Effects of pyrolysis conditions on migration and distribution of biochar nitrogen in the soil-plant-atmosphere system.

The use of biochar to amend soil has been gaining increasing attention in recent years. In this study, the N tracer technique was used together with elemental analysis-stable isotope ratio analysis and gas isotope mass spectrometry to characterise biochar, soil, plant, and gas samples in order to explore the nitrogen transport mechanisms in the biochar-soil-plant-atmosphere system during the process of returning biochar to the soil (RBS). The results showed that the nitrogen retention rate of biochar was negatively correlated with the pyrolysis temperature during the preparation process, but was less affected by the pyrolysis atmosphere. In the RBS process, the migration of biochar nitrogen to plants was significantly greater than that of straw nitrogen, and it showed an overall decreasing trend with the increase in pyrolysis temperature, but was less influenced by the pyrolysis atmosphere. At temperatures of 300-500 °C, the pyrolysis atmosphere had a slightly smaller effect on the migration of biochar nitrogen to the air, plant, and soil system, and the pyrolysis temperature was much more important. However, the activation with CO gas at a higher temperature (600 °C) significantly enhanced the pore structure of biochar, particularly the structure of small pores; therefore, biochar prepared under a CO atmosphere at 600 °C reduces gaseous nitrogen emissions better than that under a N atmosphere. In the future, more pyrolysis conditions should be examined and their optimal combination should be further explored to reduce gaseous nitrogen emissions.