Change in soil infiltration associated with leys in south-eastern Queensland

Cropping systems in south-eastern Queensland have led to degradation of soil physical properties and loss of infiltration capacity. Pasture leys are favoured for ameliorating soil physical properties because they add organic matter to the soil, create macroporosity, and help to re-aggregate soil. We measured change in hydraulic conductivity with period of ley for 5 major soil groups in south-eastern Queensland (Sodosols, light and heavy Vertosols, Red Ferrosols, and Red Chromosols/Kandosols). We characterised 2 soil layers that are susceptible to degradation when cropped: surface soil exposed to raindrop impact, and the layer immediately below the cultivated layer (0.1-0.2 m deep). A rainfall simulator was used to measure hydraulic conductivity of surface seals under high intensity rainfall. Disc permeameters and pressure plate apparatus were used to measure hydraulic conductivity of the soil matrix in the 0.1-0.2 m layer. Hydraulic conductivity of both soil layers improved with period of pasture for all but the light-textured Red Chromosols/Kandosols. The estimated period of pasture required to return hydraulic conductivity to pre-cultivated levels ranged from 5 to 40 years, depending on soil type and layer. This is about 2-3 times the period of cultivation that caused the degradation. Grazing reduced the effectiveness of pasture in ameliorating surface sealing on Sodosols. Beneficial effects of a 2.5-4.5 year, ungrazed ley pasture on surface soil persisted for up to 5 years after recultivation, but were lost in the 0.1-0.2 m layer within 1 year. These rates of decline in hydraulic conductivity were faster than observed on previously uncultivated soils. The APSIM model was used to predict the effect of measured improvements in soil hydraulic conductivity on average runoff from summer fallows. The model predicted that most benefits for fallow runoff would be achieved with 2-5 years of ley. The surface seal was the major limitation to infiltration when the soil was bare. Subsurface soil layers limited infiltration if surface sealing was reduced by ameliorating soil properties or maintaining cover on the soil surface. The results suggest that despite amelioration of soil structure with leys, appropriate tillage and cover management is still required to maintain high infiltration rates.