Biochar application rates regulate soil nutrient availability: evidence from an 8-year field study across two soils

While biochar is widely promoted as a soil amendment for enhancing nutrient availability and carbon sequestration, its long-term effectiveness across diverse soil types under intensive cultivation remains poorly understood. This study quantified temporal dynamics of biochar-induced nutrient changes across two contrasting soils (Dystrudept and Hapludult) over eight years (2018–2025), evaluating five application rates (0, 5, 15, 20, and 40 t ha−1) through 18 repeated soil samplings under continuous tobacco cultivation. Biochar application produced strong initial responses in soil pH, organic carbon (SOC reaching 83.59 g kg−1 at 40 t ha−1 in Hapludult), and available nutrients, but these effects progressively diminished, with complete convergence across all treatments by year 6–8 regardless of initial application rate or soil type—a critical finding that fundamentally challenges assumptions of permanent biochar effectiveness. Structural equation modeling identified pH-mediated pathways (λ = 0.99***) driving base cation availability, with temporal factors consistently eroding nutrient pools. Strikingly, the sandy loam Dystrudept exhibited 40% longer persistence of significant treatment effects than the clay loam Hapludult, directly contradicting conventional CEC-based predictions and revealing that crop uptake and leaching override texture-based retention under intensive management. The 20 t ha−1 application rate emerged as optimal, while rates above 40 t ha−1 accelerated nutrient depletion without extending benefit duration—demonstrating clear threshold effects previously unquantified in long-term field trials. This study suggests that biochar may function primarily as a medium-term amendment (3–5 years) rather than a permanent conditioner. In intensive cropping systems, maintaining the agronomic benefits of biochar may therefore require moving beyond single high-rate applications toward soil-specific optimization and potentially periodic reapplication strategies.