Relay intercropping cover crop combined with reduced nitrogen application improves subsequent cotton agronomic traits while maintaining yield and quality

Applying cover crops to reduce nitrogen (N) fertilizer application while ensuring crop productivity is a major challenge for sustainable agricultural production. However, few studies have quantified N fertilizer application reduction through the use of February orchid (Orychophragmus violaceus) cover crops with the goal of ensuring cotton productivity. We conducted field experiments in the Yellow River basin on two cropping systems, monoculture cotton (Gossypium hirsutum L) and cotton/February orchid under four levels of N application (0 [N0], 112.5 [N1], 168.75 [N2], and 225 [N3] kg N·ha−1), to evaluate the impacts of cover cropping on cotton growth parameters, yield, and quality under different N application levels. The results indicated that replacing winter fallow with February orchid prolonged the cotton growth period and increased plant height, leaf area index (LAI), and dry matter accumulation but did not improve cotton yield or fiber quality. The Cotton/February orchid system under a 168.75 kg ha−1 N application rate improved cotton growth, and achieved yield and quality equivalent to those of monoculture cotton under a 225 kg ha−1 N application. Further analysis revealed that February orchid cover crop and N application positively affected lint yield, whereas plant height, LAI, and dry matter accumulation (positively) and phenology (negatively) predicted lint yield and fiber quality. In conclusion, cotton relay intercropping with February orchid in combination with 168.75 kg ha−1 N application is recommended as a suitable cropping system to reduce fallow fields and ensuring cotton productivity in the Yellow River basin and other areas with similar ecological conditions. Core Ideas Replacing winter fallowing of fields with February orchid cultivation improved cotton growth parameters. Improvement in cotton growth and productivity with increasing nitrogen application rates. Relationships among cotton phenology, agronomic traits, yield, and quality were quantified. February orchid combined with 75% N fertilizer ensured cotton biomass and yield.