Black carbon as a compound agricultural stressor: impacts on plant physiology, crop productivity, and agricultural sustainability.

key messageBlack carbon is an underappreciated compound agricultural stressor that simultaneously disrupts plant physiology, agroecosystem functioning, andcrop productivity, highlighting the need for realistic exposure assessment and integrated strategies for climate-resilient agriculture. Black carbon (BC), a carbonaceous particulate generated through the incomplete combustion of fossil fuels, biomass, and biofuels, is recognized as a major short-lived climate pollutant with significant implications for atmospheric processes and agricultural sustainability. Unlike engineered biochar, atmospheric BC acts as an environmental stressor through deposition on plant surfaces and accumulation in agroecosystems, yet its direct impacts on crop physiology remain insufficiently understood. This review synthesizes current knowledge on the physicochemical characteristics, environmental pathways, and plant stress mechanisms associated with atmospheric BC while explicitly distinguishing it from intentionally applied biochar. Available evidence indicates that BC influences plant performance through multiple interconnected pathways, including reduced light availability, stomatal obstruction, disruption of photosynthesis, oxidative stress induced by excessive reactive oxygen species (ROS), chloroplast dysfunction, hormonal imbalance, and alterations in nutrient cycling and soil microbial communities. However, much of the mechanistic evidence is derived from studies on biochar, carbon nanomaterials, or other particulate pollutants, highlighting a critical gap in plant-specific evidence under realistic atmospheric BC exposure. Regional studies, particularly from the Indo-Gangetic Plain, demonstrate that elevated BC and associated aerosol loading contribute to reduced crop productivity and increased food security risks, although these impacts often reflect the combined influence of multiple atmospheric stressors rather than BC alone. The review further examines current limitations in BC exposure quantification, emphasizing the absence of agronomic dose-response thresholds and standardized field-based assessment methods. Emerging approaches, including leaf-based biomonitoring and isotopic analyses, are discussed as promising tools for future exposure assessment. Finally, key research priorities are identified, including the generation of plant-specific mechanistic evidence, development of realistic dose-response frameworks, integration of BC into crop simulation models, and implementation of long-term field studies to improve risk assessment and support evidence-based mitigation strategies for sustainable agriculture.