Distinct Mechanisms Govern Sucrose Mineralization and Soil Organic Carbon Priming in Biochar Amended Soils: Evidence from 10-Years Field Studies

While many studies have examined the role of biochar on soil organic carbon (SOC) dynamics in short-term studies, few have explored the decadal scale influences of biochar on the mineralization of new carbon (C) inputs (i.e., rhizodeposits) and native SOC. To address this knowledge gap, soils were collected from two decade-old biochar field trials sown with agricultural crops located in the United Kingdom (Cambisol) and China (Fluvisol), with each site having had three application rates (25-30, 50-60 and 75-100 Mg ha -1) of biochar plus an unamended control, applied once in 2009. We assessed biotic and abiotic mechanisms associated with sucrose mineralization and the priming effect (PE) on native SOC. Both soils amended with biochar at the middle application rate (50 Mg ha-1 biochar in Cambisol and 60 Mg ha-1 biochar in Fluvisol) resulted in greater sucrose mineralization. This might be due to the improved soil porosity and pore connectivity (revealed by synchrotron-based micro-CT), increasing the microbial accessibility to sucrose, particularly for the fast-growing bacterial genera (r-strategists) Arenimonas, Spingomonas and Paenibacillus (affiliated to phylum Proteobacteria and Firmicutes, respectively). Random forest analysis also confirmed the contribution of the phyla Firmicutes and Proteobacteria, as well as porosity (<250 μm), to sucrose mineralization. Biochar at the middle application rate, however, resulted in the lowest PE (0.3 and 0.4 mg of CO2-C g soil-1 in Cambisol and Fluvisol, respectively, after 56 days incubation), which we suggest was due to enhanced quantity of soil macro-aggregates (>250 μm). This study highlighted the interactions between soil physicochemical and biological changes in biochar amended plots, that in turn control sucrose mineralization and SOC priming, and thus decadal-scale C sequestration.