Modified biochar enhances soil fertility and nutrient uptake and yield of rice in mercury-contaminated soil

Biochar, as an emerging soil remediation technology, has gained substantial attention. However, the impact of modified biochar on soil fertility, nutrient uptake, and rice yield in mercury (Hg)-contaminated soil remains unclear. This study explores the influence of biochar on soil fertility, nutrient absorption, photosynthesis, and rice yield in Hg-contaminated soil. We analyzed soil physicochemical properties, nutrient distribution (nitrogen, phosphorus and potassium) in roots, stems, and leaves, as well as plant gas exchange traits and yield components to provide a comprehensive assessment. Two experiments were conducted using original biochar (BC), produced via direct combustion and rice husk cracking, along with six types of modified biochar [amino (BCNH2), epoxy (BCCH(O)CH), ethoxy (BCC2H5O), sulfhydryl (BCSH), selenium (BCSe) loaded, and chitosan (BCchitosan)]. These biochars were applied at four different concentrations (0.07, 0.14, 0.20 and 0.35%). Comparative analysis against the control revealed that the application of modified biochar, particularly BCNH2, BCC2H5O and BCSH, significantly enhanced soil organic matter and plant nutrients (total N, available P, available K). Furthermore, it promoted the uptake of N, P, and K by rice leaves. The biochar addition exhibited a positive impact on rice photosynthesis and yield within a specific range of concentrations. A comprehensive analysis indicates that soil fertility, photosynthesis, and rice yield were significantly enhanced when BCNH2, BCC2H5O, and BCSH were applied at 0.20% under the study conditions. These findings advance our understanding of how biochar affects soil fertility, nutrient uptake, and rice yield, offering valuable insights for practical agricultural and environmental management in Hg-contaminated fields.