Optimal nitrogen management approach for green transformation of crop-livestock mode at county level: A case study of Tangxian County

Separation of crop and livestock is the main obstacle to environmentally friendly agricultural production. Currently, research on crop-livestock production mainly focuses on large-scale and single objective, whereas research on multi-objective nutrient optimization management aimed at different environmental characteristics at county level is relatively lacking. Taking Tangxian County, Hebei Province as an example, this study used the method of multi-objective environmental zoning, collected relevant literature, farmer survey, and statistical data, then used NUtrient flows in Food chains, Environment and Resources use (NUFER) model to analyze the current environmental emission characteristics of the crop-livestock mode in Tangxian County and explore the pathways of nitrogen optimization management under the green transformation of crop-livestock mode at county level. The results showed that: 1) Total nitrogen input was as high as 35 731 t, in which the livestock manure resources were relatively abundant, reaching 13 301 t, but the environmental emissions seriously exceeded the threshold value in Tangxian County. 2) There were significant regional differences in the livestock manure resources and environmental emissions in Tangxian County, which were mainly concentrated in towns with high numbers of livestock. Using multi-objective environmental zoning, Tangxian County was divided into overloading zone with low ammonia and high surplus in the middle, non-overloading zone with high ammonia and high surplus in the north and east, and overloading zone with high ammonia and high surplus in the southwest. 3) A three-step nitrogen optimization strategy was implemented in different regions of Tangxian County to achieve environmentally friendly emissions. First, using balanced fertilization method was adopted, the proportion of towns with excessive ammonia volatilization and nitrogen surplus in Tangxian County decreased to 65% and 35%, respectively. Second, after adding different emission reduction technologies, the proportion of towns with excessive ammonia volatilization and nitrogen surplus in Tangxian County decreased to 25% and 45%, respectively. Finally, the ammonia emissions of the whole county can be reduced to the safety threshold by using the optimization of the spatial planning method, and still 20% of towns with excessive nitrogen surplus. The contradiction between crop-livestock production and environmental emissions can be solved efficiently and at a low cost through the optimal nutrient management and spatial planning, and the green transformation of crop-livestock production at county level can be promoted.