Phosphorus exports from agricultural grassland in overland flow and subsurface drainage water

Abstract

On the background of increasing phosphorus (P) concentrations in Irish rivers and lakes, this project set out to investigate several aspects of the potential of agricultural grassland to lose P to water. All study sites were located at Johnstown Castle Research and Development Centre, Wexford, Ireland. Water samples were collected from field sites, from an artificial subsurface drainage network (at the Beef Unit), and from a small stream (at the Dairy Farm). To study the effects of soil P levels on P exports in overland flow from grassland, the quantity and quality of overland flow from a low soil P (4 mg P/1 Morgan's) and a high soil P (17 mg P/1 Morgan's) field were measured for 16 consecutive months. Additionally, a medium soil P (8 mg P/1 Morgan's) site was monitored for 5 of the 16 months. A combination of open trenches and berms were used to hydrologically isolate the surface of the overland flow monitoring sites. To investigate the possibility of P losses along subsurface flow pathways, the artificial subsurface drainage network at the Beef Unit was monitored as an example of an existing subsurface drainage system. The measurements of flow and composition of the discharge from the artificial subsurface drainage network were complemented by a soil water sampling programme of limited extent. To study the combined effects of fields of different site and management characteristics on stream water quality, the stream naming through the Dairy Farm was monitored continuously at the upstream and the downstream boundary of the Dairy Farm sub-catchment. Additionally, a grab sampling programme provided information on changes of the stream water quality along the channel. Measurements of flow and quality of water in the artificial subsurface drainage network at the Beef Unit and in the Dairy Farm stream were carried out for 5 and 6 months, respectively, in consecutive years. Dissolved reactive P (DRP) and total dissolved P (TDP) were determined in most water samples from the overland flow and the subsurfece flow monitoring sites. Total P (TP) was measured in a subset of samples. DRP was the main P fraction analysed in water from the Daily Farm stream but some TDP and TP determinations were also carried out.