2.3. Chemical dispersion with removal of sesquioxides and SOC
Fe/Al oxides and SOC are major binding agents in soil aggregation in
the soils studied. We devised four specific treatments for each paddy
soil to determine the contribution of Fe/Al oxides and SOC to soil cracking:
(i) water dispersion as a control which was the same as the treatment at
0 J ml−1 in Section 2.2, (ii) removal of amorphous Fe/Al oxides from soil
with 0.2 mol L−1 oxalate solution (pH 3.0) in the dark, (iii) removal of
free Fe/Al oxides from soil with dithioni-citrate-bicarbonate (DCB) solution,
and (iv) removal of SOC from soil by 30% H2O2. The dark condition
in the oxalate treatment was assumed to have no influence on aggregate
disruption. Each treatment had three replicates. The procedure was as
follows: 60 g b2 mmair-dried soil was transferred into a 1000 ml plastic
beaker, to which 600 ml solution (soil:solution 1:10) was added. The soil
and solution were mixed with slight stirring for 10 s using a glass rod.
Then the soils were reacted with each solution for 10 h in a constant
temperature incubator with 25 °C without any disturbance. For oxidizing
SOC, 600 ml 30% H2O2was added and allowed to reactwith the sample
until no more bubbles were observed, which lasted for 5 days. For the
treatments (ii) and (iii) the standing water in the beaker were siphoned
out after 10 h reaction, and then distilled waterwas added into the beaker
again. This procedure was repeated (approx. 4–5 times) until water
electrical conductivity was close to 0 in order to remove the ion effect.
After the above procedure, a small portion of the treated soil (0.3–0.5 g)
was used to determine the aggregate size distribution by laser particle
analyzer. The soil in the beaker was transferred into petri dishes with a
diameter of 10 cm for incubation.