Water movement and macroporosity of an Australian Alfisol under different tillage and pasture conditions

We assessed the effect of different tillage practices (i.e. conventional tillage and direct drilling) and pasture conditions on the infiltration and distribution of infiltrated rain water in an Australian Alfisol. Bromide was used as a tracer for the infiltrated rain under simulated rainfall conditions. The different infiltration patterns were then related to the macroporosity of the soils.

A 25-year-old permanent pasture was found to have the highest density (number per area) of macropores and percentages of transmitting macropores. A 9-year-old pasture phase in a pasture/crop rotation did not fully restore the macroporosity of the soil. Conventional cultivation by scarifying to 0.1 m for 4 years significantly reduced macropore density as well as continuity when compared with the pasture soil. The reduced macroporosity led to increased run-off by reducing preferential flow and altered the pathway of infiltrated water movement. As a consequence, the increase in water content below 0.1 m in the cultivated soil was predominantly from downward displacement of original soil solution, resulting in more leaching. The infiltrated rain water largely remained on the surface 0.1-m layer. In contrast, macroporosity found under direct drilling was similar to that of the pasture soil.