【Objective】To investigate whether soil nutrient concentrations would increase in a poplar (Alus trabeculosa)-alder (Populus×euramericana ‘Nanlin 95’) mixed forest relative to the nutrient concentrations in a pure poplar forest under moderate nitrogen (N) concentrations, and analyze the N-fixing potential of alder, to explore the establishment of poplar-alder mixed forests as a potential strategy for mitigating a decline in soil fertility caused by successive plantation rotations.【Method】Poplar plantations and poplar-alder mixed forest sites were set up in experimental plots in the north of Jiangsu Province. A nylon mesh belt was used to investigate the mass decomposition and nutrient release from poplar and/or alder litter and the effect of litter removal and non-removal on the soil total N and available N concentrations. The in-situ resin core method was used to investigate the annual changes in net N mineralization rates in the two forests soil with and without litter.【Result】The establishment of a poplar-alder mixed forest increased soil total N and available N concentrations significantly. Litter in the pure poplar forest decomposed slowly and the N in the surrounding soil was adsorbed by the litter during decomposition. Conversely, the litter in the poplar-alder mixed forest decomposed and entered the N release state rapidly. Therefore, the soil available N decreased significantly in the poplar-alder mixed forest when the litter was removed. In the litter non-removal treatment, the annual net amount of N from ammonification per hectare of soil in the mixed forest was significantly higher than that in the pure poplar forest. However, in the litter removal treatment, there was no significant difference in N amount from ammonification between the two treatments with or without litter in the mixed forest.【Conclusion】Alder is a suitable tree species for integration with poplar. Poplar-alder mixed forest could increase soil total N and available N concentrations and facilitate the mitigation of soil fertility decline following successive rotations of pure poplar plantations. The rates of litter decomposition and N release in the mixed forests were more rapid, which would be beneficial to the N cycle in plantation ecosystems. In the litter non-removal treatment, although the nitrification N was not considerably different between the pure poplar plantation and the poplar-alder mixed forest, the ammonification N amount was higher in the mixed forest soil than that in the pure poplar soil, which indicated that the N released during litter decomposition in the mixed forest largely enhanced the soil ammonification process and weakly influenced the soil nitrification process.