The nitrogen surplus caused by excessive nitrogen input of crop and livestock production is the major source of pollution load in surface waterbody. The spatial differentiation characteristics of agricultural unused nitrogen were quantified in this study, so as to provide a basis for the zoning management of nitrogen and the effective management of agricultural nitrogen source in the river basin.

This study was conducted in Baoding, the central area of the Baiyangdian Basin. The nitrogen input and output of the crop and livestock production system were analyzed in all counties of Baoding in 2016. The nitrogen surplus in the crop production system was defined as the differential value between each input item (chemical fertilizer, organic fertilizer, atmospheric dry and wet settlement, irrigation water, seeds, non-symbiotic nitrogen fixation and straw returning to the field) and the output item (crop grain and straw); the nitrogen surplus in livestock production system was defined as the differential value between the amount of manure and the amount of organic fertilizer; and the nitrogen surplus in the agriculture and animal husbandry was defined as the sum of crop production system and livestock production system.

(1) The level of nitrogen surplus in crop production was ranging from 90.27 to 581.73 kg·hm^-2, with the lowest value in Dingxing District and the largest value in Mancheng District. Vegetables contributed to the most nitrogen surplus of crop production (31.3%), following by fruit trees (29.0%), wheat (27.8%), and maize (11.9%). Fertilizer was the primary source of nitrogen surplus (61.8%), following by organic fertilizer (16.8%), straw return to the field (8.9%), atmospheric settlement (5.2%), irrigation (3.4%), non-symbiotic nitrogen fixation (3.0%), and seeds (0.9%). (2) The nitrogen surplus level of livestock production was ranging from 0.06×10⁴ t to 2.48×10⁴ t with the lowest value in Lianchi District and the highest value in Xushui Distrct. Beef cattle accounted for 71.0% of the total nitrogen surplus of livestock production. (3) The unused nitrogen level in the agricultural and animal husbandry system was between 0.43×10⁴ t and 4.97×10⁴ t, among which the unused nitrogen amount was the highest in the Xushui area. In farming systems, farming was the main source of nitrogen (55.8% of unused nitrogen).

The nitrogen unused space varied significantly in Baoding counties, the core area of Baiyangdian Basin, and the highest nitrogen unused in Xushui District was 10.4 times that in Jingxiu District. The largest contribution to the amount of unused nitrogen in the crop and livestock production is the crop production, of which vegetable production is the most important industry.