Phosphorus Management and Fate in Artificially Drained Systems: Upper Mississippi Case Studies.

Phosphorus loss through artificial subsurface drainage can be an important mechanism of P delivery to water resources. Phosphorus loss through subsurface drainage often is much less than loss with surface runoff and erosion, but since the majority of the P in drainage is readily available P it may have a strong effect on eutrophication. We summarize several field studies conducted in the Upper Mississippi Watershed. Two multi-year Iowa studies were established in two different soils with different internal drainage to assess impacts of poultry manure (one study) and swine manure or fertilizer P (the other study) application for corn and soybean. The studies included systems with different tillage and P application frequency and rates. Annual P rates ranged from one to four times the annual P removed with grain harvest. The P application rate and drainage flow were the main factors influencing P loss. Mean annual dissolved reactive P concentration by system across sites and years ranged from 1 to 28 ug P L^-1 and annual P loss ranged from 1 to 42 g P ha^-1. Measurements at one site suggested that predominant loamy or silty-clay subsoil in the region limit lateral P flow through subsoil. A multi-year, field-scale, on-farm research project in Southwest Minnesota measured P loss in continuous corn under conventional free (CF) and controlled (CD) subsurface drainage systems. Mean annual drain flow was reduced by 51% under CD management compared to FD. Annual DRP loss from the CD system was reduced by 49% compared to FD, 123 g ha^-1 versus 241 g ha^-1. Monthly DRP loss was greater for CD than FD in April when water stored in the soil profile was drained before spring field operations. Drainage water management system and time of drainage water release were the main factors influencing P loss at the southwest Minnesota site. At another Minnesota site, researchers have monitored subsurface drainage for six years from corn silage fields receiving liquid dairy manure. Weekly flow-weighted mean (FWM) DRP concentrations ranged from 0.02 to 4.12 mg L^-1 over three years with open inlets to the subsurface drainage system and 0.01 to 1.48 mg L^-1 over three years after blind inlets were installed.  Average weekly FWM DRP concentrations over three years after blind inlet installation were 0.12 and 0.20 mg L^-1 for injection and center pivot fertigation application methods, respectively.