This study investigated the effect of simulated cattle treading on soil infiltration rate (saturated hydraulic conductivity: Ksat) and macroporosity, and the consequent loss of sediment and phosphorus (P) via overland flow from a grassland and cultivated soil used for dairy farming in southern New Zealand. Treading decreased soil macroporosity and Ksat, and hence time to ponding, which increased the volume of overland flow. Mean suspended sediment concentration was greater in the cultivated treatments (0.076 g/L) compared with the grassland treatments (0.014 g/L). In the grassland soil, sediment and particulate P fractions in overland flow increased with treading due to increased soil disturbance and decreased protection from erosion by grass cover. In contrast, for the cultivated soil, sediment and P concentration and load decreased with increasing treading, due to greater ponding which decreased the erosive power of raindrop impact. Dissolved and particulate P fractions followed similar trends, although mean total P (mostly particulate P) was greater in cultivated treatments (1.07 mg/L) than the grassland treatments (0.64�mg/L). Relationships were generated between macroporosity and the loss of sediment and P, showing the wider application of macroporosity for environmental assessment than solely an agronomic measurement.

The forms and concentrations of P in overland flow were measured from intact pastoral soils obtained from the Winchmore long-term P fertiliser trial. Treatments under evaluation were soils that received either 0, 188, 250 or 376 kg superphosphate ha -1 yr -1 , or 175 kg reactive phosphate rock (RPR) ha -1 yr -1 . The objective was to determine the magnitude of potential P transfers from soil to water following P fertilisation, and to determine if losses were different following RPR fertilisation compared with superphosphate. Overland flow was induced by the application of artificial rainfall at 15 mm h -1 , maintained for 1 h after flow commenced. Concentrations of dissolved reactive P (DRP) and total P (TP) mirrored the longterm application rates, although prior to a fresh application of P, soils with P applied in RPR form lost more P during an event than soils with the same rate of P applied as superphosphate. After a fresh application of RPR and superphosphate treatments, up to 5.4 mg TP litre -1 was lost in flow from the A03004; 376 kg superphosphate ha -1 yr -1 treatment, while P in flow from soils fertilised with RPR were commonly c. 0.11 mg litre -1 , but still greater than from the unfertilised control soils (0.02 mg litre -1 ). Regression analysis indicated that DRP concentrations in flow from the fertilised soils were elevated above that lost before fertiliser application for a period of approximately 60 days. These results support earlier studies that demonstrate the greater risks of incidental P losses from soluble P fertilisers such as superphosphate (up to 60 days), and conversely the potential environmental benefits from RPR fertilisation of soils "at-risk" of P loss (e.g., where much overland flow occurs such as in very wet soils and near stream channels). However, if good management practice is followed then the difference in P loss between superphosphate and RPR treated soils should be minimal over a period of a year.

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