Long-term changes in water quality and solute exports in headwater streams of intensively managed radiata pine and natural eucalypt forest catchments in south-eastern Australia

Abstract

The Cropper Creek Hydrology Project was established in 1975 to study the hydrology of three small catchments of mixed species eucalypt ( Eucalyptus spp.) forest. In 1980 one catchment was cleared for the establishment of a radiata pine ( Pinus radiata D. Don) plantation but a 30-m wide buffer zone of undisturbed eucalypt forest was left along the stream. This change in land use increased annual water yield by 3.5 ML/ha during the first 6 years, but only minor changes in water quality were observed. The project was resumed in 1997 to evaluate the long-term changes in hydrology and water quality of the 17 year-old plantation compared with the historic data for the original eucalypt forest prior to conversion as well as the paired catchment of undisturbed eucalypt forest. In 1998 the radiata pine plantation was thinned and treated with phosphate fertilizer; this was followed by an application of nitrogen fertilizer in 2000. The buffer zone remained untreated throughout the study. There was no evidence to indicate that water quality has been adversely affected by the change in land use from undisturbed eucalypt forest to a more intensively managed radiata pine plantation. Although there was a small increase in turbidity from the original median value from 2 to 4 NTU since conversion of the catchment to radiata pine, turbidity and suspended solids in stream water have remained within the historic range of values of the original eucalypt forest. Turbidity and suspended solids were strongly affected by flow conditions showing strong concentration–discharge hysteresis during storm events at both catchments indicating that monitoring of these water quality indicators needs to be linked to stream flow. Application of phosphate fertilizer to the plantation at a rate of 100 kg P/ha increased median P levels in stream water from 0.002 mg/L to 0.010 mg/L during 6 months immediately following treatment before returning to antecedent values. Post-treatment exports of P over a period of 5 years were less than 0.7% of fertilizer applied. Application of nitrogen fertilizer (urea) at a rate equivalent to 139 kg N/ha increased median levels of nitrate-N from 0.04 mg/L up to 0.07 mg/L and total N from 0.10 mg/L up to 0.15 mg/L during successive 6-monhtly post-treatment periods. Export of total N was estimated at 1.2 kg/ha over a period of 2.5 years. This represents less than 0.9% of the amount of fertilizer N applied and indicates a high retention rate of nitrogen in the catchment.