Optimizing grain–ecology landscapes configuration: identifying nonlinear trade-offs and threshold effects in ecosystem services

Under the global challenges of food security and the strategic need for “storing grain in the land,” it is crucial to coordinate cultivated land protection, capacity enhancement, and the “grain-ecology” balance. In most major grain-producing areas, the production intensity is approaching environmental carrying capacity thresholds, resulting in significant trade-off relationships between grain production (GP) and ecological benefits, yet their non-linear dynamic mechanisms still lack a quantitative framework. Taking the Middle-Lower Yangtze Plain—a major grain-producing region in China—as a case study, the Production Possibility Frontier (PPF) approach is applied to measure the non-linear trade-off relationships between GP and ecological benefits (Water Purification [WP], Carbon Storage [CS], Soil Retention [SR], and Habitat Quality [HQ]) and to identify the thresholds of the trade-off boundaries. Furthermore, the System Dynamics (SD) and PLUS models are combined to evaluate the optimal scenarios for mitigating the trade-off relationships between GP and ecological benefits. During the study period, trade-off states were primarily distributed in the central and eastern regions. The thresholds for CS, SR, and HQ decreased by 48.64%, 8.06%, and 8.28%, respectively, indicating a surging pressure from agricultural development on ecological benefits. Vegetation coverage and topographic slope emerged as key variables influencing the changes in trade-off states. The Ecological Protection Priority scenario increases the capacity to maintain synergistic relationships between GP and WP/SR. Under the Cultivated Land Protection Priority scenario, the negative disturbances on CS and HQ caused by the increase in GP capacity are effectively alleviated. By combining the PPF-SD-PLUS methods, this study integrates a research framework of “Non-linear trade-offs—Carrying capacity thresholds—Scenario optimization,” which expands the application boundaries of the PPF method. It not only provides a method for optimizing the trade-off boundaries between GP and ecological benefits but also offers a reproducible resource allocation scheme to implement the dual goals of food security and high-quality ecological development.