Modelling the ecosystem effects of nitrogen deposition: Model of Ecosystem Retention and Loss of Inorganic Nitrogen (MERLIN

摘要

A catchment-scale mass-balance model of linked carbonand nitrogen cycling in ecosystems has been developed for simulating leachinglosses of inorganic nitrogen. The model (MERLIN) considers linked bioticand abiotic processes affecting the cycling and storage of nitrogen. Themodel is aggregated in space and time and contains compartments intendedto be observable and/or interpretable at the plot or catchment scale. Thestructure of the model includes the inorganic soil, a plant compartmentand two soil organic compartments. Fluxes in and out of the ecosystem andbetween compartments are regulated by atmospheric deposition, hydrologicaldischarge, plant uptake, litter production, wood production, microbialimmobilization, mineralization, nitrification, and denitrification. Nitrogenfluxes are controlled by carbon productivity, the C:N ratios of organiccompartments and inorganic nitrogen in soil solution. Inputs required are:1) temporal sequences of carbon fluxes and pools- 2) time series of hydrologicaldischarge through the soils, 3) historical and current external sourcesof inorganic nitrogen; 4) current amounts of nitrogen in the plant andsoil organic compartments; 5) constants specifying the nitrogen uptakeand immobilization characteristics of the plant and soil organic compartments;and 6) soil characteristics such as depth, porosity, bulk density, andanion/cation exchange constants. Outputs include: 1) concentrations andfluxes of NO₃ and NH₄ in soil solution and runoff; 2) total nitrogen contentsof the organic and inorganic compartments; 3) C:N ratios of the aggregatedplant and soil organic compartments; and 4) rates of nitrogen uptake andimmobilization and nitrogen mineralization. The behaviour of the modelis assessed for a combination of land-use change and nitrogen depositionscenarios in a series of speculative simulations. The results of the simulationsare in broad agreement with observed and hypothesized behaviour of nitrogendynamics in growing forests receiving nitrogen deposition.