Long-term biochar application promoted soil aggregate-associated potassium availability and maize potassium uptake

Biochar is an effective ameliorator for soil quality improvement and nutrient reuse from biomass; however, the effect of biochar application on soil potassium (K) availability, plant K uptake, and the underlying mechanisms have not been well-elucidated. To address this, the variation in the soil K forms, soil aggregate stability, and aggregate-associated K concentration, as well as maize K uptake, were investigated in a field experiment after 9 years of biochar amendment. The treatments included no biochar and NPK fertilizer (CK); NPK fertilizer treatment (F); biochar applied annually at the rate of 2.625 t ha⁻¹ (C₁), and biochar applied annually at rate of 2.625 t ha⁻¹ with NPK fertilizers (C₁F); one-time biochar applied with NPK fertilizers, with biochar rate of 31.5 (C₂F) and 47.25 t ha⁻¹ (C₃F). The results showed that after 9 years of field application, biochar inhibited the downward K migration to the deeper layer, thus increasing water-soluble potassium (WSK), exchangeable potassium (EK), non-exchangeable potassium (NEK), and total potassium (TK) in 0–20 cm soil, with C₁F exhibiting better performance than C₂F and C₃F. Biochar also increased aggregate-associated EK, NEK, and TK pools, mainly due to an increase in the macroaggregate proportion (>0.25 mm). Biochar amendment promoted maize K uptake by an average of 35.69%, the path analysis indicated that the positive effect was an outcome of the synergetic effect of the increase in surface soil WSK content and promoted macroaggregate EK pools, which was primarily attributed to biochar improved soil properties, including soil organic carbon, pH, total nitrogen, total phosphorus, and cation exchange capacity. These factors explained 76% of the variance in maize K uptake. In conclusion, biochar is an effective ameliorator for improving soil K content and availability.