Topographic differences in nitrogen cycling mediate nitrogen retention in a subtropical, N-saturated forest catchment

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 N-15-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 N-15-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 N-15-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 N-15-NH4+ into recalcitrant organic N and denitrifying N-15-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.

Automated summary

The topography of forested catchments plays a significant role in controlling nitrogen cycling, impacting N retention and dissipation in forest ecosystems.