Mechanism(s) of Phosphoric Acid-Modified Biochar Enhances Carbon Sequestration in Saline-Alkali Soil

Saline-alkali soils are characterized by low fertility, high salinity and alkalinity, and poor carbon sequestration capacity, posing major challenges to agricultural productivity and ecological sustainability. Although biochar has shown potential for soil amelioration, its effectiveness in such harsh environments is often limited by its inherent properties. Chemical modification offers a promising strategy to enhance biochar functionality; however, the underlying mechanisms governing soil–microbe–plant interactions remain poorly understood. This study investigated the effects of phosphoric acid-modified corn straw derived biochar (HBC) on soil properties, microbial communities, and plant growth in saline-alkali soil. The results showed that HBC application significantly reduced soil pH and exchangeable sodium percentage (ESP), while increasing soil organic carbon (SOC), available phosphorus (AP), alkali-hydrolyzable nitrogen (AN), and the activities of urease and alkaline phosphatase. Results from 16S rRNA sequencing revealed that HBC enriched Acidobacteria, Flavisolibacter and Limisphaera, while suppressing salt stress-related genera such as Ramlibacter and Pontibacter. Redundancy analysis (RDA) identified soil carbon and nitrogen (TN, SOC, POC) as key drivers of bacterial community shifts. Transcriptomic analysis further demonstrated that HBC promoted the growth of Kosteletzkya pentacarpos by regulating genes involved in development and hormone signaling pathways (auxin, abscisic acid, gibberellin). Collectively, HBC enhances carbon sequestration and crop productivity in saline-alkali soils through synergistic improvements in soil microenvironments and nutrient cycling functions.