Drivers of cropland carrying capacity and pathways for mitigating livestock overloading in intensive agricultural systems.

Integrated crop-livestock systems support nutrient cycling and resource efficiency. Their long-term sustainability depends on whether livestock production remains within cropland's capacity to assimilate manure. Here we develop a multidimensional framework that integrates nutrient load, organic load and heavy metal accumulation to quantify livestock-cropland carrying capacity in the black soil region of Northeast China (NBSR). County-level data from 2000 to 2020 show that carrying capacity in the NBSR increased by 67.40%, largely driven by expansion of staple crop area. As staple crops have become increasingly dominant in the cropping structure, nutrient constraints have gradually eased, while the influence of heavy metal risks on integrated carrying capacity has become more pronounced. Machine learning and scenario analysis indicate that by 2034, carrying capacity will increase by only 3.93%, whereas remaining environmental capacity will decline by 6.53% and the area of cropland at severe pollution risk will expand. Optimizing nutrient inputs or adjusting cropping structure alone has limited effects on reducing livestock overloading. By contrast, stronger spatial coupling between crop and livestock systems, together with integrated optimization, can markedly reduce local overloading risk, with combined strategies delivering the strongest effect. Our results highlight a shift in the limiting factors behind rising carrying capacity in intensive farming regions and show that the main drivers of change do not necessarily match the best pathways for reducing overloading. Sustainable transition therefore requires a shift from single-factor regulation to spatial coordination and system-level optimization.