Soil C and N concentrations under hoop pine plantations were almost uniformly distributed across all 4 aggregate fractions (Fig.
Following the dispersion of water-stable aggregates, iPOM in the top 0.1 m of soil was significantly (P<0.05) higher under pasture and rainforest than the 25- and 63-year hoop pine plantations, and rainforest iPOM content was higher than the 50-year hoop pine plantation (Table 1).
Soil under the 63-year hoop pine plantation had significantly (P<0.05) higher concentrations of [Fe.sub.d-o] at all depths than the other sites (Table 2).
In contrast, the [Fe.sub.o]/[Fe.sub.d] ratio was strongly and positively correlated with total soil C at all sites except the 25-year hoop pine plantation (Fig.
In our study we cannot rule out that differences in rates of C input and changes to soil properties during site preparation and thinning have contributed to the reduced C storage in the top 0.3 m under most hoop pine plantations, compared with rainforest and pasture.
The greater concentration of iPOM under pasture and rainforest indicates that SOC is protected to a greater extent within aggregates than in the hoop pine soils, which are dominated by C associated with silt- and clay-sized particles (<53 [micro]m).
If this is also the case for plantations, then it appears that hoop pine soils, with greater proportions of total C in imSOC fractions, are dominated by a pool of old stabilised C protected by silt- and clay-sized particles, whereas physical protection of SOC in larger aggregates may not be an important mechanism for SOC stabilisation.
The greater storage of C within aggregates in pasture and rainforest soil, compared with hoop pine plantations, may be due to different root characteristics.