Modelling the ecosystem effects of nitrogen deposition: Simulation of nitrogen saturation in a Sitka spruce forest, Aber, Wales, UK

A new model for simulating nitrogen leaching from forested ecosystems has been applied to data from an experimentally manipulated 30-year-old Sitka spruce stand. The manipulation experiment (at Aber, in north-western Wales, UK) was part of the European NITREX project and involved five years of additions of inorganic nitrogen to the spruce stand. The model (MERLIN) is a catchment-scale, mass-balance model that simulates both biotic and abiotic processes affecting nitrogen in ecosystems. The structure of MERLIN includes representations of the inorganic soil, one plant compartment and two soil organic compartments. Fluxes in and out of the simulated ecosystem and transfers between compartments are regulated by atmospheric deposition, hydrological discharge and biological processes such as plant uptake, litter production, immobilization, mineralization, nitrification and denitrification. Rates of nitrogen uptake, cycling and release among pools are regulated by carbon productivity, inorganic nitrogen availability and the C:N ratios of the organic pools. Inputs to the model are temporal sequences of carbon fluxes and pools, hydrological discharge and external sources of nitrogen. The NITREX experiment at Aber began in 1990 with weekly additions of ammonium nitrate (NH4NO3) at a rate of 35 kg N ha(-1) yr(-1). Data were collected from both control and treatment plots within the stand. The site-intensive data from the control plots at Aber were augmented by data taken from a chronosequence of 20 Sitka spruce stands and data from a survey of 5 moorland catchments in the same region to provide calibration data for the model. The data were used to establish current conditions at the Aber site and to reconstruct historical sequences of carbon fluxes and pools from 1900 to the present day with which to drive the model. The reconstructed sequences included an increase in nitrogen deposition and a vegetation change from moorland to plantation forest in 1960. The calibrated model was then used to predict the effects of the experimental nitrogen additions begun in 1990. MERLIN successfully reproduced the observed increase in NO3 leaching from aging spruce stands that results from forest maturation and increased nitrogen deposition (as inferred from the chronosequence and forest survey data in the region). MERLIN also correctly predicted the increases in soilwater NO3 concentrations, the changes in nitrogen content of tree and soil organic matter pools, and the changes in nitrogen fluxes that occur in spruce stands in response to increased nitrogen inputs (as observed in the nitrogen addition experiment).