Artificial humic acid facilitates biological carbon sequestration under freezing-thawing conditions.

Freezing-thawing events contribute to the accumulation of soil organic matter and the formation of high fertility black soil. On this base, we explore the influence of the combination of liquid artificial humic acid (LA-HA) and freezing-thawing events on strengthening carbon sequestration in soils. The measurements of the total organic carbon (TOC) and dissolved organic carbon (DOC) content illustrate that the applications of LA-HA indeed largely enhanced the persistent carbon reservoirs during freezing-thawing cycles, and the highest TOC net increment was found as up to 4000 mg/kg (0.36 wt% C with the control treatment versus 0.79 wt% C with 300 mL/kg LA-HA (3LA-HA) treatment after 10 freezing-thawing cycles). Spectral analysis reveals that LA-HA treatments accelerated the formation of additional humic substances under freezing-thawing events, i.e., the transformation of labile carbon to resistant carbon. Finally, the results of highthroughput sequencing corresponding to cbbL gene demonstrate that 3LA-HA functioned to optimizing the community structure of carbon sequestration bacteria and improving the dominant position of part bacteria with strong carbon fixation ability to reduce soil carbon loss after thawing, e.g., Mycolicibacterium gadium and Starkeya novella.