Aims Crop nitrogen (N) and phosphorus (P) stoichiometry can influence food nutritive quality and many ecosystem processes. However, how and why N and P stoichiometry respond to long-term agricultural management practices (e.g., N fertilization and film mulching) are not clearly understood. Methods We collected maize tissues (leaf, stem, root, and seed) and soil samples from a temperate cropland under 30- years continuous N fertilization and plastic film mulching treatments, measured their C, N, and P concentrations (the proportion (%) relative to the sample mass), and used structural equation models to uncover the responding mechanisms for crop N and P contents (the total amount (g/m 2) in crop biomass). Important findings Long-term nitrogen fertilization increased N concentrations in all crop tissues but sharply decreased P concentrations in vegetative tissues (leaf, stem, and root), thereby reducing their C/N ratio and increasing C/P and N/P ratios. The drop in P concentration in vegetative tissues was due to the dilution effect by biomass increment and the priority of P supply for seed production. In contrast, film mulching decreased N concentration but increased P concentrations in most crop tissues, thereby increasing C/N ratio and reducing C/P and N/P ratios. Film mulching increased crop P content by increasing soil temperature and moisture; whereas, mulching showed little effect on crop N content, because a positive effects of soil temperature may have canceled out a negative effect by soil moisture. This indicated a decoupling of P and N uptake by crops under film mulching. In conclusion, N fertilization and plastic film mulching showed opposite effects of on crop N and P stoichiometry.