Carbon mineralization following additions of fresh and aged biochar to an infertile soil

Abstract Biochar (BC) aging that has not undergone soil processing before adding to soils may have a different effect on soil carbon (C) mineralization in the short term. This article focuses on studying short-term effects of fresh and aged biochars (without soil processing) on C mineralization in a typical infertile soil in the hilly red soil region of southern China. Tree bark charcoals with known ages of 10 years (BC 10years ) and 4 months (BC 4months ) were collected and fresh biochar (BC fresh ) was produced in the laboratory using the same feedstock under similar conditions. A 42 day incubation experiment was conducted with treatments consisting of soil + 2% BC 10years (SB10-2), soil + 2% BC 4months (SB4-2), soil + 2% BC fresh (SBf-2), soil + 5% BC fresh (SBf-5) and soil only (CK). Treatments with clean quartz sand (with a similar volume) instead of the soil were also conducted. During the incubation, enhanced C released as CO 2 (CO 2 -C) was observed in SB4-2, SBf-2 and SBf-5; but not in SB10-2, and no CO 2 -C was detected in the quartz sand treatments. Biochar additions increased the C amount in the treated soil. After incubation, minor changes of dissolved organic C were detected after biochar additions. The highest values of dissolved organic nitrogen and ammonium nitrogen were detected in SB10-2. Biochar additions increased microbial biomass C, and pH levels with the highest values recorded in SBf-5 (14.71 mg kg − 1 and pH 5.30). The results suggest that soil C mineralization can be enhanced by the addition of fresh (BC fresh ) or relatively young biochar (BC 4months ), but is not evident when aged biochar (BC 10years ) is added. The biochar, whether newly produced or aged for several months without soil processing, can promote C mineralization of infertile soils in a relatively short time, but this does not appear to be the case for biochar aged for 10 years without soil processing.