Abstract: The vigorous development of biogas project leads to a large number of residue and slurry production. Hydrothermal carbonization (HTC) is a proper technology to treat high-humidity solid waste. Biogas residue and slurry are rich in nitrogen. During HTC, the speciation and migration of nitrogen are closely linked to the combustion of hydrochar, the preparation of nitrogen-doped carbon-based materials, and the recovery of nutrients. In order to realize the efficient conversion of biogas residue and the directional regulation of nitrogen elements, the influence of different acid environment (citric acid, acetic acid, nitric acid, and phosphoric acid) on nitrogen speciation and migration regulation during HTC of livestock manure digestate using biogas slurry as the medium. The products of HTC were separated into hydrochar and aqueous phase, and analyzed for yield, CHNS/O, XRF, FTIR, XPS, pH, NH₄⁺-N and TKN to analyze the morphology and distribution of the nitrogen species. Results showed that acid addition significantly reduced the hydrochar yields and promoted nitrogen migration and enhanced nitrogen content in hydrochar. However, different acid environments exert significant influences on nitrogen migration and transformation during HTC, regulating its distribution among the solid, aqueous, and gaseous phases, while altering its enrichment level and speciation in hydrochar. Different acidic additives determine the morphology and content of N in hydrochar by regulating the hydrolysis, cyclization, condensation, oxidation, and crosslinking with the carbon backbone. Inorganic acids (nitric and phosphoric) markedly reduced the relative content of protein-N in hydrochar to 24.59 % and 24.71 %, respectively. By intensifying protonation, they enhanced catalytic activity, promoted condensation and Mannich reactions, and facilitated pyrrolic-N cyclization, resulting in combined pyridinic-N and quaternary-N contents of 58.52 % in HC-N and 56.78 % in HC-P, respectively. Nevertheless, the strong acidity and oxidizing ability also drove C–N bond conversion into C=N, causing part of the nitrogen to be lost as nitrogen oxides. In addition, nitric acid mainly promoted N migration toward the aqueous phase, whereas phosphoric acid simultaneously promoted N transfer to both the aqueous and gaseous phases. Under phosphoric acid conditions, divalent alkaline-earth metals, such as Ca²⁺ and Mg²⁺ can co-precipitate with NH₄⁺-N and PO₄^3- in the biogas slurry to form struvite (MgNH₄PO₄·6H₂O) or calcium phosphate minerals, thereby reducing the organic nitrogen retained in the digestate-derived hydrochar. Moreover, elevated concentrations of Na⁺ and K⁺ in the solid phase promote the volatilization or solubilization of nitrogen as NH₃ or NH₄⁺, leading to decreased nitrogen retention in the solid product. In organic-acid environment, especially, citric acid stabilized nitrogen sources by mild hydrolysis and the chelating effect of the tricarboxylic structure, curbing volatilization and excessive migration to the aqueous phase. Organic acids enhanced Maillard reactions to promote the yield and stability of pyrrolic-N and pyridinic-N. The nitrogen content of HC-M reached 2.42 %, representing a 96.75 % increase over the original digestate and a 55.13 % increase over the raw-slurry control. Although acetic acid was less effective than citric acid for solid-phase N enrichment, it still outperformed the inorganic acids. In summary, the citric-acid environment provides the best performance for stably enriching nitrogen in hydrochar, which is the most favorable condition for nitrogen fixation in the solid phase. This study provides theoretical support for the controllable synthesis of N-doped carbon materials and the valorization of byproducts of biogas engineering.