Returning biochar to soil is a heavily researched topic because biochar functions well for soil improvement. There is a significant loss of nutrients, which occurs during biochar preparation before biochar is returned to soil,thereby seriously undermining biochar’s efficacy. Therefore, the transformation mechanisms of biochar p H,mass, nutrients and metals during pyrolysis under different atmospheres and temperatures were studied such that the best method for biochar preparation could be developed. Several conclusions can be reached:(1) a CO_2 atmosphere is better than a N_2 atmosphere for biochar preparation, although preparation in a CO_2 atmosphere is not a common practice for biochar producers;(2) 350 °C is the best temperature for biochar preparation because the amount of nutrient loss is notably low based on the premise of straw transferred into biochar; and(3) transforming mechanisms of pH, N, P and K are also involved in the biochar preparation process. Returning biochar to soil is a heavily researched topic because biochar functions well for soil improvement. There is a significant loss of nutrients, which occurs during biochar preparation before biochar is returned to soil, thus undermining biochar’s efficacy. Therefore, the transformation mechanisms of biochar p H, mass, nutrients and metals were studied such that the best method for biochar preparation could be developed. (1) a CO 2 atmosphere is better than a N 2 atmosphere for biochar preparation , although preparation in a CO 2 atmosphere is not a common practice for biochar producers; (2) 350 ° C is the best temperature for biochar preparation for the amount of nutrient loss is not likely low based on the premise of straw transferred into biochar; and ( 3) transforming mechanisms of pH, N, P and K are also involved in the biochar preparation process.