Bonten, L.T.C.

Mol, J.

Reinds, G.J.

English

Name not available

Validation of VSD+ and critical loads for nutrient N

NARCIS 

Wageningen Yield 

CCE Status Report 2011 | 49
Luc Bonten, Janet Mol-Dijkstra, Gert Jan Reinds
Alterra (WUR), Wageningen, Netherlands
3.1  Introduction
The soil chemistry model VSD+ (Bonten et al. 2009) has 
been developed from the soil acidification model VSD 
(Posch and Reinds 2009), to calculate effects of 
atmospheric deposition on i) nutrient enrichment of the 
soil and subsequent effects on the vegetation and ii) 
carbon sequestration in the soil. The reasons for this are 
the changing objectives regarding the effects of 
atmospheric deposition on nature areas. Initial objectives 
were the effects of deposition of sulphur and nitrogen (N) 
compounds on the acidification of the soil and the effects 
on forest health and surface water quality. As sulphur 
deposition has decreased dramatically in most areas in 
Europe during the last decades, the focus shifted more and 
more towards the effects of nitrogen deposition on 
biodiversity (e.g., Hettelingh et al. 2008, Millennium 
Ecosystem Assessment 2005, EEA 2007), more specifically 
plant biodiversity, as a higher N deposition generally leads 
to a lower plant species biodiversity. The increasing 
awareness of climate change due to greenhouse gas 
emissions has further lead to the demand to calculate 
carbon (C) sequestration potentials and the effects of 
atmospheric deposition on this. To be able to calculate the 
effects of N deposition on biodiversity and to include soil 
carbon sequestration we added an explicit model for 
organic carbon and nitrogen to the original VSD model, 
which then became VSD+. This C and N model and the 
coupling of VSD+ to plant species biodiversity models like 
Veg (see chapter 4) has been described previously (see 
Appendix B in Slootweg et al. 2010). In this text we show a 
first validation of VSD+ results regarding N enrichment in 
the soil and the potentials of VSD+ to calculate critical 
loads for nutrient N critical limits.
3.2  Validation
Figure 3.1 shows the N fluxes within a forest ecosystem. 
The largest fluxes are possibly the internal cycling of N in 
the soil, i.e. the turnover of soil organic matter and soil 
microbial biomass. The actual sizes of these fluxes are 
however hard to measure. Also relatively large is the 
cycling of N by the vegetation, i.e. litterfall and uptake by 
the vegetation. Both the N cycling by the vegetation and 
the soil internal N cycling determine the size of the organic 
N pool in the soil. A first validation is therefore on these N 
pools. A second validation is on the leaching of N from the 
soil. This N leaching is small compared to other N fluxes in 
the system and is actually the difference of two ‘big’ 
numbers: available N and N uptake.
3
Validation of VSD+  
and Critical Loads for 
Nutrient N 
50 | CCE Status Report 2011
Validation on N pools
Figure 3.2 shows the results on N pools in fi ve chrono-
sequences in the Netherlands and southern Scandinavia. 
Although this is not a true validation because these sites 
have been used previously in the calibration of VSD+, it is 
clear that VSD+ can describe changes in N pools with a 
single parameter set for organic C and N turnover. The 
fi gure further shows that changes in organic N pools are 
small for most sites and that a sound validation requires 
really long-term datasets.
Validation on N leaching
The calculation of N leaching by VSD+ was tested for two 
diff erent datasets. First, for a set of 137 Dutch forest sites, 
for which we used national scale model calculations for N 
deposition and estimates for vegetation growth and N 
contents of the vegetation. The second dataset contains 
two forest sites, one in Solling, Germany (Bonten et al. 
2011) and one in the Hardenberg, the Netherlands, with 
more detailed information. Atmospheric deposition, forest 
growth and N contents were measured on-site. In both 
calculations we did not calibrate organic C and N turnover 
separately for each site, but we used a single parameter 
set for organic C and N turnover taken from the calibration 
on the fi ve chrono-sequences (see above).
Figure 3.3a shows a comparison between observed and 
calculated N leaching concentrations for the 137 Dutch 
forest sites. This fi gures shows that there can be a large 
deviation between measurements and calculations, 
although average concentrations seem to correspond. 
Figure 3.3b shows the frequency distributions of the 
measured and calculated N concentrations, and these 
match perfectly. This demonstrates that site-specifi c N 
leaching can not be calculated using estimates for N 
deposition and/or N uptake. As mentioned previously, N 
leaching is the diff erence between N availability and N 
uptake and a small error in one of these can lead to large 
errors in N leaching. However, average regional scale N 
leaching can be properly assessed using VSD+.
Figure 3.4 shows the N leaching concentrations for the two 
forest sites for which more site-specifi c data are available. 
The two graphs show that a much bett er agreement 
Figure 3.1  Fluxes of N within a forest ecosystem.
deposition &
xation:
2-70 kg/ha/yr
available N:
80-140 kg/ha/yr
uptake:
80-120 kg/ha/yr
lierfall:
80-120 kg/ha/yr
organic N cycling:
??? kg/ha/yr
leacing:
0-20 kg/ha/yr
Figure 3.2  Simulated and observed N pools in the soil for fi ve 
chrono-sequences.
Gejlvang
1.2
1
0.2
0
0.8
0.4
0.6
0 0.2 0.4 0.6 1.2
Npool (kg/m2) observed
0.8 1
N
po
ol
 (k
g/
m
2 )
 s
im
ul
at
ed
Sellingen
Vestskoven Spruce
Vestskoven Oak
Tönnersjöheden
Figure 3.3  3.3a Simulated and observed N leaching concentrations at 137 Dutch forest sites (left ); 3.3b Frequency distributions of 
observed and simulated N leaching concentrations at these sites (right).
10
0.01
1
0.1
0.01 0.1 10
[NO3]-simulated (mmol/l)
1
[N
O
3]-
ob
se
rv
ed
 (m
m
ol
/l)
0 0.1 10
[NO3] (mmol/l)
1
pe
rc
en
ta
ge
observed
simulated
100
80
40
20
0
60
CCE Status Report 2011 | 51
between calculations and observations can be achieved 
using site-specific data on N deposition and N uptake.
3.3  Critical loads for nutrient N
The VSD+ model includes the calculation of critical N 
deposition for criteria of N as a nutrient. Critical loads for N 
as acidifying compound were already included in VSD. In 
VSD+ critical loads can be calculated for two different 
critical limits: i) a maximum N leaching concentrations, 
and ii) a maximum N availability (defined as the sum of N 
deposition and N mineralization).
CL for critical N leaching
Figure 3.5 shows critical loads of NH3 and NOx deposition 
as a function of denitrification. Denitrification itself is 
influenced by a number of parameters like moisture, pH, 
temperature, availability of electron donor. When there is 
no denitrification all nitrogen deposition that is not taken 
up will leach. There is no distinction then between NH3 and 
NOx deposition. For high denitrification rates, the leaching 
is mainly dependent on the NH3 deposition, because most 
NOx will be lost to the atmosphere as N2 after 
denitrification.
CL for critical N availability
Nitrogen availability has been defined as the sum of N 
mineralization, N deposition and N fixation. In steady state 
the N mineralization is equal to the organic N inputs to the 
soil, i.e. litterfall and fine root turnover. Because of this, 
critical loads for a critical N availability can be calculated 
simply as:
CL(N) = Navailable,crit – Nlitterfall – Nroot turnover – Nfixation
References
Bonten L, Mol J, Reinds GJ, 2009. Dynamic modelling of 
effects of deposition on carbon sequestration and 
nitrogen availability – VSD+: VSD plus C and N 
dynamics. In: CCE Status Report 2009, pp. 69–73; 
www.rivm.nl/cce
Bonten LTC, Groenenberg JE, Meesenburg H, De Vries W, 
2011, Using advanced surface complexation models for 
modelling soil chemistry under forests: Solling forest, 
Germany. Environmental Pollution 159: 2831–2839
EEA, 2007. Halting the loss of biodiversity by 2010: 
proposal for a first set of indicators to monitor 
progress in Europe. European Environment Agency 
Technical Report 11/2007; www.eea.europe.eu 
Hettelingh J-P, Posch M, Slootweg J (eds), 2008. Critical 
load, dynamic modelling and impact assessment in 
Europe: CCE Status Report 2008, PBL Report 
500090003, Coordination Centre for Effects, Bilthoven, 
Netherlands, 230 pp; www.rivm.nl/cce 
Millennium Ecosystem Assessment, 2005. Ecosystems and 
Human Well-being: Biodiversity Synthesis. World 
Resources Institute, Washington, DC, vi+86 pp
Posch M, Reinds GJ, 2009. A very simple dynamic soil 
acidification model for scenario analyses and target 
load calculations. Environmental Modelling & Software 24: 
329–34
Figure 3.5  Critical loads of NOx and NH3 for critical N leaching 
as a function of denitrification.
CL
 N
H
3
CL NO
x
de
nt
ri
ca
tio
n
Figure 3.4  Average annual N leaching concentrations at forest in Solling, DE (left) and Hardenberg, NL (right).
[N
O
3] 
(m
m
ol
/l)
1.6
1.4
0.6
0.8
0.4
0.2
0
1
1.2
1970 1980 1990 2000 2010 1970 1980 1990 2000 2010
[N
O
3] 
(m
m
ol
/l)
0.6
0.5
0.2
0.3
0.1
0
0.4
52 | CCE Status Report 2011
Slootweg J, Posch M, Hettelingh J-P (eds), 2010. Progress 
in the modelling of critical thresholds and dynamic 
modelling, including impacts on vegetation in Europe: 
CCE Status Report 2010. RIVM Report 680359001/2011, 
Coordination Centre for Effects, Bilthoven, 
Netherlands, 182 pp; www.rivm.nl/cce 


Validation of VSD+ and critical loads for nutrient N

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