Testing Wisconsin P index assessments across cropping systems

Managing agricultural runoff phosphorus (P) often requires understanding management effects on both dissolved P (DP) and particulate (sediment-bound) P (PP). The Wisconsin P index (WPI) allows producers to assess land management effects on runoff P by estimating annual P losses in mass per unit area. We used monitoring data to test the WPI's ability to identify P loss tradeoffs between tilled corn silage and perennial forage cropping systems and WPI's sensitivity to treatment differences within each cropping system. Overall, WPI P loss estimates were supported by monitoring results, with higher total P losses for the corn system dominated by PP (84%) and smaller losses from the perennial forage system dominated by DP (86%). Notably, the WPI also distinguished most treatment differences within systems. Greater than 50% of annual runoff occurred during winter for both systems and was generally underestimated by WPI. However, when measured suspended sediment, precipitation, and runoff were used in the WPI, stronger relationships between measured and estimated PP kg ha^-1 year^-1 (r² = 0.94-0.95) and DP kg ha^-1 year^-1 (r² = 0.63-0.91) were observed. The WPI also underestimated sediment P concentration and DP loss from hay while overestimating DP loss from corn, highlighting specific opportunities for improvement. The WPI is embedded in nutrient management planning software widely used in Wisconsin (available online at snapplus.wisc.edu); however, the model can be adapted to other cold climate regions to support both crop production and environmental sustainability goals.