Migration and Transformation Mechanisms of Nutrient Elements (N, P, K) within Biochar in Straw-Biochar-Soil-Plant Systems: A Review

In recent years, biochar has attracted interest as a soil amendment. As biochar research has increased, the nutrient characteristics of biochar were gradually determined. Pyrolysis conditions are vital in regulating the nutrient characteristics of biochar in straw: the appropriate pyrolysis conditions (e.g., 400 degrees C in a CO2 pyrolysis atmosphere) reduce the migration of nutrients to liquid and gaseous pyrolysis products, thereby increasing the retention rate of nutrient elements in biochar (biochar-N, 64.94%; biochar-P, 100%; biochar-K, 100%). The correct pyrolysis conditions can also promote the change of nutrient elements in straw to Ad available nutrient forms (including NH4+-N, NO3--N, free amino acid-N, amino-N, protein-N; H2O-P, NaOH-Pi, NaHCO3-Pi, HCl-P; KNO2, KNO3, KCl). After application, biochar nutrient elements migrate in soil: the available nutrients are absorbed and utilized by the seasonal crop and only a small portion of the structurally stable mineral nutrients migrates to the soil. Much of this remains in residual biochar, which jointly builds the repository of soil nutrient elements, providing long-term fertility for crop growth. A very small amount of nutrients is retained in microbial bodies by microbial immobilization in soil, supplying material conditions for the growth and reproduction of microorganisms and the mineralization of nutrient elements.