In the last few centuries, many streams in the eastern United States have been severely disturbed by land use change and are now disconnected from their original floodplain due to the aggradation of legacy sediment. Currently, stream-floodplain reconnection is advocated as a stream restoration practice to take advantage of ecosystem services. The objective of this study is to compare two current stream restoration approaches for their nutrient flushing ability: 1) a conventional approach leaves legacy sediment on the floodplain; and 2) an ecological approach that involves removing the accumulated legacy sediment in order to restore the original floodplain surface wetland, revealing a buried A soil horizon. Soil cores were taken from the surficial legacy sediment layer and the buried A soil horizon in the floodplain of a 550-meter reach of Stroubles Creek in the Valley and Ridge province near Blacksburg, VA, to evaluate potential for flushable DOC, TDN, NO3-, NH4+, and SRP content. In addition, an inundation model was developed to evaluate the extent of flooding under the two restoration scenarios. The inundation model results and nutrient flushability levels were then used to simulate the release of nutrients as a function of stream restoration approach. Results indicate that the buried A horizon contained less flushable nutrients, but the ecological restoration would have a higher frequency of inundation that allows for more flushable nutrient release at the annual scale. Understanding the nutrient release potential from the floodplain will provide the ability to estimate net nutrient retention in different stream-floodplain reconnection strategies. / Master of Science

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Böden aus Mudde entstehen in ehemaligen Seen, meist unter Luftzutritt infolge von künstlicher Entwässerung. Entwässerung und Nutzung führen zu einer sekundären Pedogenese, einhergehend mit der Veränderung der chemischen und physikalischen Bodeneigenschaften. Die nachhaltige Nutzung dieser meist jungen Böden ist von besonderer Bedeutung, da es sich überwiegend um Niedermoorflächen mit großer Bedeutung der Filter- und Speicher- sowie der Lebensraumfunktion handelt. Dabei ist, um negative Konsequenzen ihrer Entwicklung zu verhindern, folgende Wirkungskette zu untersuchen: Entstehung des subhydrischen Sedimentes -> Entwässerung -> landwirtschaftliche Nutzung und Bodenentwicklung -> Degradation. In Nordeuropa sind Böden aus Mudde vor allem in Norddeutschland und Polen sowie Finnland und Schweden zu finden. Es wurden 60 Bodenprofile auf 13 Untersuchungsflächen in Deutschland und Polen detailliert beschrieben und untersucht. Dabei erfolgte für jeden Horizont die Entnahme von ungestörten Proben mittels Stechzylindern für die Analyse der bodenphysikalischen Parameter (pF-Kurven, Trockenrohdichte) und von gestörten Beutelproben für die chemische Analytik (Glühverlust, Kalkgehalt, Elementgehalte mittels RFA und ICP). Bisher noch nicht beschriebene Horizonte wurden vorzugsweise anhand des Bodengefüges identifiziert. Mudden unterscheiden sich in ihrer mineralischen Zusammensetzung erheblich von allen anderen Ablagerungen. Der Anteil an organischer Substanz und Kalziumkarbonat beträgt zwischen 5 und 95 %. Wegen ihres hohen Anteils an kleinen Partikeln weisen sie ein sehr großes Porenvolumen bis zu 90 % auf, wobei die Mittel- und Feinporen mehr als 50 % ausmachen. Das Porenvolumen steigt mit dem Anteil an organischer Bodensubstanz. Als Folge der Bodenentwicklung verringert sich das Porenvolumen bei gleichzeitiger Zunahme des Anteils der Feinporen auf Kosten der Mittelporen. / The formation of gyttja soils takes place in former lakes, predominantly under aeration. This phenomenon of now terrestrial, but formerly subhydric soils (so called gyttja soils) is mainly due to artificial drainage. Their cultivation led to a secondary pedogenesis, which had an effect on their chemical and physical properties. Thus, the sustainable use of this special type of very young land (subhydric soils) is of special importance. Hence, special emphasis has to be directed towards evaluating sequential changes: formation of sediment under water -> drainage/drying -> agricultural use/pedogenesis -> degradation/devastation in order to prevent negative consequences for the environment. In Northern Europe, gyttja deposits can be found in Northern Germany, Finland, Sweden, and mainly in Poland. 13 sites in Germany and Poland with gyttja soils have been investigated. A total of 60 profiles are described in detail. For each horizon, undisturbed samples for physical (tension curves for pF 1.5, 1.8, 2.0, 2.5 and 4.2, dry bulk density) and disturbed samples for chemical analysis have been taken (loss on ignition, CaCO3 content, X-ray fluorescence spectroscopy). Previously not described horizons could be visually distinguished mainly by their structure. Subhydric soils have a very specific composition, which differs greatly from other soils. The amount of organic matter and CaCO3 ranges from 5 to 95 %. Almost all sediments contain a high amount of very small particles and, as a consequence, a high pore space volume of up to 90 %. The amount of the medium and micro-pores is more than 50 %. The porosity increases with the content of organic matter. As a result of pedogenesis, the porosity decreases while the volume of micro-pores increases and the volume of medium pores decreases.

Environmental perturbations are ubiquitous features of ecosystems and shape ecological structure and function. Climate change will alter the intensity and frequency of disturbances and expose ecosystems to novel combinations of useful inputs (subsidies) and harmful inputs (stressors). Coastal wetlands are particularly vulnerable to changing environmental conditions and are increasingly exposed to effects of interacting subsidies and stressors. In particular, the Florida Coastal Everglades, which has experienced accelerated change due to a history of water management practices, is vulnerable to new disturbances associated with climate change. The low-lying Florida Everglades faces multiple disturbances from storm surge, nutrient enrichment, and sea-level rise which will influence its responses to future environmental perturbations. Microbial communities are often used to characterize environmental change because of their high surface area to volume ratio, permeable membrane, and quick turnover rates. Therefore, assessing how microbial function changes can provide insights into how subsidies and stressors interact to alter biogeochemical cycles. I tested how nutrient enrichment can alter ecosystem responses to stress and found that it did not promote recovery in mangrove plants. I examined how long-term exposure to salinity and phosphorus (the limiting nutrient in the Everglades) affected microbial enzyme activity and found that salinity alone acts as a suppressor of enzyme activity but phosphorus addition can mitigate salinity stress in sawgrass soil. I tested how pulses of salinity can affect the microbially-mediated breakdown of organic material and found that the microbial community was functionally redundant and unaffected by saltwater pulses; however, microbial activity was consistently lower in the brackish marsh compared to the freshwater marsh. I investigated how gradients of salinity and phosphorus affected freshwater and brackish soils and determined previous exposure to saltwater intrusion dominates affects microbial function and soil composition. Across these experiments, I found that environmental perturbations alter the microbial-mediated processing of nutrients and carbon, and legacies of previous disturbances influence the microbial response to new disturbance regimes.

Indiana University-Purdue University Indianapolis (IUPUI) / The transfer of nutrients from US Midwest croplands into surface waters causes eutrophication and a decline in water quality. Temporary retention of nutrient-rich runoff in constructed wetlands can help mitigate these negative impacts through physical entrapment and biological transformation of nitrogen (N) and phosphorus (P). However, with the expectation that wet-dry periods will be more frequent in the region, there is a need to better understand the mechanisms that control nutrient retention and release in US Midwest wetlands constructed on former croplands. In this study, soil cores (30 cm long, 20 cm diam) were collected from two constructed wetlands (4 and 8-yr old), and the surface (0-20 cm) and subsurface (40-60 cm) layers of a cropland where a constructed wetland will be constructed in the future. Soil cores were subjected to either a moist or a dry treatment for 5 weeks, and then flooded with stream water (water depth 6 cm). The flux of nutrients, N2O, cations, and variation in floodwater chemistry (pH and ORP) were monitored for another 5 week period. Porewater was tested during the final 3 weeks of the experiment. Nitrate (0.1-130 mg N m-2 d-1) and inorganic P (Pi) fluxes (0.09-2.9 mg P m-2 d-1) were significantly higher in the dry treatment cores. Regardless of site, the dry treatment also resulted in higher floodwater NO3- concentrations suggesting organic matter mineralization and mineral N build up during the drying phase. However, this initial NO3- release was rapidly denitrified as indicated by the sharp increase in N2O production during that period. In contrast to N, the release of Pi was significantly higher in cores from the cropland. Soil at these sites had higher water extractable Pi and total P. Contrary to the study hypothesis and the results of previous studies, Pi concentration in floodwater and porewater was not correlated with dissolved Fe suggesting that reductive dissolution was not the dominant process controlling P release in US Midwest mineral soils developed from calcareous glacial till. Rather, variation in Ca2+ concentration and its relationship with Pi suggest that dissolution of Ca-containing minerals may be more important and should be the focus of future studies examining the geochemistry of P in these constructed wetlands.

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