For the density separation of bulk soil, 10 g of air dry bulk soil (<2 mm) was mixed with 30 mL sodium polytungstate
(SPT, 3[Na.sub.2]W[O.sub.4].9W0.X[H.sub.2]O; SOMETU, Berlin) following the procedure described by Cerli et al.
, which involved shaking the soil in sodium polytungstate
with a specific gravity of [rho] = 1.7 and then allowing soil minerals to settle for 48 h.
The inter-microaggregate POM was isolated from the fine sand, the intra-microaggregate POM (iPOM), and the silt + clay fraction within the microaggregates by density flotation with 1.85 g/[cm.sup.3] of sodium polytungstate
(SPT) (Sometu Europe, Germany) following a procedure very similar to that used by Six et al.
Instead, sodium polytungstate
solutions were made varying in density up to 3.1 g/cc.
If foraminifera were not abundant, as in the Arroyo Colorado samples, they were concentrated for ease of counting using a sodium polytungstate
flotation technique (Callahan 1987).
Briefly, 20 g of air-dried soil (<2 mm) was placed in a centrifuge tube containing 40 mL of sodium polytungstate
(SPT; [Na.sub.6] ([H.sub.2][W.sub.12][O.sub.40])) with [rho] of 1.60 [+ or -] 0.03 g [cm.sup.-3] and gently inverted 20 times by hand.
Following initial separation, the free light fraction (LF) material from a 5-g subsample of the aggregate fractions >2000p-m and 250-2000 [micro]m, and a 3-g subsample from the 53-250 [micro]m fraction, was removed using density flotation in 1.6 g/[cm.sup.3] of sodium polytungstate
(Richards et al.
They showed that C-rich light fraction (180 to 210g C [kg.sup.-1] fraction), separated by flotation on sodium polytungstate
solution with specific gravity 2.0 g [cm.sup.-3] accounted for 40-45% of the C in the silt fraction of Brazilian soils (but only 5-10% of silt mass).To conclude, we were unable to identify any specific physico-chemical factor that would account for the relatively low C concentrations observed in the <5 [micro]m and 5-20 [micro]m fractions of fine textured soils.
Briefly, 10g of air-dried soil (<2 mm) was placed into 40 mL of sodium polytungstate
solution (1.6 Mg/[m.sup.3] density) in a 50-mL plastic centrifuge tube.
The SEM micrographs of samples after density separation with sodium polytungstate
([rho] = 2.3 g [cm.sup.-3]) demonstrated a problem that occurred in this step of the sequential extraction: The light fraction containing the biogenic amorphous silica was contaminated with silt- and clay-size minerals, e.g.
Briefly, 10 g of air-dried soil (<2 mm) was placed into 40mL of sodium polytungstate
solution (1.6 t/[m.sup.3] density) in a 50-mL plastic centrifuge tube.
Briefly, 10g of air-dried soil (< 2 mm) was placed into 40 mL of sodium polytungstate
solution (1.6 g/[cm.sup.3] density) in a 50-mL plastic centrifuge tube.