Crop rotation effects on soil carbon and physical fertility of two Australian soils

The effect of using different crop rotations, including legumes and fallows, on soil structural stability, unsaturated hydraulic conductivity, and the concentration of different carbon fractions was examined in a long-term rotation trial established in 1966 on a Black Earth (Pellic Vertisol) and a Red Clay (Chromic Vertisol) soil. There was a large decrease in the concentration of soil carbon fractions following cropping and cultivation on both soils. The inclusion of some legume rotation crops resulted in an increase in labile carbon concentrations compared with continuous wheat or a long fallow treatment. Aggregate stability to wetting under both immersion and tension wetting was reduced as a result of cropping and cultivation for both soil types. However, there was an improvement in aggregate stability with immersion wetting, on the Red Clay soil, for the lucerne (Medicago sativa), clover (Trifolium subterraneum), and continuous wheat (Triticum aestivium) treatments compared with the long fallow. Similar results were found for the Black Earth soil; however, on this soil the medic (Medicago scutella) rotation also showed an improvement in soil structure. On the Red Clay soil there was a decrease in hydraulic conductivity (K) with cropping, at all tensions measured. K for the Black Earth soil was higher at 30 and 40 mm tension on the cropped soil than on the uncropped reference soil, but at 10 mm tension the reference soil had a higher K value than all rotations except the lucerne. There was a significant correlation between labile carbon and all determinations of aggregate stability for the Red Clay soil. Farmers should be encouraged to eliminate long fallowing and to adopt no-till techniques combined with the return of residues from either the primary crop or rotation crops which have a slower breakdown rate, as this management is likely to have a better potential for increasing soil carbon content and improving soil structure. The investigation of ways to better increase the quantity and quality of soil organic matter and hence soil chemical and physical fertility is necessary if long-term sustainable agriculture is to be possible.