Abstract: In order to explore the effects of potassium-reduced application of biochar-based organic-inorganic compound fertilizer (BOCF) on dissolved organic matter (DOM) characteristics and potassium availability in tobacco-planting soils, pot experiments were conducted using two tobacco-planting soil types (paddy soil and upland yellow soil). Four treatments were set up, including conventional organic-inorganic compound fertilizer (CK), BOCF with equivalent nutrients (T1), BOCF with a 10% K₂O reduction (T2), and BOCF with a 15% K₂O reduction (T3). At the maturity stage of flue-cured tobacco, the physicochemical properties, the DOM spectroscopic characteristics, and the potassium fractionation of the soil were analyzed. The results showed that the response of soil potassium availability to potassium reduction varied with soil type. In paddy soil, the water-soluble potassium (WSK) content of T1, T2 and T3 was significantly reduced by 24.7%, 34.5% and 40.3% respectively, compared with CK. The non-exchangeable potassium (NEK) content of T1,T2 and T3 was lower than that of CK, with the T3 showing a significant decrease of 40.4%. The content of various forms of potassium in yellow soil was relatively stable, and there was no significant difference. Each BOCF treatment enhanced the accumulation of humic-like substances and the humification degree in DOM of both paddy and yellow soils. Correlation and redundancy analyses indicated that changes in the chemical properties of DOM in both soils partially regulated soil potassium supply. Therefore, the utilization of potassium-reduction techniques with biochar-based organic-inorganic compound fertilizer proved to be more viable in yellow soil for maintaining potassium stability, but led to the consumption of the non-exchangeable potassium pool in paddy soil.

Key words: biochar-based organic-inorganic compound fertilizer, reduction of potassium, tobacco-planting soil, dissolved organic matter, potassium availability