Abstract: A Biogas project can often lead to a large amount of residues and slurry production. Fortunately, hydrothermal carbonization (HTC) can be expected to treat the high-humidity solid wastes. Biogas residues 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. This study aims to realize the efficient conversion of biogas residues and the directional regulation of nitrogen elements. A systematic investigation was made to explore the influence of the different acid environments (citric acid, acetic acid, nitric acid, and phosphoric acid) on the nitrogen speciation and migration regulation during HTC. The livestock manure digestates were taken as the medium using biogas slurry. The products of HTC were separated into the hydrochar and aqueous phase. The morphology and distribution of the nitrogen species were characterized by CHNS/O, XRF, FTIR, XPS, pH, NH₄⁺-N, and TKN. The yield was finally evaluated after measurement. Results showed that the acid addition significantly reduced the hydrochar yields, whereas the nitrogen migration enhanced the nitrogen content in hydrochar. However, various acid environments shared significant influences on the nitrogen migration and transformation during HTC. The distribution was regulated among the solid, aqueous, and gaseous phases, while its enrichment level and speciation were altered in hydrochar. Different acidic additives were determined to regulate the morphology and content of N in hydrochar using 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. Protonation was intensified to enhance the catalytic activity, condensation, and Mannich reactions. Nevertheless, the strong acidity and oxidation also drove C–N bond conversion into C=N, leading to the part of the nitrogen to be lost as nitrogen oxides. In addition, the nitric acid mainly promoted the N migration toward the aqueous phase, whereas phosphoric acid simultaneously promoted the N transfer into both the aqueous and gaseous phases. The divalent alkaline-earth metals, such as Ca²⁺ and Mg²⁺, were co-precipitated with NH₄⁺-N and PO₄³⁻ in the biogas slurry, in order to form struvite (MgNH₄PO₄·6H₂O) or calcium phosphate minerals. Thereby, the organic nitrogen was retained in the digestate-derived hydrochar under phosphoric acid conditions. Moreover, the high concentrations of Na⁺ and K⁺ in the solid phase were promoted the volatilization or solubilization of nitrogen as NH₃ or NH₄⁺, leading to the decreasing nitrogen retention in the solid product. Especially in an organic-acid environment, the citric acid was used to stabilize the nitrogen sources by mild hydrolysis and the chelating of tricarboxylic structure. The volatilization and excessive migration were curbed in the aqueous phase. Organic acids enhanced the Maillard reactions to promote the yield and stability of pyrrolic-N and pyridinic-N. The nitrogen content of HC-M reached 2.42 %, indicating a 55.13 % increase over the raw-slurry control. Although the acetic acid was less effective than citric acid for the solid-phase N enrichment, it still outperformed the inorganic acids. In summary, the citric-acid environment can provide the best performance for the stable nitrogen in hydrochar, which is the most favorable condition for the nitrogen fixation in the solid phase. This finding can provide the theoretical support for the controllable synthesis of N-doped carbon materials and the valorization of byproducts of biogas engineering.