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

The separation of crop and livestock is the main obstacle to the environment-friendly production of agriculture. At present, the research of agriculture mainly focuses on large-scale and single objective, while the research of multi-objective nutrient optimization management aiming at different environmental characteristics in 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, and then used the Nutrient Flow in Food Chain (NUFER) 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 county-level crop-livestock mode. The results show that: 1) The total nitrogen input was as high as 28.72 Gg, in which the manure and urine resources of livestock and poultry were relatively abundant, reaching 17.55 Gg, but the environmental emission exceeded the threshold value seriously in Tangxian County. 2) There are significant regional differences in the livestock and poultry manure resources and environmental emissions in Tangxian County which mainly concentrated in towns with high numbers of livestock and poultry. By multi-objective environmental zoning, Tangxian County is divided into the low ammonia and high residual overloaded in the middle, the high ammonia and high residual non-overloaded in the north and east, and the high ammonia and high residual overloaded in the southwest. 3) The three-step nitrogen optimization strategy was implemented in different regions of Tangxian County to achieve environment-friendly emission. Firstly, the balanced fertilization method was adopted, which could reduce the number of towns exceeding the threshold of ammonia volatilization and nitrogen residual can be reduced to 45% and 35% respectively. Secondly, after adding different emission reduction technologies, the number of towns exceeding the threshold value of the two indicators decreased to 30% and 20% respectively. Finally, the ammonia emission of the whole county can be reduced to the safety threshold by using the optimization of spatial distribution method, and only 20% of towns with the excess nitrogen residual exceed the threshold is left. The contradiction between agricultural and animal husbandry production and environmental emissions can be solved efficiently and at low cost through the optimal management of county nitrogen nutrient zoning, and the green transformation of county cultivation mode can be promoted.