Nonlinear Effects of Agricultural Mechanization on Carbon Emission Intensity: Evidence from a Fertilizer Input-Efficiency Dual-Pathway Mechanism in China

Agricultural mechanization’s carbon consequences remain contested. Using provincial panel data from 30 Chinese provinces (2000–2023), we combine benchmark regression, mediation, threshold, and spatial models to examine how mechanization affects agricultural carbon emission intensity. We find a robust inverted U-shaped relationship, with an inflection point at 0.428; approximately 61% of province-year observations have entered the emission-reduction phase, while provinces with lower mechanization levels (e.g., Yunnan and Guizhou) remain on the upward slope. Mechanization operates through two opposing pathways; higher fertilizer application intensity increases emissions, while greater fertilizer use efficiency reduces them, with the intensity pathway accounting for the larger share of the observed mediated association (68% of the total mediated effect). The emission-reduction effect only materializes once fertilizer use efficiency crosses a critical threshold (−0.626 in log terms); provinces below this level face the risk of carbon lock-in as mechanization expands. A grain-oriented shift in cropping structure and spatial spillovers constitute additional channels, with the latter accounting for over half of the total effect. These findings call for regionally differentiated policies; provinces with persistently low fertilizer use efficiency should build precision fertilization capacity before scaling up mechanization, while more advanced regions can draw on cross-regional machinery networks to spread low-carbon agricultural practices.