Limited improvement of crop nitrogen management sustainability through optimal crop distributions in drylands

Efficient nitrogen (N) management is essential for sustainable global food production. Although optimal crop distributions are recognized as potential adaptive strategies to offset the adverse effects of climate change, their effectiveness in improving regional crop N management sustainability is still uncertain. Here, we quantified spatiotemporal trends of the sustainable N management index (SNMI) for optimal crop distributions (determined by maximum crop suitability per grid) in the Hexi Corridor drylands in Northwest China since the 1960s, using crop yields and N use efficiency (NUE) simulated by a process-based regional crop model (pDSSAT). We also reduced the uncertainties of model parameters and climate scenarios in yield simulations through field water-nitrogen experiments for six crops (maize, wheat, potato, rapeseed, cotton, and alfalfa) and emergent-constraint approach, respectively. Analysis of SNMI shows an upward trend over the past 60 years, but future 20 years find a significant decline. Our findings indicate a 4-29% reduction in uncertainty using constrained crop yields versus regional average yields, which improve the sustainability of crop N management in optimal crop distributions with a lower SNMI. Future climate scenarios would further intensify crop yield loss, but optimal crop distributions will decline regional crop yield by 2%, increase NUE by 10%, and decrease SNMI by 13% compared to the 2011-2020. These results demonstrate a limited improvement of optimal crop distributions on historical SNMI, whereas their effectiveness is supported in future climate conditions-a result of a noticeable enhancement in NUE that compensates the detrimental impacts of yield reduction. Therefore, we recommend revisiting more adaptive strategies alongside optimal crop distributions for consistently improve the sustainability of crop N management.