Biochar enhanced microbial carbon use efficiency, while reducing mineralization of added and native soil organic carbon

Biochar can increase long-term soil organic carbon (SOC) storage due to its polyaromatic structure. In addition to the recalcitrant carbon (C) contained in the biochar itself, biochar can also increase SOC storage by adsorption or organic matter on its surfaces, and reduced decomposition rate of native SOC (negative priming). Limited number of studies have looked at how biochar affects decomposition and stabilization of fresh C inputs, and native SOC decomposition. To fill this knowledge gap, we conducted a laboratory incubation study, where we followed the fate of added ¹³C-labeled glucose in a fine-textured agriculturally used soil (Stagnosol) amended with two different biochar levels corresponding to 15 and 30 Mg ha^-1 in field conditions. Biochar addition reduced mineralization of SOC and added ¹³C glucose, while increasing microbial biomass and microbial carbon use efficiency (CUE). Most of the added biochar, as well as remaining ¹³C were found in the free particulate organic matter (POM) fraction after 6 months, indicating that added ¹³C glucose was preserved within the biochar particles. Our closer study of ¹³C amino sugar fraction extracted from the biochar particles revealed that the microbes that had efficiently grown on the added ¹³C glucose in the presence of biochar, were retained as dead microbial residues inside the biochar pores. This microbial route could provide a way for additional formation of rather recalcitrant C in the form of microbial residues in the presence of biochar, which could with time contribute to building SOC stock in biochar amended soils beyond the C present in biochar itself. We found that biochar also increased the portion of occluded POM in the treatments without ¹³C glucose addition, demonstrating that increased soil occlusion following biochar addition reduced SOC mineralization. The effects were found to be dose-dependent, i.e. higher biochar application rate resulted in lower mineralization rate of native SOC and of added ¹³C-glucose.