The oPOM [[delta].sup.13]C values were significantly lower in the microaggregate fraction compared with large and small macroaggregates and generally increased with depth (Table 5).
The oPOM [C.sub.3]-C stocks at 0-0.1 m depth were significantly higher in leucaena rows (3.2[+ or -]0.1 t [ha.sup.-1]) compared with mid-rows (1.1 [+ or -] 0.1 t [ha.sup.-1]) and the grass site (0.6 [+ or -] 0.2 t [ha.sup.-1]) (Fig.
However, the physical protection conferred by aggregates is related to (i) their stability (size and binding agents), (ii) their pore size-distribution and (iii) the form of occluded SOM (oPOM or molecular C).
The oPOM acts as a nucleating agent for micro-aggregate formation both within and external to macro-aggregates and, therefore, represents an important store of SOM across these fractions (Six et al.
Although the oPOM occluded in macroaggregates was enriched in C and N and formed a larger percentage of whole soil mass, this fraction may be less stable and offer a lower degree of physical protection than microaggregates.
We investigated the long-term effect of pasture type (leucaena and grass) on the quantity and origin of SOC in different aggregates, the oPOM within aggregates and NHF in Vertosols.
Remarkably, the oPOM fractions show the smallest r1395 : r1724 ratios (Fig.
The negative correlation between p(C[O.sub.2]) and r1395 (oPOM <20 [micro]m) demonstrates the suppression of interactions between SOM and metals at increasing C[O.sub.2], which negatively affects stabilisation of SOM.
There were 9 samples for the fPOM, oPOM and clay-without-POM fractions, and 22 for the clay-including-POM and soil <2 mm samples.
The left r value refers to p(C[O.sub.2]), the right r value to [Ca.sup.2+] p(C[O.sub.2]) [Ca.sup.2+.sub.ex] fPOM >20 [micro]m r2930 x (0.02) x (0.02) r1724 x (0.03) -- oPOM >20 [micro]m r1724 x (0.02) x (0.04) oPOM <20 [micro]m r1395 x (0.003) x (0.02) clay\POM r2930 -- -- [Al.sup.3+.sub.ex] r fPOM >20 [micro]m r2930 -- 0.73; -0.73 r1724 -- -0.69 oPOM >20 [micro]m r1724 -- -0.73; 0.66 oPOM <20 [micro]m r1395 -- -0.83; 0.74 clay\POM r2930 x (0.03) -0.70