Aggregate hierarchy and carbon mineralization in two Oxisols of New South Wales, Australia

Abstract

The conventional model of aggregate formation suggests a hierarchy where micro-aggregates with lower porosity and therefore reduced soil organic carbon (SOC) mineralization form inside macro-aggregates. This model has however been questioned for highly weathered Oxisols where inconclusive results regarding the presence of aggregate hierarchy have been obtained to date. We hypothesized that in Oxisols (i) an aggregate hierarchy would be present (ii) the porosity of micro-aggregates would be lower than that of macro-aggregates and (iii) pore geometry of aggregates would influence SOC mineralization. We collected topsoils from Oxisols in northern New South Wales, Australia from which macro-aggregates (>250 μm), micro-aggregates (53-250 μm) and <53 μm fractions were isolated from bulk soil by wet sieving. 3D images of macro- and micro-aggregates were produced using X-ray computed tomography (μCT) showing the presence of micro-aggregates inside macro-aggregates, which confirmed the presence of an aggregate hierarchy in the Oxisols studied. Macro-aggregates were more common and SOC in higher concentrations in forest systems compared with agricultural (the cultivation or pasture) land-uses, but aggregate geometry differed little between the land-uses studied. The porosity of macro-aggregates (4%) was significantly lower than micro-aggregates (5.5%). Despite the differences in pore geometry between macro- and micro-aggregates, SOC mineralized (SOC'min') during a 2-month incubation (at 25°C) was similar in macro- (3% of SOC concentration) and micro-aggregates (2.8% of SOC concentration). We conclude that although aggregate hierarchy exists in these soils and that aggregate geometry did differ between aggregate size classes, there was no evidence to support the porosity exclusion principle and the assumption that SOC is preferentially stabilized within micro-aggregates in these soils.