Biochar carbon dynamics in physically separated fractions and microbial use efficiency in contrasting soils under temperate pastures

There is overwhelming evidence for the long-term persistence of biochar in soil. However, the partitioning of biochar into light and heavy carbon (C) fractions and microbial biomass C (MBC), and the dynamics of C use efficiency (CUEE: net incorporation of biochar into MBC per unit of biochar-C consumed, including microbial death and recycling of biochar-derived microbial metabolites) in planted soil systems are poorly understood. A 13C-labelled wood biochar (δ13C: −36.7‰) was incorporated into topsoil (0-10 cm) in an Arenosol, Cambisol and Ferralsol under C3 dominated temperate pastures (δ13C: −25 to −27‰). The partitioning of biochar-C into the various soil C pools and CUEE were measured at 4, 8 and 12 months. The results showed that 8.6-28.2% of the biochar-C in the top soils was distributed to the heavy fraction (HF) within 4 months, which increased to 11.0-33.3% at 8 and 12 months. Biochar-C recovery in the HF was the highest in the Ferralsol (cf. Arenosol and Cambisol), possibly due to greater interaction of biochar and biochar-derived microbial metabolites with soil minerals. Biochar significantly increased MBC across the three soils. Biochar-derived MBC ranged from 22 to 93 mg C kg−1 soil over time (Arenosol < Cambisol < Ferralsol), representing 11-20% of the total MBC pool. Biochar CUEE was 0.20-0.27 at 4 months, which decreased over time, possibly due to lowering of biochar-C availability to microbes. Further, although biochar-derived MBC was higher, biochar CUEE was lower in the Ferralsol (cf. Arenosol and Cambisol), likely supported by higher microbial respiration and turnover, and lower recycling of microbial metabolites via greater organo-mineral interaction. Here, the study advanced our understanding of key C cycling processes, such as CUEE and the temporal fate of biochar-derived C in an organo-mineral fraction with relevance for biochar sequestration in contrasting soils under planted field conditions.