Deep drainage rates of Grey Vertosols depend on land use in semi-arid subtropical regions of Queensland, Australia

Changes in land use can affect the soil water balance and mobilise primary salinity. This paper examines changes in soil chloride (Cl) and deep drainage under pasture and annual cropping on five gilgaied Grey Vertosols in southern inland Queensland, Australia, comparing them to remnant native vegetation. Transient soil Cl mass-balance (CMB) was used for crop and pasture sites, as it is suitable for determining the long-term, low rates of drainage since clearing some 40-50 years ago. Steady-state CMB was used for native vegetation. Large masses of salts and Cl were stored under native vegetation (31-103t/ha of Cl to 3.2m), and deep drainage was low (0.10-0.27mm/year). The Cl profiles were generally of a normal shape for matrix flow (e.g. no bypass flow). Soil Cl was lost under cropping (average 65% lost to 1.4m) and pasture (32%) compared with native vegetation. This lost Cl was not stored within the top 4-5m of soil, indicating movement of water below 4-5m. Deep drainage averaged 10mm/year under cropping for both gilgai mounds and depressions (range 2.7-25mm/year), and 3.3 and 5.1mm/year under pasture for mounds and depressions, respectively. Subsoil (depth 1.5-4+m) was generally dry under native vegetation and wetter under cropping and pasture. Deep drainage over the last 40-50 years was stored in the unsaturated zone (to deeper than 4+m), indicating a long time lag between land-use change and groundwater response. Steady-state CMB greatly underestimated drainage for crop and pasture sites