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Bottom-Up Modeling of Agricultural Phosphorus Mass Balance in the Little Otter Creek Headwaters Watershed, Vermont
Selenis, Sloane
Selenis, Sloane
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Abstract
Excess phosphorus (P) applied to croplands can accumulate within agricultural soils over time. Managing agricultural soil P is critical to prevent P runoff into receiving waters, where it can degrade environmental quality and negatively impact related socioeconomic systems. Mass balance offers a method to quantify agricultural soil P flows, which is conducted in this study in Vermont’s Little Otter Creek watershed. This study applies a bottom-up method, in which spatially heterogeneous soil test and P management data are extrapolated from a subset of agricultural fields with available data to all agricultural fields in the watershed. Extrapolation is conducted within clusters of fields, which are grouped based on crop type, soil clay content, and slope. A Monte Carlo simulation of 10,000 repeated extrapolations is conducted to identify the central tendency for partial P mass balance and other metrics. The model produced a net P mass balance of 8.21 ± 0.90 kg P ha-1 yr-1 for all agricultural land in the watershed, with a higher mass balance surplus for hay compared to corn on average. Results include evidence of good nutrient management practices, with the distribution of excess agricultural P biased toward soils with a greater capacity to retain the added P. However, an overall P surplus, if continued, will continue to grow the legacy P pool stored within the watershed’s soils and pose long-term risks to receiving waters. Current Vermont policy focuses primarily on limiting P mobility from agricultural soils but does not consider watershed P mass balance. Future efforts to improve P management at the watershed scale could include reduction of synthetic P fertilizer use and new approaches to reduce and manage surplus P to achieve greater nutrient management circularity in Vermont’s working landscape.
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2026-05-08
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Sloane Selenis Thesis - Final Submission 5.8.26.pdf
Adobe PDF, 959.61 KB
- Embargoed until 2028-05-08
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Environmental Sciences