Runoff Losses of Phosphorus and Nitrogen In a Corn Silage System: A Field-Scale Paired Watershed Approach.

Transport of P, N, and sediment via runoff from crop fields, especially where manure has been applied, can contribute to eutrophication and degradation of surface waters. We established a paired-watershed field site to evaluate surface runoff losses of nutrients and sediment from different manure/crop/tillage management systems for silage corn production. During the 2-yr calibration period the four 1.6-ha watersheds, or fields, were treated identically with fall dairy manure application and chisel plowing; and runoff was monitored, sampled, and analyzed for suspended sediment and total and dissolved forms of P and N. With the beginning of the treatment phase, that management was maintained as a control in one field, while alternative management systems were initiated on the three treatment fields -- fall manure/chisel plowing with field-edge grass-legume buffer/waterway; fall surface-applied manure with spring chisel plowing; and fall-seeded rye cover crop with spring manure and chisel plowing. During the calibration period both concentrations and loads of suspended sediment and total and dissolved P and N varied by field and over 50% of runoff and dissolved P and N was from snowmelt runoff. Linear regressions of treatment fields against the control field were highly significant for runoff and concentrations and loads of all constituents. The estimated minimum detectable change (difference between means) was 10 to 30% for most parameters, indicating a reasonable probability of success in detecting treatment effects. Early treatment period results suggest a vegetative buffer reduced losses of sediment and total P, while fall surface-applied manure increased concentration and load of dissolved P.