient retention, processing, and removal from the landscape. In the early 1990s the U.S. adopted a “no net loss” policy towards wetlands in order protect these remaining wetland resources from the impacts of development. Under this policy the practice of wetland mitigation banking became the preferred method of compensating for “unavoidable” structural and functional impacts to natural wetlands. Created and restored wetlands, however, do not necessarily develop the same structural or functional capacity of their natural counterparts in the required five to ten year monitoring period. This study investigated four created non-tidal freshwater wetlands of varying ages and two natural non-tidal freshwater reference wetlands in the northern Virginia Piedmont. The purpose of this study was to identify how the design features of hydrologic connectivity and microtopography influence nitrogen cycling in created wetlands and investigate whether created wetland soils develop with age in order to support nitrogen processing and removal comparable to that of natural wetlands. Ammonification, nitrification, and net nitrogen mineralization (ammonification + nitrification) were determined using in situ incubation of modified ion exchange resin cores and denitrification potential was determined with denitrification enzyme activity. Principal component analyses were conducted on hydrologic and soil variables to identify hydrologic connectivity, soil moisture, and soil condition indices. Total nitrogen sedimentation and ammonification rates increased with the hydrologic connectivity index. Nitrification and denitrification potential increased with soil moisture, soil condition, and microtopographic indices. Net nitrogen mineralization also increased with soil condition index. Nitrogen flux rates demonstrated age-related patterns, with younger created wetlands having lower rates of ammonification, nitrification, nitrogen mineralization, and denitrification potential than older created wetlands, which had flux rates similar to natural reference wetlands. Results demonstrated a clear, but variable age-related trajectory of coupled soil and nitrogen cycling development in created wetlands that trend toward natural wetlands. Findings of this study support the incorporation of hydrologic connectivity and microtopography into the design and regulatory evaluation of created wetlands, as well the monitoring of soil development indicators in these wetlands, in order to improve their ecosystem service of water quality improvement through the functional development of nitrogen cycling.Item Urban Stream Restorations Increase Floodplain Soil Carbon and Nutrient Retention along a ChronosequenceNapora, Katrina; Ahn, ChangwooStream restoration is a common management practice to meet regulatory or voluntary efforts to improve water quality via carbon and nutrient retention, including in the Chesapeake Bay watershed. However, restoration projects have few quantifiable measures of project success, no standard metrics, and rarely collect pre-restoration data. Storage of nutrients, such as phosphorus (P), and carbon (C), in floodplain soils of restored streams can act as an easily quantifiable indicator of restoration success, particularly when the project goals include improved water quality. To determine how floodplains of restored streams change in their phosphorus and carbon storage as time since restoration increases, floodplain surficial soil samples (10 cm depth) were collected from 18 streams in the urbanized Piedmont region of northern Virginia, representing a chronosequence of time (1-10+ yrs.) since restoration as well as unrestored and reference streams. The samples were analyzed for total carbon (TC), total nitrogen (TN) and total phosphorus (TP) storage, whereas CO2 mineralization potential and equilibrium phosphorus concentration (EPC0) were measured as metrics of nutrient and carbon loss. These metrics were compared to time since restoration and potential environmental drivers, including soil moisture, pH, particle size, organic matter content, and degree of phosphorus saturation. These stream restorations demonstrated increasing nutrient storage for TC, TN, and TP along the chronosequence to values greater than both unrestored or reference streams, as well as decreasing C mineralization turnover and no significant changes in EPC0. Soil wetness and organic matter, key drivers in nutrient retention, also increased as restoration projects aged increasing nutrient and C storage. Overall, stream restoration did improve carbon and nutrient retention in floodplains as compared to unrestored sites and exceeded those of low urbanization 'reference' sites.