4.7 Article

Impact of legacy soil phosphorus on losses in drainage and overland flow from grazed grassland soils

Journal

SCIENCE OF THE TOTAL ENVIRONMENT
Volume 575, Issue -, Pages 474-484

Publisher

ELSEVIER
DOI: 10.1016/j.scitotenv.2016.07.063

Keywords

Legacy phosphorus; Water quality, overland flow; Drainage; Grassland; Soil moisture

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Rates and quantities of legacy soil phosphorus (P) lost from agricultural soils, and the timescales for positive change to water quality, remain unclear. From 2000 to 2004 five 0.2 ha grazed grassland plots located on a drumlin hillslope in Northern Ireland, received chemical fertiliser applications of 0, 10, 20, 40, 80 kg P ha(-1) yr(-1) resulting in soil Olsen P concentrations of 19, 24, 28, 38 and 67 mg P L-1, respectively, after which applications ceased. Soil Olsen P and losses to overland flow and drainage were monitored from 2005 to 2011 on an event and weekly flow proportional basis, respectively. Soluble reactive P and total P time series were synchronised with daily rainfall and modelled soil moisture deficits. From 2005 to 2011 soil Olsen P decline was proportional to soil P status with a 43% reduction in the plot at 67 mg P L-1 in 2004 and a corresponding 12% reduction in the plot with lowest soil P. However, there was no significant difference in the flow-weighted mean concentration for overland flow among plots, all of which exceeded 0.035 mg L-1 in >98% of events. Strong interannual and event variations in losses were observed with up to 65% of P being lost during a single rainfall event. P concentrations in drainage flow were independent of Olsen P and drain efficiency was potentially the primary control on concentrations, with the highest concentrations recorded in the plot at 38 mg L-1 Olsen P in 2004 (up to 2.72 mg L-1). Hydrological drivers, particularly antecedent soil moisture, had a strong influence on P loss in both overland and drainage flow, with higher concentrations recorded above a soil moisture deficit threshold of 7 mm. This study demonstrates that on some soil types, legacy P poses a significant long term threat to water quality, even at agronomically optimum soil P levels. (C) 2016 Elsevier B.V. All rights reserved.

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