Biochar-mediated uptake of secondary- and micro-nutrients in crops: a meta-analysis

Secondary- and micro-nutrients are essential for the growth, high yield, and high-quality production of crops, and their deficiency in arable soils has become a global constraint in agriculture. As a novel soil amendment, biochar shows great potential in regulating soil nutrient availability and crop nutrient uptake, but its effects on crop secondary- and micro-nutrient acquisition are inconsistent due to varied biochar properties and environmental conditions. This meta-analysis systematically evaluated biochar effects on the uptake of seven key secondary- and micro-nutrients (Ca, Mg, S, Fe, Mn, Cu, and Zn) and the underlying regulatory mechanisms. Results showed that biochar significantly increased crop aboveground biomass and total secondary-/micro-nutrient accumulation without significantly altering their overall concentrations. Biochar exhibited element-specific effects: it reduced Mg, Mn and S concentrations, increased Fe, Cu, and Zn concentrations, and had no significant effect on Ca concentration. Biochar properties (pyrolysis temperature, feedstock, application rate, and pH) were the dominant regulators. The optimal conditions were 400 °C-500 °C pyrolysis temperature, wood-based feedstock, ≤1% (w/w) application rate, and alkaline pH, which affected the regulatory intensity but not the response direction. Soil properties, experimental methods, and crop types did not change the response pattern. Biochar stably regulated nutrient uptake across soils and crops, with the strongest promotion in vegetables. This study clarifies the universal regulatory characteristics of biochar and provides a scientific basis for its rational application to improve crop nutrient status in sustainable agriculture.