Improved subsoil macroporosity following perennial pastures

Biopores left in the soil by perennial and annual pastures and their effects on macroporosity, water infiltration and the water use and productivity of subsequent wheat and canola crops were investigated in a field experiment on a Sodosol in southern New South Wales. Phases of both lucerne (4 years) and phalaris (10 years) improved the macroporosity and water infiltration into the dense B horizon compared with continuous annual crops and pastures. After removal of lucerne and phalaris, the subsoil (> 12 cm depth) contained similar numbers of pores > 2 mm diameter (228 and 190/m2, respectively) compared with a mean of 68/m2 after annual crops. However water infiltration rate after lucerne was greater than after phalaris, apparently because of more numerous pores > 4 mm, rather than a change in total porosity. The subsoil after phalaris on the other hand contained more pores 0.3 mm in diameter and a higher total porosity, possibly because of more roots around this diameter, and a longer period without traffic or cultivation. The number of lucerne biopores in the subsoil remained unchanged (170–180/m2) for at least 2 crops after the lucerne was removed although the average size decreased. The volume of water extracted from the subsoil by crops following lucerne was similar to that following annual crop/pasture for 10 of the 12 crop comparisons made. For 2 of the crops, more subsoil water (22 and 24 mm) was used after lucerne than after annuals, and in 1 season this was associated with higher yield of canola. During the 3-year study there was no winter waterlogging or post-anthesis water stress, so there was little opportunity for yield responses to improved subsoil structure. The results confirm speculation that the unfavourable structure of dense subsoils can be improved by the biological action of perennial pasture roots, although reduced wheel traffic and cultivation during the pasture phases may also play a role. Further studies will be necessary to demonstrate associated yield improvements.