Transformation of biochar into extracted humic substances under short-term laboratory incubation conditions: Evidence from stable carbon isotopes

Agricultural waste can be made into biochar, which serves to raise soil fertility and fix carbon (C). When biochar is added to soil, a small part of biochar could be transformed into soil organic C in a short time to promote soil fertility. However, it is still unknown how much biochar can be stored in the soil as stable C and whether it plays a role in C sequestration. In this study, a 540-day incubation experiment was conducted to evaluate the transformation of maize straw biochar into humus fractions using a 13C isotopic labelling technique. The treatments were CK (no biochar applied) and AB (21.3 g kg-1 of biochar + CK). The results showed that: 1) Compared with those in the CK treatment, the concentrations of organic and humus C in the bulk soil were significantly higher under the AB treatment. 2) The δ13C values of soil humus C were higher (ranging between 0% and 23.45%) in the AB treatment than those in the CK treatment. 3) The proportions of biochar transformed into humic acid (HA), fulvic acid (FA), water-soluble substances (WSS), and water floating substances (WFS) were 2.66–4.30%, 1.11–1.57%, 0.49–0.88%, and 0.68–1.40%, respectively. The remaining biochar (59.44–98.00%) remained in the crude humin form, retaining the inert nature of the biochar itself. Although biochar is relatively stable, approximately 4.22–5.40% of biochar was transformed into extracted humic substances (HA+FA). We concluded that a relatively small amount of biochar could be transformed into soil organic matter under short-term (540 days) incubation conditions. •Biochar increased the carbon (C) concentrations of soil and its humus fractions.•Biochar significantly increased the δ13C values of soil and its humus fractions.•Only 4.2–5.4% of biochar could be transformed into extracted humic substances.•Most of biochar (59–98%) were retained in soil in the form of solid C.