Application of the 15N tracer method to study the effect of pyrolysis temperature and atmosphere on the distribution of biochar nitrogen in the biomass–biochar-plant system

Biochar nitrogen is key to improving soil fertility, but the distribution of biochar nitrogen in the biomass–biochar–plant system is still unclear. To provide clarity, the 15N tracer method was utilised to study the distribution of biochar nitrogen in the biochar both before and after its addition to the soil. The results can be summarised as follows. 1) The retention rate of 15N in biochar decreases from 45.23% to 20.09% with increasing pyrolysis temperature from 400 to 800 °C in a CO2 atmosphere. 2) The retention rate of 15N in biochar prepared in a CO2 atmosphere is higher than that prepared in a N2 atmosphere when the pyrolysis temperature is below 600 °C. 3) Not only can biochar N slowly facilitate the adsorption of N by plants but the addition of biochar to the soil can also promote the supply of soil nitrogen to the plant; in contrast, the direct return of wheat straw biomass to the soil inhibits the absorption of soil N by plants. 4) In addition, the distribution of nitrogen was clarified; that is, when biochar was prepared by the pyrolysis of wheat straw at 400 °C in a CO2 atmosphere, the biochar retained 45.23% N, and after the addition of this biochar to the soil, 39.99% of N was conserved in the biochar residue, 4.55% was released into the soil, and 0.69% was contained in the wheat after growth for 31 days. Therefore, this study very clearly shows the distribution of nitrogen in the biomass–biochar-plant system.