Optimized crop-livestock coupling to reduce agricultural manure-N surplus and greenhouse gas emissions in China

China is under great pressure for livestock-related environmental degradation, as evidenced by the rising regional manure-N surplus and greenhouse gas (GHG) emissions. The coupling of crop planting and livestock breeding (CPLB) has become the common practice to address this issue. However, the current CPLB coupling considers only the relationship between livestock manure-N supply and crop-N demand, it could not reflect the greater mitigation potential through optimizing spatial distribution and structure of livestock. Here, we attempted to assess the CPLB coupling status in 2020 and its trajectory for 2030 to achieve multi-objectives: minimizing manure-N surplus, mitigating GHG emissions, and enhancing regional livestock product self-sufficiency, based on governmental nationwide survey data. The results indicated that: (1) The estimated cropland carrying capacity of breeding livestock for the years 2020 and 2030 were 0.52 and 0.55, but the associated manure-N surplus were 0.71 and 0.88 Tg, which indicated that there was a spatial mismatch between livestock and cropland. Therefore, it was necessary to optimize the distribution of livestock from a national perspective. (2) The optimal scenarios that allocated livestock breeding capacity across provinces based on crop-N demand (CND), food demand of residents (FDR) and GHG emissions mitigation requirement (GER) reduced manure-N surplus to 0 Tg, 1.29 Tg, and 0.88 Tg, and mitigated the GHG emissions by 1.49% (7.88 Tg), 6.80% (35.94 Tg), and 14.91% (78.78 Tg), respectively. (3) The spatial redistribution in terms of multi-objective optimal scenario (MOO) that comprehensively balanced regional crop-N demand, livestock product self-sufficiency, and GHG mitigation from multiple stages of livestock breeding, manure management, and transport processes could not only eliminate the manure-N surplus but also mitigate GHG emissions by 17%. Therefore, optimizing CPLB coupling through spatial reallocation and structural adjustment of livestock breeding based on crop, diet, and livestock species would provide a scientific basis for developing livestock-related environmental protection policies in China.