Dissolved and particulate phosphorus in leachate from structured soil amended with fresh cattle faeces

Abstract

Substantial transfer of phosphorus from land to aquatic environments has been observed in regions supporting intensive animal husbandry. We characterised leaching of P derived from fresh cattle faeces located at the surface of a structured clayey soil. Fresh cattle faeces were placed on top of an undisturbed column of structured soil (diam. and height 0.5 m) subjected to steady, saturated flow (1 mm h^-1) with a synthetic rain solution. After 0.01 pore volumes (PV, 1PV = 46.5 1) or 2.5 h of flow the effluent concentrations of both dissolved and particulate P forms increased rapidly, and after 0.15 PV (37 h) a maximum of 12 mg P l^-1 was observed for total P. This rapid, strong and permanent leaching of P from applied faeces was ascribed to preferential flow through continuous macropores. Dye tracing showed that especially earthworm burrows were active flow paths. Of the faeces derived P observed in the effluent, dissolved inorganic P (DIP, measured as molybdate reactive P) constituted the largest fraction (37%) followed by particulate organic P (POP, 33%) and particulate inorganic P (PIP, 22%) while dissolved organic P (DOP) accounted for the remaining 8%. Leaching of IP from faeces appeared to be greater than from inorganic sources. This may be ascribed to co-leaching of P complexating compounds, or compounds that compete with faeces derived P for sorption sites. When the flow regime was shifted from saturated to unsaturated conditions (after 5 days), DIP temporarily became undetectable in the effluent, and the concentration of PIP and POP decreased markedly, while DOP remained almost unchanged and became the main P fraction leached. Average effluent concentration of total P for the whole 12 day leaching period was 4.1 mg P l^-1. This high concentration suggests well-drained soils with high animal densities to constitute future field research targets.