Effects of cattle treading and soil moisture on phosphorus and sediment losses in surface runoff from pasture

Abstract

Cattle treading is known to influence surface runoff contaminants such as phosphorus (P) and suspended sediment (SS). The objective of this work was to determine how simulated cattle treading affects losses of P and SS in surface runoff from four soils (Brown, Melanic, Pallic and Recent Gley) with contrasting resistance to treading damage and drainage properties. Soils were excavated, placed in boxes (0.8 m long and 0.20 m wide) and one of three moisture treatments were imposed (10, 50 and 90% of available water holding capacity) before surface runoff was induced via simulated rainfall; P fraction and SS were determined in the resulting runoff. The Pallic and Recent Gley soils lost the most P and SS compared with the better structured Brown and Melanic soils. Greater losses of particulate phosphorous (PP) and SS were observed at higher soil moisture contents. In contrast, dissolved P in runoff was greater at lower soil moisture contents, especially dissolved reactive phosphorus (DRP), which accounted for up to 90% of total P losses at 10% soil moisture in the Recent Gley soil. Measurement of soil water soluble P, a surrogate for DRP at a range of soil moisture contents, suggests this could be due to lysis of soil microbial biomass upon drying. Although concentrations and loads of DRP in surface runoff were greatest at 10% moisture, the risk of surface runoff is less than the risk posed by PP and SS losses with treading on wet soils, where surface runoff is more likely. This was particularly true for the Pallic and Recent Gley soils. It is therefore recommended that, to minimise SS and P losses to surface water, care be taken when grazing these soils under wet winter–spring conditions. Although collected under controlled conditions, these data help to explain the wide range of potential SS and P losses that may occur in a field due to hydrologic and soil variation.