Several biochars have a considerable amount of essential plant nutrients; however, the release of those nutrients in soil is not well understood. Therefore, in this study, six biochars with varying nutrient contents were produced from Japanese larch (Larix kaempferi, JL), dairy manure (DM), and chicken manure (CM) at 300 and 500 °C and incubated in a temperate clay loam soil to investigate their nutrient release dynamics. The available N, P, and K release patterns of the biochars (2% dry-basis) were compared with the recommended fertilizer dose of sweet corn (Zea mays convar. saccharata) for 120 days. The results indicated that only chicken manure biochars (C/N ratio < 8) have the potential to satisfy plant needs by releasing sufficient N, P, and K. Fourier-transform infrared analysis revealed that N-containing amide dissolution was the major mechanism behind the 49% N release from the CM300 biochar. A higher production temperature (500 °C) diminished most of these amides, resulting in lower N release from CM500 (8.9%). Nitrification-induced reduction of soil pH caused Ca/Mg-P dissolution from manure biochars and enhanced P release. Much of the released phosphates later became unavailable in the soil by aluminosilicate complex formation. DM biochar also released adequate P and K, and production at 500 °C generally performed better than at 300 °C. Effect of coexisting ions on some nutrient release was found, though was statistically nonsignificant. Overall, results revealed that low-temperature (≤500 °C) CM and DM biochars can become comprehensive or complementary sources of plant nutrients, respectively, with some enhanced nutrient-retaining potential, although inherent soil properties may play a significant role.