Soil chloride and deep drainage responses to land clearing for cropping at seven sites in central Queensland, northern Australia

Soil cores were taken at seven paired sites (native vegetation and adjacent dryland cropping on cracking clay soils) which had been cropped for 10–65 years in the Fitzroy Basin in central Queensland, northern Australia. Levels of soil chloride (Cl −) and nitrate nitrogen (NO 3–N) were determined in 0.3 m increments to a depth of 5 m where possible. The amounts of Cl − in the soil (0–1.5 m depth) under native vegetation were generally high (10–23 t ha −1 at six of the seven sites). The amounts of Cl − that had leached below 1.5 m depth during dryland cropping varied from 2.2 to 16.8 t ha −1 or 19–91% of the original totals at 0–1.5 m. Leaching of salt from the crop rooting zone in combination with higher rates of deep drainage can lead to outbreaks of soil salinisation but can also increase the soil plant available water capacity (PAWC). NO 3–N had also been leached below crop rooting depth at three sites. Such leaching not only contaminates the groundwater but also wastes crop nutrients. The transient chloride mass balance approach was used to determine mean annual rates of deep drainage below crop rooting depth (1.5 m). At all seven sites annual deep drainage was low under native vegetation (0.2–1.7 mm yr −1) but increased under dryland cropping (1.6–27.5 mm yr −1). Drainage losses showed an inverse relationship with plant available water content (PAWC). Drainage losses waste the limited supply of water available for dryland cropping but can be reduced by practising opportunity cropping or by growing ley (temporary) pastures in rotation with annual crops.