Forest-atmosphere exchange of reactive nitrogen in a remote region – Part II: Modeling annual budgets
Authors: Wintjen Pascal , Schrader Frederik , Schaap Martijn , Beudert Burkhard , Kranenburg Richard , Brümmer Christian 
Published: March 2022

Abstract. To monitor the effect of current nitrogen emissions and mitigation strategies, total atmospheric nitrogen deposition to forests is commonly estimated using chemical transport models or canopy budget models in combination with throughfall measurements. Since flux measurements of reactive nitrogen (Nr) compounds are scarce, dry deposition process descriptions as well as the calculated flux estimates and annual budgets are subject to considerable uncertainties. In this study, we compared four different approaches to quantify annual dry deposition budgets of total reactive nitrogen (ΣNr) at a mixed forest site situated in the Bavarian Forest National Park, Germany. Dry deposition budgets were quantified based on (I) 2.5 years of eddy-covariance flux measurements with the Total Reactive Atmospheric Nitrogen Converter (TRANC), (II) an in-situ application of the bidirectional inferential resistance scheme DEPAC (Deposition of Acidifying Compounds), here called DEPAC-1D, (III) a simulation with the chemical transport model LOTOS-EUROS (LOng Term Ozone Simulation – EURopean Operational Smog) v2.0 using DEPAC as dry deposition module, and (IV) a canopy budget technique (CBT). Averaged annual ΣNr dry deposition estimates determined from TRANC measurements were 4.7 ± 0.2 and 4.3 ± 0.4 kg N ha-1 a-1 using DEPAC-1D only, and the Mean-Diurnal-Variation method in combination with DEPAC-1D as gap-filling approaches, respectively. DEPAC-1D modeled dry deposition, using concentrations and meteorological drivers measured at the site, was 5.8 ± < 0.1 kg N ha-1 a-1. In comparison to TRANC fluxes, DEPAC-1D estimates were systematically larger during summer, and in close agreement in winter. Modeled ΣNr deposition velocities (vd) of DEPAC-1D were found to increase with lower temperatures, higher relative humidity, and in the presence of wet leaf surfaces, in particular from May to September. This observation was in contradiction to TRANC-observed fluxes, leading to the conclusion that the parametrizations may need revision. LOTOS-EUROS modeled annual dry deposition was 6.5 ± 0.3 kg N ha-1 a-1 for the site-specific weighting of land-use classes within the site’s grid cell. LOTOS-EUROS showed substantial discrepancies to measured ΣNr deposition during spring and winter, which was related to an overestimation of ammonia concentrations within the grid cell. LOTOS-EUROS predicted an averaged ΣNr concentration of 5.0 ± 3.3 μg N m-3. Ammonia (NH3) contributed most to modeled ΣNr concentrations but the modeled NH3 concentrations were overestimated by a factor two to three compared to measured values. Annual deposition estimates were in the range of minimum and maximum estimates determined from CBT being at 3.8 ± 0.5 and 6.7 ± 0.3 kg N ha-1 a-1, respectively. By adding locally measured wet-only deposition, we estimated an annual total nitrogen deposition input between 11.5 and 14.8 kg N ha-1 a-1, which is at the upper end of the critical load ranges proposed for deciduous and coniferous forests. In this work, we conducted one of the first comparisons of micrometeorological and ecological methods for estimating annual nitrogen dry deposition to a remote mixed forest.