Biochar has the potential to sequester carbon and mitigate greenhouse gas emissions, and it may also contribute nutrients for plant growth in temperate climates. Nutrient availability in biochar-amended soil was assessed in a 338 d incubation study. The clay soil prepared with 4% w/w (dry basis) compost or without compost, then amended with wood-based biochar made at different pyrolysis temperatures [maple bark (Acer saccharum) at 400 (M400), 550 (M550), and 700 °C (M700)] on a dry-rate basis of 5% (w/w). After moistening the soil mixture to 44% volumetric soil water content (equivalent to 70% water-filled pore space), soil mixtures were incubated in the dark at 22 °C. Soil was sampled at days 9, 16, 23, 44, 86, 23 170, and 338 of the incubation. Biochar amendment increased the Mehlich-3 phosphorus, potassium (K), magnesium, and copper concentrations, and reduced the Mehlich-3 aluminum and iron concentrations at each sampling date, and M400 had the most significant effect on Mehlich-3-extractable nutrient concentrations. Compost addition also increased the amounts of extractable nutrients. These results suggested that M400 and carbon-rich compost promoted microbial growth and mineralization in amended soil. In addition, soil mixed with compost and amended with biochar had more Mehlich-3-extractable K than when compost or biochar was applied alone, probably due to greater growth and activity of soil K-solubilizing microorganisms. Overall, our study indicated that co-application of wood-based biochar and compost could improve soil fertility in temperate regions by increasing the availability of most plant macronutrients and micronutrients.