Water budgets and nutrients in a native Banksia woodland and an adjacent Medicago sativa pasture

Abstract

In southern Australia, the spread of dryland salinity can be traced to increased leakage of water through the root zone to the groundwater, associated with clearing of perennial vegetation and its replacement with annual crops and pastures. Agricultural activity, through fertilizer addition and subsequent nutrient export, has also changed the nutrient status of the soils, often causing increases in soil acidity. In this trial, an area of native vegetation on a deep sandy soil in south-western Australia (dominated by Banksia prionotes Lindley), and an adjacent introduced perennial pasture (Medicago sativa L.), were compared in terms of their water balance and nutrient fluxes for the period between August 1998 and March 2001. Initially, the Banksia woodland vegetation maintained a drier soil profile below 2 m than the establishing lucerne pasture, and leakage beyond 4 m in 1999 was 80 mm under the Banksia woodland and 180 mm under the lucerne. However, in 2000, lucerne's rate of water use during winter was faster than any other vegetation observed on this soil type, possibly due to direct groundwater extraction, and it dried the soil to the same level as the Banksia woodland vegetation. Nutrient (nitrate, phosphate, sulphate, potassium, calcium, magnesium and sodium) fluxes under both systems were generally low, reflecting the inherently low fertility of the soil type. However, sodium and nitrate appeared to accumulate in soil at a depth of 4 m under the Banksia woodland (particularly between the Banksia canopies), but not under the lucerne, possibly due to a history of leaching under the lucerne. Whilst both vegetation types effectively controlled excess water leakage, the differences in nutrient cycling and production levels suggests that some aspects of native perennial vegetation function may not be suitable for incorporation into agricultural systems.