Controlled drainage to improve edge-of-field water quality in southwest Minnesota, USA

Wet, poorly drained soils throughout the northern Cornbelt are often artificially drained to improve field conditions for timely field operations, decrease crop damage resulting from excess water conditions, and improve crop yields. Drainage has also been identified as a contributing factor to water quality impairments in surface waters. Our objective was to quantify drain flow volume, nitrogen and phosphorus loss, and grain yield 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 22ha, 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) drain flow volume was reduced on average 63%, 141 to 52 mm. There was also evidence that annual nitrate-nitrogen, total phosphorus, and orthophosphorus loads were reduced by 61, 50, and 63%, respectively. However, the reasons for a 33% increase in flow weighted mean total phosphorus concentration under controlled drainage are unclear. The use of CD showed environmental benefits compared to FD but has not resulted in a consistent yield benefit at this site to date.