DIRECT EFFECTS OF STUBBLE BURNING ON SOIL HYDRAULIC AND PHYSICAL-PROPERTIES IN A DIRECT DRILL TILLAGE SYSTEM

Stubble burning is widely practiced in cropping systems and is often u tilised as a means of reducing stubble loads on the soil surface. Howe ver, the short-term effects of these burning regimes on soil surface p roperties are largely unknown. Therefore, a study was carried out at C owra, N.S.W., Australia, to investigate the direct effects of stubble burning on the hydraulic and physical properties of a hardsetting red- brown earth soil. Five paired 1 m(2) plots were set up in a paddock th at had been direct drilled for 15 years. Stubble in one plot of each p air was burnt by a low intensity fire (2.8 Mg ha(-1) fuel load), while that in the other plot remained unburnt. Two days later the soil hydr aulic properties of the burnt and adjacent unburnt plots were measured using disc permeameters at a supply potential of - 10 mm. The volumet ric moisture content and bulk density of the 0-40 mm layer were also m easured using undisturbed cores. Disturbed soil samples were taken fro m the 0-20, 20-50 and 50-100 mm depths from both treatments for labora tory determinations of water repellency, organic carbon, clay dispersi on and aggregate stability. There was a significant (P < 0.05) decreas e of approximately 50% in sorptivity, final infiltration rate and hydr aulic conductivity in the burnt plots relative to the adjacent unburnt plots. However, fire had no significant effect on volumetric moisture content or bulk density in the 0-40 mm layer. No water repellency was recorded before or after fire. Clay dispersion and aggregate stabilit y tests found no significant differences between burnt and unburnt plo ts. These findings indicate that low intensity fires used for removing stubble can have an immediate, direct effect on soil hydraulic proper ties. However, the mechanisms causing these effects are as yet uncerta in. (C) 1997 Elsevier Science B.V.