Effects of Biochar and Its Fractions on Soil Nitrogen Forms and Microbial Communities Under Freeze-Thaw Conditions

Biochar shows potential for regulating nitrogen cycling in cold-region soils, but the roles of its different fractions during freeze-thaw cycles (FTCs) remain unclear. To elucidate the regulation of cold-region soil environments by biochar at the fraction scale, we examined the effects of biochar and its fractions (dissolved and undissolved) on soil nitrogen forms and microbial communities under simulated FTCs. The experiment included a constant-temperature control, a freeze–thaw control, and three biochar treatments with pristine biochar (PBC), dissolved biochar fraction (DBC), and undissolved biochar fraction (UBC), respectively, maintained in triplicate at five FTC frequencies (0, 1, 5, 10, and 20). Changes in soil physicochemical properties and nitrogen forms were measured at five FTC frequencies, and microbial community composition was analyzed by high-throughput sequencing after the 20th cycle. Both biochar fractions reduced inorganic nitrogen, with ammonium nitrogen decline resulting from joint action and nitrate nitrogen (NO3−-N) reduction dominated by UBC. PBC alleviated microbial biomass nitrogen stress by relying primarily on its undissolved fraction to enhance soil water retention, organic carbon, and total nitrogen. Redundancy analysis indicated that total nitrogen and NO3−-N were the key factors affecting microbial community composition. Partial least squares structural equation modeling results suggested that soil physicochemical properties influenced microbial community structure characteristics more strongly than nutrient properties. These findings provide a new perspective on the regulatory mechanism of biochar on the agricultural soil environment in cold regions.