Effect of Tile Spacing and Depth on Drainage Discharge and Phosphorus Losses under Corn and Soybean Rotation

Abstract

Abstract : Phosphorus (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 drainage water management systems (regular free drainage, RFD, vs. controlled drainage/subirrigation, CDS) with a corn-soybean rotation. Under RFD with the 4.2 m tile spacing, soil P losses with the tile depth of 0.65 m were 0.021 kg ha -1 for dissolved reactive P (DRP) and 0.275 kg ha -1 for total P (TP) in runoff, and 0.382 kg ha -1 for DRP and 5.43 kg ha -1 for TP in subsurface drainage. With the tile depth of 0.85 m, soil P losses increased by 92 fold for DRP and 28 fold for TP in runoff and by 8.8 fold for DRP and by 1.5 fold for TP in subsurface drainage, because of the increased of both discharge and flow weighted mean DRP and TP concentrations. Effects of tile depth on soil DRP and TP losses under CDS followed similar patterns to those under RFD in runoff, except for the greater increases, while in subsurface drainage there were slight increases in DRP and decreases on TP with increased tile depth, related to redirection of discharges from subsurface to surface. Regardless of water management, increased tile depth increased soil DRP loss in both runoff and subsurface drainage at similar extents. However, CDS reduced soil TP loss in combined runoff and subsurface drainage by 15.7 %, relative to RFD. Effects of tile spacing on soil TP loss were less pronounced than those of tile depth. With tile depth of 0.65 m and increased tile spacing from 4.2 m to 7.5 m, Soil TP increased in runoff at similar extents for both RFD and CDS. In contrast, increased tile spacing decreased TP loss in subsurface drainage under both RFD and CDS. Consequently, increased tile spacing increased soil TP loss in combined surface and subsurface drainage, regardless of water management. There appeared to be a greater effect of tile depth than tile spacing on both DRP and TP losses, of which the effects were predominately attributed to the redirection of field water discharge, followed by changes in P concentration. Installing shallower tile depths, or managing the outlet at shallower levels, may be an effective practice to mitigate soil P loss from tile drained agricultural lands.