Effect of returning biochar from different pyrolysis temperatures and atmospheres on the growth of leaf-used lettuce

The aim of this study is to evaluate the effects of biochar on the plant's growth. A pot experiment was carried out in our study. Rice straw-derived biochar were charred at two heating temperatures (400 °C/800 °C) and two oxygen-limited atmospheres (CO₂/N₂), respectively. The FESEM/EDS technique (field emission scanning electron microscopy with X-ray energy-dispersive spectroscopy) was used to study soils, biochar and plant samples. FESEM images indicated that the structure of the biochar was highly heterogeneous with larger macropores, which can enhance soil porosity. Fine soil mineral particles blocked the biochar inner pores and channels after returning biochar to soil. EDS analysis indicated that the Al and Fe contents increased on the surface of biochar after their returning, which reduced the toxicity of Al and Fe in the soil. The returning straw directly inhibited the growth of leaf-used lettuce. Four returning biochar all significantly improved leaf-used lettuce growth, and the effects of biochar prepared under 400 °C and a CO₂ atmosphere were better than those prepared under 800 °C and a N₂ atmosphere. Changes of nitrogen content in the biochar before and after their returning were consistent with the improvement of soil available nitrogen, and plant growth was positively correlated with the nitrogen content of biochar. This study explored the impact of biochar on soil nutrients and revealed the mechanism of biochar returning to the field to promote plant growth. It is of great significance in studying and improving the characteristics of soil nutrients.