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 due to low hydraulic conductivity of the surface soil and especially the subsoil will lead to abundant surface runoff. The abundant runoff will induce soil erosion and phosphorus losses. To determine the influence of improved subsurface drainage (IMP) on soil erosion, phosphorus losses and nitrogen leaching, a heavy clay soil with a 29 year old subdrainage system was fitted with new drains, with topsoil or wood chips used as backfill in the drain trenches. Before IMP, drainage water constituted only 10–40% of the total runoff (drainage+surface runoff) but after IMP 50–90%. Where topsoil was used as backfill, the estimated soil erosion and particulate P and dissolved orthophosphate P losses from 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 and particulate P losses were not reduced. Owing to the increased drainage discharge, nitrogen leaching during barley cultivation was much higher after IMP (14 vs. 7 kg ha⁻¹a⁻¹ ).