Mechanisms of protection of soil organic matter under pasture following clearing of rainforest on an Oxisol

Vegetation change from rainforest (C3) to pasture (C4) was used to study the nature and capacity for organic matter protection in an Oxisol from eastern Australia. As much as 17.1% and 31.0% of the organic carbon in the 0–0.075 and 0.075–0.15m horizons, respectively, derived from C3 rainforest was still present even after 90 years under continuous C4 pasture. A considerable proportion of the organic carbon is therefore protected in the surface horizons of these oxidic soils. High energy uv photo-oxidation was used to determine the nature of the protection. With increased time of photo-oxidation, the relative proportion of C3-derived OC in the < 53 μm fractions increased under the pasture demonstrating that microaggregates had a significant role in protection of the older C3-derived OC against decomposition. Although tannins or tannin-like materials were present within the < 53 μm microaggregates under rainforest and hence were resistant to photo-oxidation, they were not detectable under the pasture using solid-state 13C NMR spectroscopy. The aryl-C components under pasture appear to be derived from lignin rather than tannin but cannot be attributed to the presence of charcoal. Lipids of microbial origin may also contribute considerably to C3-derived OC under pasture through chemical recalcitrance to decomposition rather than physical protection. There is no evidence that a significant proportion of plant-derived lipids survive in these soils and contribute to a stable pool of soil OC. The work presented here demonstrates that protection within microaggregates is the major mechanism for protection of OC in this Oxisol.