Testing Wisconsin P index assessments across cropping systems

Authors: GOOD, LAURA - University Of Wisconsin; BLAIRE, HAVA - University Of Wisconsin; Sherman, Jessica; Young, Eric

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2025
Publication Date: 6/3/2025
Citation: Good, L., Blaire, H., Sherman, J.F., Young, E.O. 2025. Testing Wisconsin P index assessments across cropping systems. Journal of Environmental Quality. https://doi.org/10.1002/jeq2.70042.
DOI: https://doi.org/10.1002/jeq2.70042

Interpretive Summary: Agricultural runoff phosphorus (P) contains dissolved and particulate (sediment-bound) P that varies by cropping system factors. The Wisconsin P Index (WPI) estimates agricultural runoff P losses with routinely collected nutrient management data via the SnapPlus nutrient management software program developed by University of Wisconsin researchers. Periodic evaluation of the WPI with measured runoff data from sites across Wisconsin is critical to test model performance. We determined WPI’s ability to predict erosion and P transport differences between fields in corn silage versus hay forage production. Total P, dissolved reactive P, suspended sediment, and runoff volume were monitored year-round from four fields for a ten-year period. Total annual P losses from corn (2.8-4.2 kg ha-1yr-1) were higher than perennial forage (1.4 -1.5 kg ha-1yr-1). Runoff P from corn was mainly particulate P (94-98%) whereas dissolved P dominated perennial forage runoff (76-81%). When measured sediment and runoff were used with the WPI strong relationships were found (R2=0.64-0.82) between measured and modeled dissolved and total P. The WPI also identified periods of high P loss risk during the non-growing season and snowmelt periods. Results stress the need for P loss indices to account for non-growing season P loss transport processes in cold climates and tradeoffs between cropping systems.

Technical Abstract: Managing agricultural runoff phosphorus often requires understanding management effects on both dissolved P (DP) and particulate (sediment-bound) P (PP). The Wisconsin P Index (WPI) is intended to allow producers to estimate the effect of land management practices on runoff P losses by estimating annual losses in mass per unit area. We tested the WPI’s ability to correctly identify P loss tradeoffs between two management systems: tilled corn silage and perennial forage. Year-round monitoring for total P (TP), dissolved reactive P (DRP), suspended sediment (SS), and runoff volume (RO) was conducted in four watersheds with each system present for five years separated by a transition period. We identified PP as TP-DRP. Total annual P losses from the tilled corn system (2.8-4.2 kg ha-1yr-1) were higher than those for perennial forage (1.4 -1.5 kg ha-1yr-1) and were dominated by PP (94-98%), while losses from the perennial forage system were dominated by DP (76-81%). The WPI adequately described differences in P loss pathways between these two systems. When measured SS and RO were used in the WPI, we observed strong relationships (R2=0.64-0.82) between measured and estimated DP and PP kg ha-1yr-1 across the two systems. The WPI method for estimating runoff volume was also tested using monitored weather data. It correctly identified higher frozen soil/snowmelt runoff from perennial forage systems that led to greater overall runoff volumes. The WPI is embedded in nutrient management planning software widely used by Wisconsin producers and agronomists and is used for a range of water quality improvement programs.