LAUSR

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85 ha) subsurface drainage plots in Illinois, USA were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four years. Three plot-months were identified as DRP control points (one export and two transport control points) where each plot-month contributed > 10% of the annual DRP load from the field. These control points occurred on separate plots, and in both the growing and non-growing seasons, but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The non-growing season was hypothesized to be the most significant period of P loss, but this was only documented in two of four years. A cereal rye (Secale cereale) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing-thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small (< 3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point). This article is protected by copyright. All rights reserved.