Effects of biochar application frequency on soil nutrients and crop quality: evidence from eight-year field experiments in two contrasting soils

Eight-year consecutive field experiments were conducted at two locations (Heishi and Linquan, Bijie, Guizhou, China) to investigate the optimum biochar application frequencies for sustaining soil nutrients and crop quality. A randomized complete block design with three treatments (annual, biennial, triennial) and three replicates was established using flue-cured tobacco as a test crop. Biochar derived from tobacco stems was applied at a rate of 7.5 Mg ha⁻¹ per application according to treatment frequency, and multi-component fertilizer was applied each year. Soil samples were collected at 12 times for nutrients and enzyme activity analyses. Results showed that annual application significantly increased soil pH by 0.67 to 0.70 compared to triennial treatments, and soil organic carbon (SOC) by 38%–62% in Hapludult and 17%–46% in Dystrudept. Annual application substantially increased available K by 35%–53% increases over triennial treatments. However, annual application also elevated soluble chloride (Cl⁻) by 56%–134%. Structural equation modeling revealed that frequency primarily controlled pH, SOC, and soluble Cl. Soil enzyme activities increased by 18%–45% under annual versus triennial applications. For tobacco leaves, annual application increased total sugar accumulation, while biennial and triennial applications improved potassium/chlorine (K/Cl) ratios. Site-specific responses were evident, with Hapludult showing stronger frequency effects than Dystrudept. These findings suggest that a 2 to 3 years biochar application interval, tailored to soil type, provides optimal balance between soil nutrient enhancement, microbial activity, and tobacco quality while mitigating excessive Cl accumulation. ● Eight-year field study quantifies biochar application frequency effects ● Biochar increased soil K (35–53%) but also elevated Cl (56–134%) ● Frequent application increased soil pH and SOC, especially in Hapludult ● Frequency regulates soil pH, SOC, and soluble Cl accumulation ● A 2–3 year interval optimizes soil fertility while limiting Cl risk