Pasture-based grazing systems contribute to the excessive nutrients found in some streams in south-eastern Australia. This study investigated phosphorus (P) exported in runoff from a rain-fed dairy pasture (Darnum) and 4 bays of irrigated dairy pasture (MRF). Runoff was monitored for 7 years at Darnum and 2 years at the MRF to identify factors associated with the variation in total P (TP) concentrations between events.
The flow-weighted mean annual P concentrations in runoff varied between 3.3 and 28.2 mg TP/L for Darnum and 6.2 and 31.5 mg TP/L for the MRF. The relationships between TP concentrations in runoff and days between fertiliser application and runoff, days between grazing and runoff, and total storm flow were examined using an additive component model that explained 61% and 70% of the variation in log-transformed TP for Darnum and the MRF, respectively. The interval between application of fertiliser and runoff and the effect of year were highly significant and explained most of the variation in TP. Grazing and fertiliser application were identified as the major factors that may affect TP concentrations that the land manager can control (preventable). The estimates of year effect (i.e. the component of TP not explained by the other variables and over which the land manager had no apparent means of control) ranged from 1.60 mg (s.e. 1.99) to 7.14 mg (s.e. 1.90) TP/L in non-drought years (>45 kL/ha runoff annually). The year effect averaged 5.7 and 6.9 mg TP/L for Darnum and the MRF, respectively. It is shown that an additive component model provides a useful structure for investigating similar, field-scale data.
Cropping is one of the many industries contributing to the excessive loading of nitrogen (N) and phosphorus (P) to rivers and lakes in Australia. Nitrogen and P exports from cropping systems have not been systematically investigated to the same extent as those from other agricultural sectors, such as dairy pastures. Therefore, this review relies heavily on information derived from agronomy and other fundamental studies on soil-nutrient interactions to determine the potential for nutrient export from high rainfall zone (HRZ) cropping. There is a great deal of variation in environmental and management strategies across cropping in the HRZ, which suggests that nutrient exports could occur under a range of scenarios. The potential for exports is therefore discussed within a conceptual framework of nutrient sources, mechanisms for mobilization, and transport pathways in HRZ cropping. Transport refers to nutrient movement by flowing water after it has been mobilized, and export refers to the transfer of nutrients from one landscape compartment (e.g., a soil) to another (e.g., a stream or lake). The transport of nutrients from HRZ cropping can occur through surface and/or subsurface pathways depending on factors such as landform and infiltration and nutrient sorption characteristics of the soil profile. Surface pathways are likely to be more significant for phosphorus. For N, subsurface movement is likely to be as significant as surface movement because nitrates are generally not bound by most soils. Information about mechanisms of nutrient mobilization is essential for developing management strategies to control nutrient exports from HRZ cropping.
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