LAUSR

Abstract Biochar amendment causes immediate increases in soil organic carbon (SOC), but long-term effects are unclear. Biochar properties change with time (biochar aging) potentially affecting how efficiently SOC remains in soil after decomposition, indicated by microbial carbon use efficiency (CUE, the ratio of microbial growth over carbon uptake). Effects of fresh and aged biochar amendments on microbial CUE and biomass turnover time (BTT) were determined using the H218O-DNA incorporation method. Biochar aging increased CUE (control vs. fresh vs. aged of 0.6 vs. 0.6 vs. 0.7) but decreased BTT (51 vs. 37 vs. 27 days) due to changes in soil pH. Moreover, a greater CUE but shorter BTT was found in the sandy-textured Tenosol compared to the clayey Dermosol, possibly due to protection by clay minerals. Biochar aging and associated increase in soil pH may promote soil biogenic carbon sequestration through increasing microbial CUE or decreasing BTT.