This study intended to evaluate the impact of biochar
amendment on some of the physical and hydraulic properties of
a clay soil. Equivalent hydraulic conductivity values were obtained
for unamended and amended soils for both saturated and
unsaturated hydraulic conductivity at the same pressure head.
Independent of the values of initial soil water content, higher Kfs
values were always detected for the highest biochar concentration,
following the sequence C30 > C5 > C0 > C20 > C10. The relative
changes to the control between amended and unamended soils
ranged between
8%/+18% and
7%/+32%, respectively for low and
high ui values. Relatively similar results were obtained under
unsaturated conditions, although the higher K(h) values were
detected for the unamended soil (C0 > C30 > C10, and max
RC =
61%). Significant increases of soil water retention were
observed only close to water saturation (i.e., 0 < h <
10 cm) for the
highest biochar concentration, C30. For the latter, the RC value of
saturated water content was equal to +11%. Soil amendments also
appreciably modified the capacitive indicators. The highest biochar
concentration significantly increased macroporosity and air
capacity, as well as significantly decreased available water and
relative
field capacity. Similar
findings were obtained for C10, but
only for Pmac. In line with the reference values in the literature and
for the studied clay soil, our results show that an excessive
concentration of biochar may cause an imbalance between the
liquid and gaseous phases. This can be clearly seen from relative
field capacity results, since detected RFC values were lower than
the critical limit of 0.6. While C10 improved the relative
field
capacity, thus creating an optimal balance between water and air,
C30 significantly worsened it, with the result that insufficient
water was available for agricultural soils. A logical implication of
these
findings is that the biochar addition has the potential to
improve the physical properties of soils with low levels of
macroporosity. The highest saturated hydraulic conductivity
values of C30, extrapolated at the
field capacity of the regression
between Kfs and ui, support this interpretation. However, since
these significant differences were found in undisturbed soil
samples of similar bulk density, our results suggest that small
differences in rb may also result in appreciable modifications in
soil water retention. However, although a comprehensive and
thorough evaluation of the biochar effects on physical and
hydraulic properties of the soil is time consuming and could also
appear tedious, our investigation also confirms the need of testing,
during accurate preliminary investigations, the effects of these soil
conditioners before
field use, especially when their expected
effects are biochar and soil-specific. The simulations carried out
with DSSAT suggest that, a moderate addition of biochar (not
higher than 10 g kg1), has the potential to increase the durum
wheat yield (mean increase
standard deviation, 236
126 kg
ha1). However, this result must be interpreted with caution as it
needs to be verified under real
field conditions. Since the present
study gives an account of the results obtained using a biochar
applied to a clay soil, further
field research is planned (i.e., under
long-term
field experiments or considering different soil texture
and biochar types), in order to evaluate whether the use of biochar
can be integrated into sustainable agricultural systems, as well as
to evaluate its effects on both crop yields and the physical quality of
the soil.