Characterizing Phosphorus Dynamics in Eastern Wisconsin Tile-Drained Agroecosystems.

Artificial subsurface drainage provides an avenue for the rapid transfer of phosphorus (P) from agroecosystems to surface water. Through the University of Wisconsin Discovery Farms program, long-term surface and subsurface water monitoring stations were established on four fields in eastern Wisconsin to measure surface and tile drainage P losses. These sites, which received frequent manure applications, represent a range of crop management practices [two chisel plowed corn fields (CP1, CP2), a no-till corn-soybean field (NT) and a grazed pasture (GP)]. Subsurface drainage was the dominant pathway of water loss at each site accounting for 66 to 96% of total discharge. Long-term subsurface flow-weighted total P (FW-TP) concentrations were 0.75, 0.55, 0.22, and 1.31 mg L-1 for sites CP1, CP2, NT, and GP, respectively. High TP concentrations measured at the grazed site were likely caused by the field’s use as an overwintering paddock with high animal densities throughout the winter months. Low TP concentrations at the NT site were due to tile drain interception of groundwater flow where large volumes of tile drainage water diluted the FW-TP concentraitons. Long-term surface TP-FW concentrations were 3.59, 2.69, 4.08, and 6.36 mg L-1 for sites CP1, CP2, NT, and GP, respectively. Subsurface pathways contributed between 17 and 41% of the TP loss across sites. Only at the NT site did P export clearly appear to be driven by incidental P-loss following manure applications. Regression analyses indicate a strong positive relation between event subsurface dissolved reactive P export and event tile discharge at the tilled sites (R2=0.70, CP1; R2=0.69, CP2). This relation was weak at the no-till and grazed site, (R2= 0.05, NT; R2= 0.36, GP). Based on these field measures, P losses from artificial subsurface pathways must be integrated into field level P budgets and P loss calculations on heavily manured soils.