Drainage System Management Impacts On Phosphorus Loss.

Previous and ongoing research indicates that phosphorus losses through subsurface drainage systems can be an important transport pathway for the movement of phosphorus to surface waters. Phosphorus lost from agricultural land means economic loss for producers and may also contribute to water quality impairments. One of the objectives of this research was to quantify drain flow volume and phosphorus loss from a conventional free-drainage (FD) compared to a controlled drainage (CD) system in Minnesota, USA. A field study was conducted from 2006-2009 on a tile-drained Millington loam soil (fine-loamy, mixed, calcareous, mesic Cumulic Haplaquoll). The field site consisted of two independently drained management zones, 15 and 22 ha, respectively.  The project used a paired design approach to statistically evaluate treatment effects. During the calibration period (2006-2007) each zone was managed the same. The treatment phase of the experiment began in 2008 with one zone managed in FD mode and the other managed in CD mode.  During the two year treatment period (2008-2009) total outflow volume from CD was reduced on average 63%, 141 to 52 mm compared to FD. There was also evidence that annual total phosphorus and ortho-phosphorus loads were reduced by 50 and 63%, respectively. However, the reasons for a 33% increase in flow weighted mean (FWM) total phosphorus concentration under controlled drainage are unclear. The results also showed differences in phosphorus-sorption capacity between the two drainage water management systems. Controlled drainage effectively reduced annual drainage outflow and annual total and ortho-phosphorus loss but increased FWM total phosphorus concentration.