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