Nitrogen (N) export from forested catchments has been documented even under relatively low atmospheric N deposition, challenging the traditional view that temperate forest ecosystems remain persistently N limited. To understand potential mechanisms underlying this apparent decoupling between external N inputs and N retention, we developed a minimal yet mechanistically explicit carbon–nitrogen (C–N) modeling framework. The model is implemented as a multi-agent-based system in which heterotrophic microbes, nitrifiers, and plants are represented as interacting agents competing for dissolved inorganic nitrogen (DIN) under dynamically changing carbon conditions.
The framework explicitly couples labile carbon turnover with stoichiometric microbial growth, allowing net mineralization or immobilization to emerge from substrate C:N and microbial N demand rather than being prescribed. Microbial agents differ in turnover and resource-use strategies, while plant uptake is formulated as a state-dependent process that can decline with physiological aging. Nitrification and leaching processes connect ammonium dynamics to nitrate export, with ammonium accessibility and physicochemical constraints influencing the propagation of carbon-driven signals into inorganic N pools. In this structure, N retention capacity is not imposed as a fixed system property but emerges from competition among biological agents and their access to carbon and nitrogen resources. Transient labile carbon pulses are introduced to investigate how shifts in substrate quantity and quality alter microbial growth, DIN partitioning, and downstream nitrate fluxes. In the current framework, nitrification and nitrate leaching fluxes are strongly influenced by parameters controlling microbial nitrogen demand, particularly microbial C:N ratios and carbon use efficiency. Carbon pulses enhance microbial assimilation and transiently shift DIN interception from plants to microbes, suppressing nitrification-driven export. Under the present parameterization, this shift is temporary, and plants regain long-term dominance in nitrogen uptake, despite continued nitrate export. These results suggest that sustained nitrate loss reflects the balance between biological interception (plants and microbial immobilization) and nitrification-driven pathways, highlighting the potential importance of carbon state and biological competition in regulating long-term N export.
This multi-agent, stoichiometrically explicit framework provides a platform for hypothesis testing on the mechanisms governing sustained N loss in aging or disturbed forest ecosystems and offers a theoretical basis for interpreting long-term watershed observations under shifting carbon regimes.