Nitrogen-based pollution from agriculture has global environmental consequences. Excessive use of chemical nitrogen fertilizer, incorrect manure management and rural waste treatment are key contributors. Circular agriculture combining cropland and livestock is an efficient channel to reduce the use of chemical nitrogen fertilizers, promote the recycling of livestock manure, and reduce the global N surplus. The internal circulation of organic nitrogen resources in the cropland-livestock system can not only reduce the dependence on external synthetic nitrogen, but also reduce the environmental impacts of organic waste disposal. Therefore, this study tried to clarify the reactive nitrogen emissions of the crop-swine integrated system compared to the separated system from a life cycle perspective, and analyze the reasons for the differences in nitrogen footprints of the two systems. The results showed that the integrated crop production and swine production increased the grain yield by 14.38% than that of the separated system. The nitrogen footprints of crop production and swine production from the integrated system were 12.02% (per unit area) and 19.78% lower than that from the separated system, respectively. The total nitrogen footprint of the integrated system showed a reduction of 17.06%. The reduction was from simpler waste manure management and less agricultural inputs for both chemical fertilizer and raw material for forage processing. In conclusion, as a link between crop planting and pig breeding, the integrated system not only reduces the input of chemical fertilizers, but also promotes the utilization of manure, increases crop yield, and decreases environmental pollution. Integrated cropland and livestock is a promising model for agriculture green and sustainable development in China.

Organic materials incorporation and nitrogen utilization are both important measures to enhance soil fertility and crop productivity. To evaluate the effect of different organic materials on soil N utilization and crop yield, three kinds of organic materials, including straw (ST), pig manure (PM), and biogas residue (BR), were incorporated into the soil with equal N in the wheat (Triticum aestivum L.)-maize (Zea mays L.) cropping system with chemical fertilizers (CFs) as a control in Wuqiao County in the North China Plain. The contents of soil N, nitrate N, ammonium N, N use efficiency, N partial productivity (NPFP), N uptake efficiency (NUPE), and N harvest index (NHI) were measured and calculated. The results of 2-yr field trials suggested that, compared with CF, soil total N (TN) content of ST, PM, and BR treatment increased by 3.04, 8.99, and 9.10%, respectively. An even larger increase was found in nitrate N, which was 25.87, 44.67, and 44.93% in ST, PM, and BR, respectively. For N utilization, PM significantly increased NPFP, NUPE, and NHI by 4.01, 5.86, and 3.04%, respectively, meanwhile decreasing soil N dependency rate by 4.64%; BR improved NUPE by 5.15%, while ST showed little improvement in the utilization of soil N. Overall, organic materials promoted soil N maintenance and increased N utilization.Among the kinds of organic material amendment in the study, PM had a better performance in improving soil N utilization and crop yield than BR and crop ST return in field experiments. INTRODUCTIONIn crop and soil systems, N has been suggested to be a factor in crop growth and yielding ability by participating in plant genetic metabolism (material synthesis and signal transduc-