INFLUENCE OF IMPROVED SUBSURFACE DRAINAGE ON PHOSPHORUS LOSSES AND NITROGEN LEACHING FROM A HEAVY CLAY SOIL

Without proper subsurface drainage of heavy clay soil, water logging d ue to low hydraulic conductivity of the surface soil and especially th e subsoil will lead to abundant,surface runoff The abundant runoff wil l induce soil erosion and phosphorus losses. To determine the influenc e of improved subsurface drainage (IMP) on soil erosion, phosphorus lo sses and nitrogen leaching, a heavy clay soil with a 29 year old subdr ainage system was fitted with new drains, with topsoil or wood chips u sed as backfill in the drain trenches. Before IMP, drainage water cons tituted only 10-40% of the total runoff (drainage + surface runoff) bu t after IMP 50-90%. Where topsoil was used as backfill, the estimated soil erosion and particulate P and dissolved orthophosphate P losses f rom ploughed soil during winter were lower after IMP than before (1168 vs. 1408 kg ha(-1), 0.58 vs. 0.69 kg ha(-1), 0.09 vs. 0.12 kg ha(-1), respectively). Where wood chips were used as backfill, soil erosion a nd particulate P losses were not reduced. Owing to the increased drain age discharge, nitrogen leaching during barley cultivation was much hi gher after IMP (14 vs. 7 kg ha(-1)a(-1)).