Abstract: Nitrogen overload is a key factor driving eutrophication in water bodies. Constructed wetlands are widely used for the remediation of polluted water, but their effluent quality remains unstable, necessitating improvements in nitrogen removal efficiency. In this study, an indoor microcosm experiment was established by introducing benthic macroinvertebrates. Four treatments were set up: Myriophyllum elatinoides wetland (Group L), M. elatinoides with Bellamya aeruginosa wetland (Group L+T), Pontederia cordata wetland (Group S), and P. cordata with B. aeruginosa wetland (Group S+T). Using stable isotope ¹⁵N tracing combined with mass balance analysis, the migration and transformation processes of nitrogen among water, plants, sediment, and snails were quantitatively assessed. The results showed that in the Group L+T and S+T , the removal rates of total nitrogen (TN) and ammonium nitrogen NH₄⁺-N increased by 2.38%~17.10% compared with the snail-free groups. The fate of nitrogen was dominated by system loss (55.13%~59.03%), mainly driven by microbial transformation), followed by plant uptake (28.28%~32.87%), sediment retention (8.91%~9.57%), and snail assimilation (3.09%～3.12%). Random forest analysis identified water TN and NH₄⁺-N as the dominant factors affecting nitrogen removal in the wetlands. Overall, the introduction of B. aeruginosa restructured nitrogen migration pathways and their contribution patterns by regulating the microenvironment and biological interactions, directing more exogenous nitrogen toward plant and animal assimilation while reducing the proportion of microbial-driven gaseous loss. This study provides a novel regulatory strategy for enhancing nitrogen removal performance and promoting nitrogen resource utilization in constructed wetlands.