The porosity could also be effectively improved by application of the
biochar and hydraulic conductivity as well. Asai et al. (2009) indicated
that the incorporation of biochar into rice-growing soils changed the
pore-size distribution, which increased water permeability. Regarding
the porosity and hydraulic conductivity of the amended soils, we considered
the redistribution of the proportion of soil aggregate sizes to
be a critical factor in influencing the physical and chemical properties
of the soil (Table 2). The incorporated biochar could function as a binding
agent that connects soil microaggregates to form macroaggregates.
The oxidized biochar surface, which included hydroxyl groups and carboxylic
groups, could adsorb soil particles and clays (Fig. 4c) to form
macroaggregates under acidic environments. Our incubation study
showed that the biochar-amended soils seemed to have larger soil aggregates
than the control after 21 d although significant difference of
MWD was just found after 63 d between the amended soils and the control. Changes of soil aggregate appeared to be determined by microbial
activity during the incubation period (Figs. 2g; 3). The largestMWD
of aggregate for each treated soil occurred at 21 d, while maximum
MBC contents were also found at that time. Consistently significantly
higher MBC content for 5% biochar-amended soil throughout the incubation
duration obviously facilitated the aggregation of soil particles
at the end of the incubation.