A framework for assessing the carbon-water-energy nexus of cropping systems, applied to tobacco production

Carbon, water, and energy use are tightly interconnected in crop production, and integrated carbon-water-energy assessments have been used to evaluate these interactions. However, less attention has been paid to whether the three subsystems develop in a coordinated manner, which subsystem constrains nexus performance, and which variables are most strongly associated with its spatiotemporal variation. This study integrates carbon footprint (CF), water footprint (WF), and emergy input (EM) accounting with a coupling coordination degree (CCD) model, elasticity-based sensitivity analysis, and a random forest model to diagnose the carbon-water-energy nexus of cropping systems. The framework was applied to tobacco production in 17 Chinese provinces from 2004 to 2021. Producing 1 kg of dry tobacco leaf emitted on average 5.9 kg CO 2 -eq, consumed 1.6 m 3 of water, and required 6.4 × 10 12 sej of emergy inputs. The curing stage accounted for 70% of CF and 36% of EM, while green water dominated WF. The national CCD mean increased from 0.77 in 2004 to 0.80 in 2010, declined to 0.66 in 2015, and recovered to 0.78 in 2021. Northern provinces were generally better coordinated than southeastern provinces. The water subsystem was the main internal constraint, while curing-energy consumption and nitrogen fertilizer input showed the strongest predictive associations with CCD. Site-specific irrigation optimization, targeted nitrogen management, and clean-energy transition in curing are critical to enhancing the sustainability of tobacco production. The framework can be adapted to other crops when crop-specific inventories and parameters are available, and expanded to broader ecological indicators.