Sustainability for Virginia dairy farms requires balancing phosphorus (P) imports and exports at the farm-gate level. Balancing P helps prevent further accumulation of P in farm soils through routine applications of manure, which over time contributes to surface water quality issues. The objectives of this research centered on guiding dairy farms in Virginia toward lower, more sustainable P balance, and without adversely impacting profitability. First, the state of P balance had to be determined for a sample of dairy farms, including risk factors for excessively high P balance. Second, a repeated assessment of P balance on those dairy farms sought to determine any key factors of change in P balance between years. Lastly, a small Virginia dairy farm was used as a case farm to evaluate whether or not it could reduce its P balance while maintaining or improving farm profitability. An initial assessment of 58 dairy farms in Virginia showed that 75% of farms could operate with a P balance less than 18.7 kg ha-1. The two risk factors that led to excessively high P surpluses were the use of poultry litter and excessive P imported with purchased feed. The repeated assessment included 30 of the 58 original dairy farms. Increases of 1.0 kg P ha-1 of total P imports and exports were respectively correlated to a mean P balance increase of 0.76 kg ha-1 and a mean P balance decrease of 0.43 kg ha-1, suggesting that changes in P imports affect changes in P balance more than changes in P exports. Reduced poultry litter use was highly correlated to reduced P balance, and increasing cow manure exports also reduced P balance for the farms with the opportunity. As a significant portion of the farms assessed were small (less than 200 milking cows), a case farm of 105 cows on 100 acres was used to explore how farm profitability could be affected as P balance was reduced through additional acres, increased crop production, and with a grazing-based farming strategy. Results from partial budget analysis showed that after expanding the land base from 100 to 150 acres for crop production, the change in potential net return ranged from $-0.90 to $1.26/cwt of milk, with accompanying changes in P balance ranging from -9.0 to -14.7 lbs/ac. The analysis also showed that changes in potential net returns after converting to a grazing-based system ranged from $-2.14 to $1.39/cwt, with greater change in P balance ranging from -9.7 to -17.8 lbs/ac. The most profitable strategy, generally, for this farm seemed to be expanding the land base and growing a cash crop. Phosphorus balance on Virginia dairy farms can be reduced, potentially without negative impacts on farm net return, though challenges remain for farms with limited land or areas with high density of animal agriculture. / Doctor of Philosophy / Phosphorus (P) is a vital nutrient for crops and livestock, but too much of it in soils leads to surface water quality problems. Sustainability for Virginia dairy farms requires balancing P imports and exports at the farm level. This research centered on guiding dairy farms in Virginia toward lower, more sustainable P balance. An initial assessment of 58 dairy farms in Virginia helped establish a zone of operation, a feasible target toward which the 25% of farms with high P balance could aim. Avoiding poultry litter as a fertilizer choice and limiting P imported with purchased feed were both ways in which some of these farms could lower their P balance. A repeated assessment in a second year showed that reductions in P imported were more likely to reduce P balance than were increasing P exports. In this, reducing the use of poultry litter as a fertilizer was again an effective way of lowering P balance. As a significant portion of the farms assessed were small (less than 200 milking cows), a case farm of 105 cows on 100 acres was used to explore how farm profitability could be affected as P balance was reduced through various management changes. Results showed that after expanding the land base by at least 50 acres for additional crop production, P balance could be significantly reduced while maintaining or increasing the potential net return to the farmer, especially if the farmer can ensure a high milk yield grazing-based operation or if they grow a cash crop like corn or soybeans. Overall, the research suggests that P balance on Virginia dairy farms can be reduced, and that these farms can operate more sustainably, though challenges remain for farms with limited land or areas with high density of animal agriculture.

Natürliche Niederungsgebiete wirken als effektive Phosphor Senke. Landwirtschaftliche Nutzung und Drainage führt zur Transformation zu Phosphor Quellen. Dem Spreewald, ein Feuchtgebiet und seiner Funktionsweise als Phosphor Quelle oder –Senke, kommt im Einzugsgebiet der Spree eine wichtige Rolle zu. Die vorhandenen Kenntnisse der Phosphor Umsatzprozesse und des Phosphor Austausches zwischen den Flächen und dem Fließgewässernetz sind jedoch gering. Praktikable Ansätze zur Beschreibung des Phosphor Austauschverhaltens von Nährstoffen in den ausgedehnten Polderregionen liegen nicht vor. Anhand hydrologischer, geohydraulischer und biogeochemischer Prozessuntersuchungen in einer Polderlandschaft mit typischer Stauhaltung konnten die Phosphor Freisetzungs- und Retentionsprozesse ausgewiesen werden. Unter Nutzung verschiedener Modellansätze (geohydraulische Modellierung, Stofftransportmodellierung und Statistische Modelle) und der Berechnung von Phosphor Prozessraten werden horizontale und vertikale Phosphor Fluxe in den Flächen-Wasser Übergangszonen quantifiziert. Die Ergebnisse gehen in ein Phosphor Bilanzmodell ein, das die P-Quellen und Senkenfunktion von Polder Teilflächen (überstaut, genutzt) und des gesamten Polders in Monatsschritten abbildet. Die biogeochemischen Phosphor Umsatzprozesse sowie der horizontale und vertikale Transport werden wesentlich von den hydrologischen und den klimatischen Bedingungen gesteuert. Sie stellen die primären Einflussgrößen der P Senken oder -Quellenfunktion dar. Im Polder wird die Phosphor Netto Freisetzung der genutzten Bereiche durch eine hohe Phosphor Netto Retention in den überstauten Flächen abgemindert. Szenario Untersuchungen zeigen, dass sinkende Grabenwasserstände zu einer erheblichen Erhöhung der Phosphor Emissionen aus dem Grundwasserpfad führen. Ansteigende Phosphor Fluxe bei sinkenden Wasserständen legen ein zunehmendes Eutrophierungsrisiko in den Gräben sowie für unterhalb gelegene aquatische Systeme nahe. / Natural wetlands effectively retain phosphorus. Agricultural cultivation and drainage by ditches transform them to phosphorus sources. In the Spree catchment, the Spreewald, a large scaled lowland has a strategic importance operating as a phosphorus sink or phosphorus source. Anyhow, knowledge of its phosphorus turnover processes and the phosphorus exchange behavior between the plain and the river and ditch network are marginal. Practicable approaches to reproduce the phosphorus exchange behavior in its typical polder areas are missing. Based on process investigations in a polder area with typical weir regulation, phosphorus retention and remobilization processes are characterized. Combining different model approaches (groundwater modeling, matter transport modeling and statistical models) and calculating process rates, the horizontal and vertical phosphor fluxes in the water soil/sediment transition zones are quantified. The outcomes are used as input data for a phosphorus balance model reproducing the phosphorus source and sink character of used and rewetted polder areas and for the whole polder in monthly time steps. Results from process and transport investigations point out that biogeochemical turnover processes and horizontal or vertical phosphorus transport are driven by hydrological and climatological conditions. Net phosphorus release found in the extensive used polder areas is counteracted by significant net phosphorus retention in the rewetted parts. Scenario analyses identify decreasing ditch water levels to cause a considerable increase of phosphorus emissions from the groundwater pathway. Consequently, the increasing phosphorus fluxes at decreasing water levels provoke a growing risk for eutrophication in the ditches but also in the downstream aquatic systems.