Maize stover biochar increases urea (¹⁵N isotope) retention in soils but does not promote its acquisition by plants during a 4-year pot experiment

Biochar as a soil amendment has been shown to improve soil quality and crop growth. However, biochar's effect on urea-N use efficiency in long term is not well elucidated. Here we studied urea-N (¹⁵N isotope) allocation in plants and soil in the presence of maize (Zea mays L.) stover biochar (equivalent to 46 t ha-1) during a 4-yr pot trial. Results showed that biochar only increased maize biomass (about 9%) with high amount of urea addition, which indicates the increased maize dry weight by biochar application could be attributed to synergistic effects between biochar and urea. Soil total N contents and fertilizer N retention were increased by 20% and 10.47% to 94.52%, respectively, indicating that biochar was more capable for fertilizer N retention than promote plant adsorption. Moreover, inorganic N content in biochar treatment was greatly increased, which implies the increased N mineralization. In total, we concluded that biochar application was a potential urea enhancer during plant production