Mitigation potential for carbon and nitrogen emissions in pig production systems: lessons from the North China Plain

China produces approximately half of the world's pork at a high environmental cost. Implementing mitigation measures requires a better understanding of pig farming systems that are often diverse in practice. Nutrient uses and losses can be largely altered due to these variations but have not, however, been fully evaluated; moreover, attention is rarely paid to pollution swapping. Our study evaluated carbon (C) and nitrogen (N) flows among pig farms on the North China Plain using the mass flow approach. The impacts of advanced mitigation measures on nitrogen use efficiency (NUE) and on C and N emissions were further analyzed via scenario analyses. The results showed that large variations among farms were observed by comparing the best-performing farms ("top 20%") with the "other" farms; the comparisons showed 31.3 vs. 21.3% for the system NUE, 3.0 vs. 5.7 kg N ·100 kg liveweight gain (LWG)^-1 for manure N losses, and 108.1 vs. 146.4 kg CO₂-eg·100 kg LWG^-1 for greenhouse gas (GHG) emissions, respectively. Lower system NUE was caused by lower NUEs of pigs, followed by high N emissions from manure and excessive manure application. Scenario analyses indicated that the total N loss of systems can be mitigated by 10-13% through dietary manipulation and by 26%, 27%, and 13% by low-emission storage, biogas production with improved storage, and balanced fertilization, respectively. Anaerobic digestion was solely effective for GHG mitigation (46% reduction), but no impacts were observed for all other low-NH₃ measures. Combining mitigation measures simultaneously decreased total N and GHG losses by 56% and 54%, respectively, and increased the system NUEs by 89%. The wide variations among farms suggest largely attainable improvements in productivity and nutrient use by closing the management gaps related to these factors. Proper combinations of advanced measures are further needed to achieve more effective mitigation goals for multiple pollutants.