Phosphorus (P) is a limiting nutrient for crops in many parts of the world, especially in the tropics, due to low P availability in soils. Biochar has attracted increasing attention as a potential soil conditioner, due to its ability to increase P availability in soils. However, how biochar contributes to P availability and influences P transformation in soils is largely unknown. This PhD research project (a) investigated changes in biochar characteristics as a function of feedstock type and pyrolysis temperature, including biochar chemical composition and structures; (b) assessed P availability in four biochar thermosequences and P transformation during pyrolysis using a sequential extraction scheme in combination with 31P NMR studies; (c) assessed the ability of different biochars to retain phosphate and investigated corresponding retention mechanisms, and (d) investigated effects of biochar amendments on P availability and transformation in tropical soils under precisely-controlled laboratory conditions, over a short time period (12 weeks), and under field conditions over a longer time period (≥1 years).