Biochar aging increased microbial carbon use efficiency but decreased biomass turnover time.

• Biochar aging increased microbial carbon use efficiency (CUE) due to increase in pH. • Biochar aging decreased soil microbial biomass turnover time (BTT). • CUE was soil specific with higher CUE in sandy-textured soil than clay rich soil. • BTT was longer in a clay rich soil than in a sandy soil. 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 H 2 18O-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. [ABSTRACT FROM AUTHOR]