THE EFFECTS OF DIRECT DRILLING AND STUBBLE RETENTION ON WATER AND BROMIDE MOVEMENT AND EARTHWORM SPECIES IN A DUPLEX SOIL

Water infiltration and movement into soil are important factors for cr opping systems in texture contrast soils subject to runoff and erosion under unsteady rainfall. This study considers the effects of 10 years of direct drilled (with stubble retained) wheat, in a duplex soil (lu visolic type soil with sandy clay loam A horizon and clay subsoil) in southeastern Australia, on water penetration into both dry and wet soi l profiles, in comparison with conventional tillage (soil cultivated 1 00 mm after stubble burnt). Earthworm populations, biomass, and specie s diversity were determined under each tillage system. Surface-applied bromide in solution was used in the field, under ponded conditions, t o trace water movement into the soil profile and to detect differences in leaching patterns between tillage systems. In dry soil conditions, conventional tillage retarded the movement of water, and the soil sur face quickly became saturated. Bromide penetration and movement was in creased below 100 mm by direct drilling, in comparison with convention al tillage. Bromide added to a wet soil tended to concentrate at the s urface 50 mm under conventional tillage, compared with direct drilling . Preferential flow, where a proportion of infiltrating water bypassed the soil matrix, apparently occurred in the 0 to 100-mm depth under d irect drilling. Unsaturated flow and impeded water movement were evide nt near the surface of the conventional tillage. It is suggested that different soil structures and soil stabilities directly influenced wat er infiltration, although antecedent moisture conditions appeared to s trongly influence leaching patterns. Characterization of earthworm abu ndance and species indicated that conservation tillage increased the p resence of Aporrectodea trapezoides and Aporrectodea rosea, which form s extensive, vertical burrows. Overall, direct drilling improved the m ovement of water into the A horizon of duplex soils.