Field soil and ditch sediment phosphorus dynamics from two artificially drained fields on poorly drained soils

The installation of artificial drains alters soil permeability such that migrating water interacts with soil and sediment biogeochemistry to mobilise or attenuate phosphorus (P). Soil and ditch sediment P chemistry was explored at two artificially drained sites with similar land use, management and drainage class. Site A was characterised by high total P content (282–1437mgkg−1) and elevated water soluble P (WSP) (10.11mgkg−1) in a Humic topsoil. Subsurface horizons contained high amounts of leached aluminium (Al) and iron (Fe) and P sorption capacities expressed by the Freundlich K term increased with depth from 338 to 942mgkg−1. Site B was characterised by low TP (58–476mgkg−1) and low P sorption capacities (40–173mgkg−1) that decreased with depth, owing to a high% sand and low Al. Bankside and sediment in the ditch were mostly higher or comparable to P sorption properties measured in subsurface soil horizons from adjacent fields. Dissolved reactive P (DRP) concentrations were monitored in the open ditch, end-of-pipe and in-field piezometers and highest values were recorded in the open ditch (0.03–0.183mgl−1) at Site A, potentially due to diffuse and point sources on the farm. Higher P concentrations were recorded at end-of-pipe locations compared to piezometers at similar depth, and attributed to a larger contributing area reaching the pipe from the surface and surrounding subsurface layers. Attenuation of WSP by subsoil at Site A was evidenced by low piezometer values (0–0.003mgl−1). Low P sorption in the ditch at Site B suggests that dredging could expose low P sorbing layers, but adding topsoil could enhance P sorption. Drainage design, maintenance, and measures for P mitigation require an assessment of surface and subsurface P dynamics to ensure a ‘right measure right place’ approach.