Limited potential for soil carbon accumulation using current cropping practices in Victoria, Australia

The extent to which soil C storage can be increased in Australian agricultural soils by adoption of improved management practices is poorly understood. There is a pressing need for such information in order to evaluate the potential for soil C sequestration to offset greenhouse gas emissions. In this study we used the RothC model to assess whether soil C accumulation under cropping using stubble retention and pasture rotations could be a significant offset for greenhouse gas emissions. We chose eight regions to represent the climatic range of the Victorian cropping industry: Walpeup, Birchip, Horsham, Bendigo, Rutherglen, Lismore, Bairnsdale and Hamilton (annual rainfall 330–700 mm). For each region, we chose two representative soil types, varying in clay and total organic C contents. For each region × soil combination, we compared the effects of five rotations: Canola–wheat–pulse–barley (C–W–P–B); Canola–wheat–triticale (C–W–T); Canola–wheat–barley–5 year perennial pasture (C–W–B–Pt5); Canola–wheat–fallow (C–W–F) and Continuous pasture (Pt). We compared the cropping rotations with cereal stubble burnt and with cereal stubble retained and, for two regions, with cereal stubble grazed by sheep. The results of the simulations showed that, across all scenarios, the equilibrium C density varied between 19 and 135 t C/ha to 300 mm depth, with potential soil C change being strongly influenced by crop yield, crop rotation, climate, initial soil C content, stubble management and continuity of management. The simulations suggested that soil C stocks could be increased under a crop-pasture rotation (C–W–B–Pt5) with stubble retention, with rates of increase of 0.3–0.9 t C/ha yr over 25 years. If all of Victoria's cropland were converted to C–W–B–Pt5 rotation with stubble retention, and if 50% of the modelled potential C change were achieved, this would represent 3.0–4.5 MtCO2-e/year, equivalent to 2.5–3.7% of Victoria's greenhouse emissions. Less C accumulation would be possible under continuous cropping with stubble retention; even using the most conservative rotation (C–W–T) rates of C change varied from loss of 0.3 t C/ha yr to accumulation of 0.5 t C/ha yr over 25 years. If all of Victoria's cropland were converted to C–W–T rotation with stubble retention, and if 50% of the modelled potential C change were achieved, this would be equivalent to 0.8–2.3 MtCO2-e/year, or 0.7–1.9% of Victoria's greenhouse emissions. It would generally take 10–25 years for the soil C changes to become measurable using conventional soil sampling and analytical methods. Thus we conclude that, with current technology, the potential for significant and verifiable soil C accumulation in Victoria's croplands is limited.

► The RothC model was used to assess the potential for soil organic C accumulation under various cropping practices. ► Crop-pasture rotations with stubble retention generally accumulated C. ► Continuous cropping with stubble retention resulted in loss or accumulation of C. ► The potential for accumulation of C in soil to abate greenhouse gas emissions is limited. ► The amounts of C accumulation realistically expected with modifications to arable cropping are unlikely to be measurable within less than 25 years.