Declines in soil physical, chemical and biological fertility have occurred in the vertisols in northwestern New South Wales after conversion of natural pasture land to monoculture wheat cropping. The effectiveness of a pasture phase, including Mitchell grass (Astrebla lappacea), in restoring the fertility of degraded cropped soils in the semi-arid subtropical environment was evaluated on 2 different vertisols. For a non-sodic brown clay with higher soil organic carbon level, significant increase in soil organic carbon was detected 2 years after establishment of the pasture and this was accompanied by significant increases in available nitrogen and microbial biomass and improvement in soil structural stability. However, these changes were detected only in the top 5 cm layer even 4 years after the establishment of the pasture. Improvement in subsoil structure as indicated by a nearly 4-fold increase in sorptivity was also observed due to the creation of burrows formed by the return of native earthworms when the cropped soil was returned to pasture.However, no change in organic carbon level and therefore all associated improvements in soil fertility were detectable in a sodic grey clay with low organic carbon level even 4 years after establishment of the pasture.
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