Foliar nitrogen application improved cotton yield from allometric root-shoot relationship and delayed senescence

Nitrogen (N) management is crucial for sustaining crop yield, improving profitability, and ensuring environmental stewardship. Multiple soil N application is facing quite a few challenges, especially when the canopy is closing, such as low efficiencies in both N use and application, disturbance or even drop of the plant fruits during application. We suppose that N foliar spraying could be a potential candidate to partially replace soil application. Therefore, the aim of this study was to verify if the foliar N application could improve cotton yield, and its possible mechanisms in N absorption, root architecture and root-shoot relationship. A 2-season (2020–2021) field experiment with a randomized complete block design was conducted to evaluate the effects of foliar N spraying on cotton yield and its components, root development, biomass accumulation and N uptake, in comparison with the multiple soil application and the conventional farmers practice as well. The results showed that multiple soil and foliar N application at flowering period improved cotton yield over the conventional N managements, while a similar performance was observed between soil and foliar strategies. Further, foliar N spraying increased shoot biomass partitioning, establishing an allometric root-shoot relationship that ultimately resulted in a 5.3 % increase in total N uptake. Meanwhile, foliar N application reduced root allocation in the topsoil while promoting deeper root development, which confirmed as a critical factor for yield improvement by SHapley Additive exPlanations (SHAP) analysis. Overall, split foliar N application increased cotton yield by higher shoot partitioning, combined with a deeper and thinner root architecture, thereby alleviating leaf senescence. These findings highlighted the potential of foliar nitrogen strategies to support mechanised cotton production by improving nitrogen use efficiency and production sustainability, particularly in regions where soil-based fertilisation is constrained by climatic or management limitations like semi-arid regions.