Carbon sequestration under subtropical perennial pastures II: Carbon dynamics

Here we take advantage of the stable carbon isotope shift that occurs when a C4 plant is sown into a soil previously dominated by C3 vegetation, to explore the movement and fate of newly sequestered soil organic carbon (SOC) following establishment of subtropical perennial pastures in temperate regions of Australia. In kikuyu-based pastures up to 33 years of age, SOC accumulated exclusively in the coarse size fraction (>50 μm) in the sandy soils of southern Western Australia. In South Australian loams, regardless of pasture age, new SOC was found to accumulate in both the coarse and fine (<50 μm) size fractions. These differential results suggest that in soils with low clay content, new SOC remains in an unprotected form that is highly vulnerable to loss through decomposition and erosion. The Rothamsted Carbon Model, modified to track changes in stable isotopes, was able to represent the changes in total SOC stocks in both regions; however, the model over-predicted the incorporation of the new C4-SOC into the soil. This difference between data and model output could be reconciled if a greater proportion of new SOC is rapidly mineralised without being incorporated into any sort of stabilised pool.