[Objective] The impact of different litter inputs on soil nitrogen fractions during the process of returning farmlands to forests in typical black soil regions was explored, in order to provide a theoretical basis for soil nutrient regulation, tree species selection for afforestation, and vegetation restoration. [Methods] The experiment included three treatments (Populus alba litter, Larix gmelinii litter, and maize straw) and a blank control. The litter from each treatment was mixed with air-dried soil and incubated at 25 ℃ for 28 days. The content of soil nitrogen components and the activities of the two enzymes 〔β-glucosidase (βG) and urease (URE)〕 were then measured. Variance analysis and Pearson’s correlation were used to investigate how different litter inputs affect soil nitrogen and enzyme activities in black soil regions. [Results] Variance analysis showed that at 3, 7, 14, and 28 days, the total nitrogen (TN) values in the P. alba litter treatment were significantly higher than those in the control, L. gmelinii litter, and maize straw treatments (p<0.05). The input of the three kinds of litter did not significantly impact the soil available nitrogen (AN) content. For microbial biomass nitrogen, inorganic nitrogen, and enzyme activities (MBN), the P. alba litter treatment displayed a significantly greater increase at 7 and 14 days compared to the control and the other two treatments (p<0.05). At 28 days, the MBN values in both the P. alba and L. gmelinii litter treatments were significantly higher than those in the control and maize straw treatments (p<0.05). All litter treatments significantly reduced the soil’s inorganic nitrogen content. βG was highly or significantly negatively correlated with NH⁺₄-N and NO^-₃-N; it was highly positively correlated with URE, TN, and AN. URE was significantly or highly significantly correlated with TOC, MBN, and TN and negatively correlated with NH⁺₄-N. [Conclusion] The decomposition of the three types of exogenous inputs was closely linked to the soil’s TN, the AN, MBN, inorganic nitrogen, and enzyme activities. Litter input promoted soil nutrient cycling and accumulation, with litter input from the broadleaf tree species P. alba having the most significant impact on improving soil nitrogen and enzyme activities.