Bulk soil samples were taken by using a spade in the cultivated
horizon (Ap, 0–150mm) from five sites in each plot after the
harvest of second rice in 2011. The soil samples were mixed to
obtain a composite sample, which were manually separated into
20–40mmaggregates along the natural failure surfaces after being
air-dried to the plastic limit. After air drying, part of the soils was
ground to pass a 0.25-mm sieve to measure SOC concentration by a
wet oxidation method with K2Cr2O7 and concentrated H2SO4
Measurements were repeated at 15 sites randomly distributed in
each plot. The readings of resistance force were used to calculate
sinkage resistance by dividing the plate surface area and to calculate
penetration resistance by dividing the maximum area of the cone.
The readings of 15 replicates were averaged for each depth. The
measurement using penetrometer can only be carried out without
interference of crop. Therefore, thismethod was not applied during
the rice growth period.