The WHC was determined using a method adapted from Case et al.
(2012). 150 g dried soil or soil-biochar mixture (2% w/w) was added
to plastic cylinders (inner diameter 5 cm, height 7 cm), with the bottom
end covered in a fine mesh. Each cylinder was submerged in a water
tank for 8 h. The cylinders were covered with plastic film to limit evaporation.
The samples were then removed, permitting drainage by gravity
for 3 h, weighed after three hours, heated to 105 °C for 12 h and reweighed.
The WHC of soil and soil-biochar mixture are shown in Table
S1. Dissolved organic matter (DOC) was extracted from the mixture of
biochar and soil with distilled water (1:1 w/v) for 30 min. The mixture
was then centrifuged with a rotation speed of 3000 rpm for 15 min and
the supernatant was collected and filtrated through a 0.2 mm membrane.
DOC content was measured with a Shimadzu TOC-VCPH analyzer.
Compositional changes of surface functional groups in control and biochar-amended
soils were observed by Fourier Transform infrared spectroscopy
(FTIR, Thermo scientific, Nicolet 6700). Chemical and physical
properties of the soils are shown in Table 1. Other properties of biochar
are shown in Table S1.