The effects of direct drilling and stubble retention on hydraulic properties at the surface of duplex soils in north-eastern Victoria

Abstract

Soil hydraulic properties in the sandy clay loam surface horizon of duplex soils were assessed under cultivated, stubble burnt (CCB); direct drilling, stubble burnt (DDB); and direct drilling, stubble retained (DDR) systems for three wheat-lupin rotations, established from 7 to 10 years, in north-eastern Victoria. Disc permeameters were used to determine sorptivity and steady state infiltration in each rotation. A rainfall simulator was used on the 10 year rotation to characterize saturated infiltration and surface soil stability. Cultivation caused an increase in soil bulk density and decreased organic C at the soil surface. Application of a -40 mm water supply potential removed macropore flow from the infiltration process and mainly characterized water flow in the soil matrix. Differences in sorptivity among tillage treatments at this potential mainly reflected initial soil moisture, rather than soil structure. Sorptivity measured on soil cores was related (T = 0.74, P = 0.01)= to sorptivity measured in the field. Under Rainfall simulation , DDR increased sorptivity, wetting depth and time to runoff, and decreased runoff rate and sediment loss, compared with DDB and CCB. Overall, steady state infiltration rate was controlled over time by the permeability of lower soil horizons. The combination of disc permeameter and rainfall simulation measurements provided a useful description of unsaturated and saturated infiltration under field conditions. The DDR system improved the potential for saturated infiltration, maximized rainfall storage in the surface horizon, and increased the stability of macroporous infiltration.