<p>Converting a nutrient management plan from commercial fertilizers to poultry litter helps effectively utilize waste from the nearly 10 billion broiler birds across the United States. Nine field scale watersheds from the USDA ARS Grassland, Soil and Water Research Laboratory near Riesel, TX were evaluated for P inputs and P outputs to determine phosphorus budgets for 15 years of annual application of poultry litter ranging from 75 – 219 kg P ha^-1 yr^-1 on cultivated and pasture/grazed fields. The cumulative net P continued to increase regardless of the application rate and had a positive relationship with soil level P (Mehlich-3 P) and flow weighted mean concentration (FWMC) for dissolved reactive P for both cultivated and pasture managed fields. We assessed hydrological connectivity within two nested watersheds by using the before-after-control-impact (BACI) design. Results showed hydrological connectivity during high rainfall years whereas low rainfall years had minimal connectivity compared to the controls. These results suggest the P contributions from upstream fields receiving poultry litter, even at high application rates, did not exhibit a treatment effect during the low rainfall years at downslope monitoring stations. </p><p><br></p> <p>As nutrient source variability increases in nutrient management plans, improving our ability to differentiate P sources and their fate in soils is critical. We evaluated soils with unique P inputs: inorganic P, poultry litter, and cattle grazing for isotopic signatures by forming silver phosphate and determining the δ¹⁸O_P. Isotopic signatures of the oxygen molecules which are strongly bound to P, provided signatures of 17.09‰, 18.00‰, and 17.20‰ for fields receiving commercial fertilizer, poultry manure, and cattle grazed, respectively. Significant effort was made to determine critical steps in the method to successfully precipitate Ag₃PO₄for analysis. Results show adding a cation removal step as well as monitoring and adjusting pH throughout the method increases probability of successful Ag₃PO₄precipitation. Findings from this study provide a valuable framework for future analysis to confirm unique δ¹⁸O_P signatures which can be used to differentiate the fate of different phosphorus sources in agricultural systems.</p>

Die Arbeit besteht aus mehreren Studien zur Quantifizierung des Grundwasserstroms in Seen (Exfiltration; engl.: lacustrine groundwater discharge, LGD) und damit verbundener Nährstoffeinträge. In zwei einleitenden Kapiteln dieser Arbeit werden eine Gründe für die Vernachlässigung der Grundwasserexfiltration (LGD) in Seen und der daran gekoppelten Nährstoffeinträge identifiziert. Diese Literaturstudien fassen den aktuellen Kenntnisstand zum Einfluss des Grundwassers auf die Hydrologie von Seen und ihre Nährstoffhaushalte zusammen. Den Kern der vorliegenden Arbeit bilden zwei empirische Studien, die sich mit der Quantifizierung der grundwasserbürtigen Phosphor (P)-Fracht in den Arendsee in Deutschland befassen. Das Gesamtvolumen des Grundwasserzustroms wird basierend auf der Grundwasserneubildung im Einzugsgebiet des Sees ermittelt. Lokale Muster der Grundwasserexfiltration werden anhand von Temperaturtiefenprofilen des Seesediments bestimmt. Eine Kombination der Ergebnisse ermöglicht es, die quantitativen Daten mit lokalen Informationen zu unterstützen. Die Untersuchung der Grundwasserqualität zeigt, dass die P-Konzentrationen im Grundwasser im besiedelten Bereich teilweise stark erhöht sind. Als Konsequenz daraus haben die grundwasserbürtigen P-Frachten einen Anteil von mehr als 50% an der gesamten externen P-Last des Arendsees. Das Grundwasser ist damit eine maßgebliche Ursache für die Eutrophierung des Gewässers. Drei weitere Studien widmen sich der Entwicklung und Optimierung von Ansätzen zur qualitativen und quantitativen Bestimmung der Grundwasserexfiltration in Seen. Die kritische Auseinandersetzung mit den Ergebnissen der Studien zeigt die Notwendigkeit weiterer Forschung zur Verbesserung und Standardisierung der Methoden zur Bestimmung von LGD und damit verbundenen Stofftransporten auf. Der Fall des Arendsees sollte alle, Wissenschaftler und Praktiker, dazu motivieren, das Grundwasser als relevante Eutrophierungsquelle in Betracht zu ziehen. / The present work is a collection of studies on lacustrine groundwater discharge (LGD) and groundwater-borne phosphorus (P) loads. For a number of reasons, groundwater exfiltration (i.e., LGD) is often not considered in water and nutrient budgets of lakes. This is also and especially true for P which was often regarded to be immobile in groundwater until recently. Two chapters review the scientific literature regarding the impacts of groundwater on hydrology and nutrient budgets of lakes, respectively. They present mechanisms and processes of LGD as well as techniques and methods to measure LGD and related nutrient transports. Moreover, numbers of LGD volumes and loads reported in literature are presented. The core of the present work is represented by two case studies dealing with the quantification of P loads from LGD to a lake in Germany. A combination of different methods is applied to overcome the problem of quantitative large scale LGD determination without losing local spatial information. P concentrations in groundwater and LGD are investigated by detailed spatial water sampling. The results reveal that P is actually present in concentrations far above natural background concentrations in the urban groundwater. LGD-derived P loads account for more than 50% of the overall external P loads to the lake and by that contribute significantly to lake eutrophication. Three further studies are devoted to the development and improvement of approaches to determine LGD. Critical reviews of the above mentioned studies reveal the need for further research in order to standardize and improve methods for LGD and mass load determination. It is found that the appropriate method for LGD determination depends on the spatial scale of interest. The identification of P introduced by LGD as a main driver of lake eutrophication is an important finding which should encourage scientists, policy makers, and lake managers to consider groundwater as a relevant P source for lakes.