Forest–atmosphere exchange of reactive nitrogen in a remote region – Part II: Modeling annual budgets
<p>To monitor the effect of current nitrogen emissions and mitigation strategies, total (wet <span class="inline-formula">+</span> dry) atmospheric nitrogen deposition to forests is commonly estimated using chemical transport models or canopy budget models in combination...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-11-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/19/5287/2022/bg-19-5287-2022.pdf |
| Summary: | <p>To monitor the effect of current nitrogen emissions and
mitigation strategies, total (wet <span class="inline-formula">+</span> dry) 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 (N<span class="inline-formula"><sub>r</sub></span>) 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 (<span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span>) 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 flux model
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).</p>
<p>Averaged annual <span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span> dry deposition estimates determined from
TRANC measurements were 4.7 <span class="inline-formula">±</span> 0.2 and 4.3 <span class="inline-formula">±</span> 0.4 kg N ha<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>, depending on the gap-filling approach. DEPAC-1D-modeled dry
deposition, using concentrations and meteorological drivers measured at the
site, was 5.8 <span class="inline-formula">±</span> 0.1 kg N ha<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>. In comparison to TRANC
fluxes, DEPAC-1D estimates were systematically higher during summer and in
close agreement in winter. Modeled <span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span> deposition velocities
(<span class="inline-formula"><i>v</i><sub>d</sub></span>) of DEPAC-1D were found to increase with lower temperatures and higher
relative humidity and in the presence of wet leaf surfaces, particularly
from May to September. This observation was contrary to
TRANC-observed fluxes. LOTOS-EUROS-modeled annual dry deposition was
6.5 <span class="inline-formula">±</span> 0.3 kg N ha<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span> for the site-specific weighting of
land-use classes within the site's grid cell. LOTOS-EUROS showed substantial
discrepancies to measured <span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span> deposition during spring and
autumn, which was related to an overestimation of ammonia (NH<span class="inline-formula"><sub>3</sub></span>)
concentrations by a factor of 2 to 3 compared to measured values as a
consequence of a mismatch between gridded input NH<span class="inline-formula"><sub>3</sub></span> emissions and the
site's actual (rather low) pollution climate. According to LOTOS-EUROS
predictions, ammonia contributed most to modeled input <span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span>
concentrations, whereas measurements showed NO<span class="inline-formula"><sub><i>x</i></sub></span> as the prevailing
compound in <span class="inline-formula">Σ</span>N<span class="inline-formula"><sub>r</sub></span> concentrations. Annual deposition estimates
from measurements and modeling were in the range of minimum and maximum
estimates determined from CBT being at 3.8 <span class="inline-formula">±</span> 0.5 and 6.7 <span class="inline-formula">±</span> 0.3 kg N ha<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>, 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<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>, which is within the critical load
ranges proposed for deciduous and coniferous forests.</p> |
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| ISSN: | 1726-4170 1726-4189 |