MERLIN (Model of Ecosystem Retention and Loss of Inorganic Nitrogen) is a catchment scale model of linked C and N cycling in ecosystems. The model is split in to two plant compartments, namely active (plant) and structural (wood) biomass, and two soil organic compartments termed labile (LOM) and recalcitrant organic matter (ROM). Fluxes in and out of the ecosystem as well as between compartments are regulated by processes such as atmospheric deposition, hydrological discharge, plant uptake, litterfall, wood production, microbial N-immobilisation, mineralisation, nitrification, and denitrification. The rates of fluxes are controlled by the C/N ratios of organic compartments as well as the inorganic N concentrations in the soil solutions.
The physical structure of the soils (depth, bulk density, ionic adsorption) and hydrological discharge (amount, timing and pattern through soil layers) are used in conjunction with the organic fluxes to determine the retention and/or leaching characteristics of inorganic nitrogen at any time in the simulation. Two species of inorganic nitrogen are considered, NO3 and NH4. Both have a dissolved phase in soil water and both are modelled as having an adsorbed phase (cation and anion exchange) on the soil matrix.
Plant and soil organic matter pools are highly aggregated in MERLIN in keeping with the focus of the model on catchment scale dynamics. The plant pool in MERLIN encompasses not only photosynthetically active tissues but also fine roots and new shoots. The labile organic matter pool is the compartment in the soil that receives litter input and thus may be associated with the forest floor. The litter flux, however, also includes fine root death and the interpretation of the LOM compartment may be somewhat different in non-forested systems. In general the interpretation of each aggregated compartment will vary with the application depending on the quality and quantity of data available from the site. The key feature of MERLIN is an emphasis on the coupling and interaction of hydrological and abiotic processes affecting N, with the biotic cycling of N within the ecosystem. Saturation and leaching of N from the forested ecosystem can be viewed as the result of a "competition" between hydrological transport processes, biotic uptake, and immobilisation. The more "efficient" group of processes wins more of the resource. In this approach MERLIN is unique among models of N cycling in ecosystems.