Effects Of Tile Drain Depth and Spacing On Phosphorous Losses Under Corn and Soybean Rotation.

Phosphorous (P) losses from tile drained agricultural lands may differ with tile depth and spacing.   Studies were conducted on clay loam soils using large field plots equipped with automatic flow volume measurement and sampling systems over a 4-year period to evaluate tile depth (0.65 m vs. 0.85 m) and spacing (4.2 m vs. 7.5 m) on P losses under two water management systems (regular free drainage, RFD, vs. controlled drainage and sub-irrigation, CDS) with a corn-soybean rotation. Under RFD with 4.2m tile spacing, soil P losses with the tile depth of 0.65m were 0.021 kg ha^-1 for dissolved reactive P (DRP) and 0.275 kg ha^-1 for total P (TP) in surface runoff, and 0.382 kg ha^-1 for DRP and 5.43 kg ha^-1 for TP in sub-surface runoff. Increased tile spacing to 0.85m, soil P losses increased by 92 folds for DRP and 28 folds for TP in surface runoff and by 8.8 folds for DRP and by 1.5 folds for TP in sub-surface runoff, because of the increased water flow discharge, by 56%. Effects of tile depth on soil DRP and TP losses under CDS followed the similar patterns to those under RFD in surface runoff, except for the greater increases, while in tile drainage there were slight increases in DRP and decreases on TP with increased tile depth, due to the redirection of water discharge from sub-surface to surface. Regardless of water management, increased tile depth increased soil DRP loss in both surface and sub-surface runoff at the similar extent. However, CDS reduced soil P loss totaled in surface and sub-surface runoff by 17.3%, relative to RFD. Effects of tile spacing on soil P loss were less profound than those of tile depth. With tile depth of 0.65m, increased tile spacing from 4.2m to 7.5m increased TP loss in surface runoff at a similar extent for both FRD and CDS, but at a greater degree for DRP under CDS than under FRD in both surface and sub-surface runoff. In contrast, increased tile spacing decreased TP loss in sub-surface under both FRD and CDS. Consequently, increased tile spacing increased soil P loss summed in surface and sub-surface runoff, regardless of water management. There appeared greater effects of tile depth than tile spacing on soil P loss, of which the effects were predominately attributed to the redirection of field water discharge, followed by the changes in P concentration. Managing tile depth can be an efficient practice to mitigate soil P loss from tile drained agricultural lands.