N leaching and gaseous N emissions from forested catchments are controlled by soils differing in nitrogen (N) status and turnover depending on landscape position. To understand the impact of topography on N retention and dissipation in forested catchments suffering from high atmospheric N deposition, we carried out an ex-situ ¹⁵N-tracing study with soils from a hillslope (HS) and a hydrologically connected groundwater discharge zone (GDZ) of an N-saturated subtropical forest in South China. Despite being severely N-saturated, soil from HS incorporated a substantial amount of added ¹⁵N–NH₄⁺ instantly into recalcitrant organic N. The remaining NH₄⁺ was cycled via a microbial loop of fast N immobilization and re-mineralization, slowly releasing NH₄⁺ for autotrophic nitrification. Heterotrophic nitrification was only observed right after tracer application. Added ¹⁵N–NO₃^- cycled between soil microbial biomass and dissolved organic N without being stored in the recalcitrant organic N pool, explaining the strong propensity of HS soils for NO₃⁻ leaching. By contrast, the soil from GDZ acted as a sink for added N by incorporating ¹⁵N–NH₄⁺ into recalcitrant organic N and denitrifying ¹⁵N–NO₃^- to gaseous N. Here, N immobilization exceeded N mineralization, suggesting N limitation. Heterotrophic nitrification was the main pathway of NH₃ oxidation in the GDZ soil, and N₂O–N contributed substantially to N removal. Abiotic processes played a role in NO₃⁻ incorporation into organic N but not in N₂O production, while DNRA was negligible in either soil. Overall, our findings suggest strong topographic control on N cycling, which might explain the unexpectedly high N retention and removal from N-saturated forests in subtropical China.