Phosphorus flows and legacy accumulation in an animal-dominated agricultural region from 1925 to 2012

Phosphorus (P) is a scarce but critical input for agriculture, yet its overuse can lead to water quality degradation. Most P applied as fertilizer and manure binds to soils, accumulating over time, constituting a legacy source with implications for mitigating nutrient pollution. To investigate how the flows and balance of P evolved over a period of rapidly changing technology, agricultural practices, and land cover, we modeled P flows in Vermont's dairy-dominated agricultural system at county- and state-levels from 1925 to 2012. An important dairy exporter, Vermont faces water quality challenges complicated by a mismatch between the scale of the market and that of policymaking, a common occurrence in export-oriented agricultural regions. Over the period analyzed, agricultural soils accumulated at > 1000 tonnes of P annually, accruing a legacy stock > 230,000 tonnes. The peak surplus of 4439 tonnes occurred in 1950, declining to 1493 tonnes per annum in 2012. Legacy P accumulation at the state-level ranged from < 1 to > 16 kg ha(-1), depending on year and measurement method. The decline in total P surplus reflects an 82% decline in fertilizer use that was partly offset by an increase in animal feed imports, the largest source of P entering Vermont since 1982. Despite declining inputs, milk output doubled, evidence of increased P use efficiency. Simultaneously, animal unit density increased by > 250%, enabled by rising feed imports. While feed is imported and milk exported, manure remains in Vermont; hence, Vermont soils continue to accrue legacy P at rates > 5 kg ha(-1), undermining efforts to reduce P runoff and achieve water quality targets. We discuss the governance, management, and policy implications, outlining opportunities to improve input accountability to address the persistent P imbalance. We highlight constraints facing regional policymakers due to increased embeddedness in commodity trade networks.