Long‐Term Combined Application of Biochar and Chemical Fertilizers Enhances Potassium Supply Capacity in Continuous Cropping Soils

Sustaining soil potassium (K) supply in intensive continuous cropping systems is increasingly challenging because long‐term crop removal often exceeds the natural replenishment capacity of soil K‐bearing minerals, leading to progressive depletion of plant‐available K pools. Biochar has been proposed as a promising amendment for improving soil nutrient retention and K cycling, yet its long‐term effects on the mineralogical and kinetic mechanisms regulating soil K supply remain poorly understood. Here we report a 9‐year field experiment in a continuous maize monoculture on brown soil, comparing biochar applied alone (C) or coapplied with fertilizer (CNPK) against fertilizer alone (NPK) and an unfertilized control. Relative to sole application of fertilizer, CNPK increased maize K uptake and grain yield by 35.9% and 7.7%, respectively. Compared with NPK, CNPK enhanced water‐soluble, exchangeable, and nonexchangeable K by 147.2%, 28.6%, and 6.0%, respectively. Mechanistically, x‐ray diffraction analysis of the < 2 μm clay fraction revealed a 4.8 percentage‐point increase in illite abundance under CNPK relative to NPK, accompanied by K redistribution from edge sites (e‐sites; −0.8%) to more readily releasable planar sites (p‐sites; +1.6%). Kinetic modeling with organic‐acid extractants demonstrated that CNPK enhanced both cumulative K release and apparent rate constants in fast and slow phases, indicating improved K bioavailability. These coupled mineralogical and kinetic effects strengthen the intensity, capacity, and potential of soil K supply, enabling more efficient long‐term K use. Our findings suggest that biochar‐fertilizer coapplication represents a viable strategy for enhancing K availability in intensive maize systems, with future research needed to optimize biochar characteristics and application protocols across diverse pedoclimatic conditions.