Priming of soil structural and hydrological properties by native woody species, annual crops, and a permanent pasture

Impermeable subsoil is a major constraint to root growth and water infiltration in most duplex soils of Australia, but can be ameliorated by channels or biopores created by dead and decomposed roots of plant species that are adapted to these soils. In the current study, we evaluated whether a 6-year phase of native woody species planted in belts created sufficient biopores to significantly improve the soil structure of a yellow Chromosol, when compared with either continuous annual crop rotations or a permanent grassy pasture. At 10 months after belt removal, we found no difference between belt-soil and cropping-soil in terms of total number of biopores, air-filled porosity, and total porosity, all of which were less than the values measured for the pasture-soil. The belt-soil, however, had significantly more large pores (diam. greater than or equal to2.0 mm) and marginally higher hydraulic conductivity than the other 2 treatments, indicating some improvement in permeability. The limited amelioration of the belt-soil at this time was due to (1) slow decay of the thick roots of woody species, and (2) increased bulk density arising from soil shrinkage due to prolonged drying during the years of active growth by the woody species. Following a further decomposition of roots 20 months after belt removal, however, the total number of pores of all sizes was 55% higher, and of large pores 25% higher, in the belt-subsoil than in the cropping-subsoil. At the same time, estimated hydraulic conductivity was 27% greater, and air-filled porosity 23% greater, for the belt-soil than for the cropping-soil. Preferential flow technique with a food dye solution at 20 months after belt removal also suggested a 51% increase in the macroporosity for the belt-soil compared with the cropping-soil.