Nitric acid-modified biochar ameliorates saline-alkali soil and promotes cotton growth by regulating microbiomes and metabolites.

Biochar is extensively utilized for saline-alkali soil remediation; however, the effects of feedstock origin on its performance remain unclear. Moreover, there is a growing need to develop modified biochar with improved efficiency in saline-alkali soil. Therefore, cotton straw biochars derived from saline-alkali soil (SBC) and non-saline-alkali (NBC), along with their nitric acid-modified forms (HSBC and HNBC), were prepared and assessed via a cotton pot experiment. The results showed that the pH and total soluble salts (TSS) of SBC were significantly higher than those of NBC by 0.33 units and 306.80%, respectively. Nitric acid modification significantly reduced the pH value, salt and alkaline mineral content of biochar, and enriched acidic oxygen-containing functional groups. Meanwhile, SBC treatment increased the contents of Na+, HCO3- and CO32- in soil and showed no promoting effect on cotton growth. In contrast, the acid-modified biochar treatment reduced soil pH by 0.25-0.32 units, decreased TSS by 9.81%-12.11%, and significantly increased cotton dry biomass by 3.63-3.88 times. In addition, acid-modified biochar increased the complexity of the soil microbial network and promoted the enrichment of acid-producing microorganisms as well as stress-resistant microbial groups, such as Actinobacteria and Nocardioides. Simultaneously, it significantly induced the production of metabolites that enhance plant tolerance to saline-alkali stress, such as flavonoids and betalamic acid. In summary, acid-modified biochar enhances its beneficial effects on soil improvement and cotton growth through the coordinated regulation of rhizosphere acid-producing microorganisms and stress-responsive metabolites. This work highlights the scientific significance of biochar modification for advancing sustainable agriculture in saline-alkali regions.