WANDA, a regional dynamic nitrogen model (With Aggregated Nitrogen DynAmics) for nitrate leaching from forests

International audienceNitrate concentrations in recently infiltrated groundwater in forested areas in the Netherlands are slowly increasing towards the EU limit of 50 mg NO3 l?1. The origin of this nitrate is thought to be leaching from nitrogen (N) saturated semi-natural ecosystems in these areas. To simulate nitrate leaching on a regional scale, the empirical model WANDA (a regional nitrogen model With Aggregated Nitrogen DynAmics) is introduced. The model is built around the concept that in forests the C:N ratio of the organic layer is indicative of the amount of nitrate leaving the system in drainage. WANDA was tested on a regional dataset of an infiltration area of 10 km2. This infiltration area consisted of 350 forest stands draining to the catchment "Edese Bos" used by a drinking water company. In 75 of the 350 forest stands the C:N ratio of the organic layer was measured. In 30 of these 75 stands, the nitrate concentrations below the rooting zone were measured. A hydrological model calculated water fluxes. The C:N ratio of the 75 stands ranged from 15.7 to 31.3 g C g?1 N. Scots pine stands had the highest C:N ratios in the organic layer. Nitrate concentrations varied in the 30 stands from 0.6 to 70 mg NO3 l?1. The nitrate concentrations in the Douglas fir stands were higher, and in the beech stands lower, than those observed in the locations with other tree species. WANDA predicted nitrate concentrations well, but improvements are required to simulate nitrate fluxes. The simulation of the water fluxes in WANDA is probably where more focussed modelling effort is now required. Keywords: nitrogen, nitrate, forest, model, WANDA, C:N ratio, organic laye

Applying MERLIN for modelling nitrate leaching in a nitrogen saturated Douglas fir forest in the Netherlands after decreased atmospheric nitrogen input.

International audienceThe MERLIN model was applied on the results of a field-scale manipulation experiment with decreased nitrogen (N) deposition in an N saturated forest ecosystem in the Netherlands. The aim was to investigate the mechanisms that could explain the observed rapid response of nitrate as a result of the decreased N input. Calibrating the model to pre-treatment data revealed that, despite the high atmospheric N input, the trees relied on N mineralised from refractory organic matter (ROM) for their growth. MERLIN could simulate only the fast response of nitrate leaching after decreased input if this ROM mineralisation rate was decreased strongly at the time of the manipulation experiment

Towards reduced uncertainty in catchment nitrogen modelling: quantifying the effect of field observation uncertainty on model calibration

International audienceThe value of nitrogen (N) field measurements for the calibration of parameters of the INCA nitrogen in catchment model is explored and quantified. A virtual catchment was designed by running INCA with a known set of parameters, and field "measurements" were selected from the model run output. Then, using these measurements and the Shuffled Complex Evolution Metropolis algorithm (SCEM-UA), four of the INCA model parameters describing N transformations in the soil were optimised, while the measurement uncertainty was increased in subsequent steps. Considering measurement uncertainty typical for N field studies, none of the synthesised datasets contained sufficient information to identify the model parameters with a reasonable degree of confidence. Parameter equifinality occurred, leading to considerable uncertainty in model parameter values and in modelled N concentrations and fluxes. Fortunately, combining the datasets in a multi-objective calibration was found to be effective in dealing with these equifinality problems. With the right choice of calibration measurements, multi-objective calibrations resulted in lower parameter uncertainty. The methodology applied in this study, using a virtual catchment free of model errors, is proposed as a useful tool foregoing the application of a N model or the design of a N monitoring program. For an already gauged catchment, a virtual study can provide a point of reference for the minimum uncertainty associated with a model application. When setting up a monitoring program, it can help to decide what and when to measure. Numerical experiments indicate that for a forested, N-saturated catchment, a fortnightly sampling of NO3 and NH4 concentrations in stream water may be the most cost-effective monitoring strategy. Keywords: INCA, nitrogen model, parameter uncertainty, multi-objective calibration, virtual catchment, experimental desig

Dynamic nitrogen deposition thresholds during forest stand development in a Douglas fir forest analysed with two nitrogen models SMART2 and MERLIN

In contrast to the classical critical load (CL) concept, based on long-term steady-state conditions, a dynamic deposition threshold (DDT) is introduced. This DDT takes into account all relevant dynamic aspects of vegetation development/forest growth, mineralisation, immobilisation and denitrification, depending on the successional stage of the forest. DDT values for nitrogen were determined for a Douglas fir rotation by two process-based nitrogen models SMART2 and MERLIN using three different criteria for critical nitrogen leaching. During most of the rotation time, the predicted DDT values were higher than the corresponding traditional CL. SMART2 and MERLIN predicted a maximum DDT of 4.9 and 4.6 kmol N per ha per year (69 and 64 kg N per ha per year, respectively), when accepting a critical N leaching level of 1.73 kmol N per ha per year related to impacts on ground water quality. This is due mainly to relatively high tree uptake during the first 50 years of a forest rotation, compared to a long-term estim

Modelling the ecosystem effects of nitrogen deposition: Model of ecosystem retention and loss of inorganic nitrogen (MERLIN)

A catchment-scale mass-balance model of linked carbon and nitrogen cycling in ecosystems has been developed for simulating leaching losses of inorganic nitrogen. The model (MERLIN) considers linked biotic and abiotic processes affecting the cycling and storage of nitrogen. The model is aggregated in space and time and contains compartments intended to be observable and/or interpretable at the plot or catchment scale. The structure of the model includes the inorganic soil, a plant compartment and two soil organic compartments. Fluxes in and out of the ecosystem and between compartments are regulated by atmospheric deposition, hydrological discharge, plant uptake, litter production, wood production, microbial immobilization, mineralization, nitrification, and denitrification. Nitrogen fluxes are controlled by carbon productivity, the C:N ratios of organic compartments and inorganic nitrogen in soil solution. Inputs required are: 1) temporal sequences of carbon fluxes and pools- 2) time series of hydrological discharge through the soils, 3) historical and current external sources of inorganic nitrogen; 4) current amounts of nitrogen in the plant and soil organic compartments; 5) constants specifying the nitrogen uptake and immobilization characteristics of the plant and soil organic compartments; and 6) soil characteristics such as depth, porosity, bulk density, and anion/cation exchange constants. Outputs include: 1) concentrations and fluxes of NO3 and NH4 in soil solution and runoff; 2) total nitrogen contents of the organic and inorganic compartments; 3) C:N ratios of the aggregated plant and soil organic compartments; and 4) rates of nitrogen uptake and immobilization and nitrogen mineralization. The behaviour of the model is assessed for a combination of land-use change and nitrogen deposition scenarios in a series of speculative simulations. The results of the simulations are in broad agreement with observed and hypothesized behaviour of nitrogen dynamics in growing forests receiving nitrogen deposition

Spatial variability of throughfall water and chemistry and forest floor water content in a Douglas fir forest stand

International audienceThis study focuses on spatial variability of throughfall water and chemistry and forest floor water content within a Douglas fir (Pseudotsuga menziesii, Franco L.) forest plot. Spatial patterns of water and chemistry (NH4+, NO3?, SO42-, Cl?, Mg2+, Ca2+, Na+ and K+) were compared and tested for stability over time. The spatial coefficient of variation (CV) was between 18 and 26% for amounts of throughfall water and ions, and 17% for forest floor water content. Concentrations and amounts of all ions were correlated significantly. Ion concentrations were negatively correlated with throughfall water amounts, but, except for NH4+, there was no such relation between throughfall water and ion amounts. Spatial patterns of throughfall water fluxes and forest floor water contents were consistent over time; patterns of ion fluxes were somewhat less stable. Because of the spatial variability of forest floor thickness and drainage, it was not possible to relate patterns in throughfall water directly to patterns in water content. The spatial variability of throughfall nitrogen and forest floor water contents can cause significant variability in NO3? production within the plot studied. Keywords: nutrient throughfall, forest floor water, spatial variability, time-stability, nitrogen</p