Long-Term Sustainability of Direct Straw Return Challenged by Soil Carbon Saturation and Emission Trade-Offs.

Sustainable residue management underpins agricultural productivity and climate resilience. Direct straw return (DSR) is widely adopted to enhance soil organic carbon (SOC) and fertility, yet its long-term sustainability remains uncertain due to trade-offs between C sequestration and greenhouse gas (GHG) emissions. By synthesizing 375 studies across China, we evaluate DSR's long-term impacts on SOC, crop yield, and net carbon benefits (NCB) and propose an integrated strategy (DSR-C) combining optimized tillage, nitrogen reduction, and microbial inoculation. Model-simulated dynamics show that SOC accumulation and yield gains plateau over time, whereas the national-scale NCB eventually turns negative due to persistent GHG emissions. Pareto optimization reveals an optimal trade-off threshold (Knee point) at 1.4-5.5 years across regions. Significant regional heterogeneity governs these dynamics: upland wheat/maize systems sustain positive NCB for 6.4 to over 50 years, whereas rice-dominated regions require temporal limits (1.6-3.5 years) to prevent transitioning from a net carbon sink to a net carbon source. Beyond these critical windows, yield increments stagnate while ecological trade-offs intensify. By highlighting the dynamic, transient nature of agricultural carbon sinks, these findings provide a quantitative blueprint for region-specific adaptive residue management, offering broad global relevance for scaling climate-smart agriculture and accelerating decarbonization under future global change.