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

• Effects of biochar aged without soil processing on soil C mineralization are studied.
• Biochar aged for years has few effects on soil C mineralization.
• The effect of biochar aged for months on soil C mineralization alters little.

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 (BC10years) and 4 months (BC4months) were collected and fresh biochar (BCfresh) 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% BC10years (SB10-2), soil + 2% BC4months (SB4-2), soil + 2% BCfresh (SBf-2), soil + 5% BCfresh (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 CO2 (CO2-C) was observed in SB4-2, SBf-2 and SBf-5; but not in SB10-2, and no CO2-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 (BCfresh) or relatively young biochar (BC4months), but is not evident when aged biochar (BC10years) 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.