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The aim of this study was to determine target loads for acidification for representative forest ecosystems in Flanders (Belgium) using dynamic modelling. Target loads were calculated for 83 forest stands for which sufficient measurements were available. This dataset is considered to be representative for the Flemish forest area. It is concluded that, despite the inherent uncertainty in modelling soil acidification at a regional level, important N and S deposition reductions are needed to allow recovery of the Flemish forest soils.

The input of nitrogen (N) and sulphur (S) to terrestrial and aquatic ecosystems has strongly increased over the past century due to human activities. This particularly holds true for forest ecosystems in Europe (

For forest and nature policy it is important to know the highest deposition level below which no harmful effects on forest soils occur. Therefore, the concept of critical loads has been adopted,

The N and S deposition in Flanders (northern Belgium) is amongst the highest in Europe (

Critical loads and target loads were calculated for 83 forest stands of the Flemish forest condition (Level I) and intensive monitoring (Level II) networks according to the harmonized methodology of the Coordination Center of Effects (_{4} ^{+} leaching; and (iv) lumping the exchange of all base cations in one term. Furthermore, it uses a new approach of modelling N immobilisation. More information on VSD is given in

Input data were as much as possible derived from site-specific measurements or regional data, following guidelines of ^{+}, Ca^{2+}, and Mg^{2+}), estimated from the measured soil type, (ii) cation exchange, derived from the measured cation exchange capacity and the Gapon exchange model, with exchange coefficients calibrated by the VSD model, (iii) denitrification, estimated from soil drainage status and assuming total nitrification, and (iv) nitrogen immobilisation, modelled based on the measured carbon to nitrogen (C:N) ratio of the litter and upper 0-20 cm soil layers. Time series of atmospheric N and S deposition were provided by EMEP (Convention on Long-range Transboundary Air Pollution, CLRTAP). Base cation and chloride deposition were assumed to be constant over time and were derived from throughfall data corrected for ion exchange within the canopy. The growth uptake of nitrogen and base cations was calculated using tree species-specific annual wood production and nutrient contents. The annual percolation flux below the rooting zone was quantified by combining regionally interpolated rainfall with species-specific rainfall interception and actual evapotranspiration values. Model sensitivity was assessed for all stands by varying the input data and model parameters within reasonable ranges.

Critical loads and target loads of acidifying deposition indicate a sum of N and S deposition that allow the soil and soil solution to reach a chemical criterion in the long term or in the target year, respectively. If only S contributes to soil acidification, the acceptable S deposition is called the maximum critical or target load of S. If the S deposition is zero, the acceptable N deposition is the maximum critical or target load of acidifying N (

Target loads depend on the target year by which the preset chemical criterion (Al:Bc = 1) in the soil solution should be achieved at last. Target loads are higher for more future target years (

The median (n = 83) target load of S for 2030, 2050, and 2100 amounted to 58, 65, and 86% of the median critical load of S (being 1829 eq ha^{-1} yr^{-1}), respectively (

The critical and target loads were lower for deciduous than for coniferous stands (^{-1} yr^{-1} for deciduous stands (n = 25) and 1638 eq ha^{-1} yr^{-1} for coniferous stands (n = 27). The acceptable acidifying deposition was lowest for forests on sandy soils because of the lower mineral weathering rates compared to loamy or clayey soils (

According to the sensitivity analysis (

Exceedances of target loads were calculated based on projected EMEP depositions for 2010. For target year 2050, the projected depositions were too high for 88% of the study sites. The median exceedances of acidifying N and S deposition for the target years 2030, 2050, and 2100 were 1953, 1443, and 1202 eq ha^{-1} yr^{-1}.

In the present study, target loads were calculated for 83 forest stands for which sufficient measurements were available. Thanks to the systematic site selection of the forest condition network, this dataset is considered to be representative for the Flemish forest area in general, although coniferous stands were slightly under-represented. This was confirmed by comparing the distribution of critical loads for acidification for the 83 sites with those for a larger database of 1438 plots (

This study was funded by the Flemish Environment Agency (VMM - MIRA) and implemented at the Research Institute for Nature and Forest (INBO). The first author is currently granted as a post-doctoral fellowship at Ghent University by the Research Foundation Flanders (FWO-Vlaanderen).

Relationship between target loads for 2030, 2050, and 2100 and critical loads for (A) sulphur, S, and (B) acidifying nitrogen, N, for 83 Flemish forest sites. Note: the diagonal indicates a 1:1 ratio.

Boxplots (n = 83) of target loads for (A) sulphur, S, and (B) acidifying nitrogen, N, for target year 2050 for the main combinations of forest type and soil type in Flanders. Note: The closed rectangle of the boxplots indicates the interquartile range (25-75%) of the distribution, the dot being the median. The small x and large X left (right) of the rectangle show the 5^{th} (95^{th}) and 10^{th} (90^{th}) percentiles, respectively. The [ ] range comprises all values closer to the median than 1.5 times the interquartile range. Values outside the [ ] range are shown individually as open circles.

5^{th}, 50^{th} (median), and 95^{th} percentile values of the target load for three target years and critical load (eq ha^{-1} yr^{-1}) for sulphur (S) and acidifying nitrogen (N) for 83 Flemish forest sites.

Loads | S (eq ha^{-1} yr^{-1}) |
N (eq ha^{-1} yr^{-1}) |
||||
---|---|---|---|---|---|---|

5% | 50% | 95% | 5% | 50% | 95% | |

Target load 2030 | 0 | 811 | 2464 | 0 | 1938 | 7947 |

Target load 2050 | 41 | 1324 | 2873 | 747 | 2564 | 8011 |

Target load 2100 | 478 | 1598 | 2885 | 1691 | 2958 | 8064 |

Critical load | 919 | 1829 | 2885 | 2162 | 3115 | 8374 |

Median (n = 83) target load (TL, target year 2050) and critical load (CL) for sulphur (S) and acidifying nitrogen (N) using modified input parameters and variables. All values are expressed as the difference (%) compared to the reference approach values (eq ha^{-1} yr^{-1}). (-): Parameter not used for calculating critical loads.

Input parameter | Modification | Median TL 2050 | Median CL | ||
---|---|---|---|---|---|

S | N | S | N | ||

Reference approach | 1324 | 2564 | 1829 | 3115 | |

Mineral weathering rate (BC_{w}) |
BC_{w} + 20% |
14.5 | 9.4 | 3.2 | 3.3 |

BC_{w} - 20% |
-21.7 | -10.5 | -12.9 | -4.9 | |

Denitrification factor (f_{de}) |
f_{de} + 20% |
2.7 | 9.4 | 0.0 | 14.9 |

f_{de} - 20% |
-3.1 | -7.6 | 0.0 | -9.3 | |

Minimal denitrification | f_{de} = 0.1 |
-13.2 | -15.0 | 0.0 | -19.6 |

Acceptable N immobilisation (N_{i,acc}) |
N_{i,acc} + 20% |
0.4 | 2.4 | 0.0 | 1.9 |

N_{i,acc} - 20% |
0.0 | -2.0 | 0.0 | -1.9 | |

Minimal N immobilisation | N_{i,acc} = 1 kg ha^{-1} yr^{-1} |
-0.9 | -8.2 | 0.0 | -5.2 |

Deposition of base cations (Bc_{dep}) |
Bc_{dep} + 20% |
48.5 | 38.5 | 23.6 | 25.7 |

Bc_{dep} - 20% |
-56.2 | -33.4 | -24.6 | -24.3 | |

Growth uptake of nitrogen (N_{u}) and base cations (Bc_{u}) |
N_{u} and Bc_{u} + 20% |
-22.4 | -10.4 | -14.1 | -7.2 |

N_{u} and Bc_{u} - 20% |
17.1 | 12.0 | 5.3 | 9.2 | |

Bc_{u} + 20% |
-23.2 | -14.8 | -14.1 | -9.2 | |

Bc_{u} - 20% |
20.7 | 17.6 | 5.3 | 11.3 | |

N_{u} + 20% |
1.3 | 4.8 | 0.0 | 3.3 | |

N_{u} - 20% |
-1.3 | -7.3 | 0.0 | -3.3 | |

Drainage flux (Q) | Q + 20% | -1.8 | -0.3 | -0.3 | -0.5 |

Q - 20% | 1.3 | -0.2 | 0.2 | 0.4 | |

Parameters for Al^{3+}-H^{+} concentration relationship (logK_{Alox}, expAl) |
logK_{Alox} + 0.5 |
-9.0 | -5.1 | -4.6 | -4.2 |

logK_{Alox} - 0.5 |
10.6 | 5.4 | 5.9 | 5.8 | |

Dutch empirical values | ( |
-11.9 | 10.4 | 13.0 | 23.0 |

Dissociation of organic anionsConcentration (m·DOC) | m·DOC + 20% | -4.9 | -1.1 | -1.7 | -1.7 |

mDOC - 20% | 4.5 | 1.1 | 1.6 | 1.7 | |

mDOC = 0 | 14.1 | 11.7 | 7.4 | 8.5 | |

Fixed dissociation constant (pK_{1}) Factor partial_{CO2} pressure (pCO_{2}) |
pK_{1} = 4.5 |
7.7 | 2.7 | 3.7 | 3.8 |

pCO_{2} = 25 |
-0.4 | -0.1 | -0.2 | -0.2 | |

pCO_{2} = 5 |
0.9 | 0.1 | 0.2 | 0.2 | |

Cation exchange capacity (CEC) | CEC + 20% | -8.3 | -4.3 | - | - |

CEC - 20% | 5.9 | 5.7 | - | - | |

Base saturation (BS) | BS + 20% | -6.9 | -2.1 | - | - |

BS - 20% | 3.7 | 6.0 | - | - | |

Cation exchange model | Gaines-Thomas | -31.8 | -25.7 | - | - |

N deposition (N_{dep}) |
N_{dep} + 20% |
-2.7 | -3.0 | - | - |

N_{dep} - 20% |
2.9 | 5.2 | - | - | |

S deposition (S_{dep}) |
S_{dep} + 20% |
-21.1 | -7.5 | - | - |

S_{dep} - 20% |
11.9 | 7.8 | - | - | |

N and S deposition | N_{dep} and S_{dep} + 20% |
-23.8 | -8.0 | - | - |

N_{dep} and S_{dep} - 20% |
15.9 | 11.5 | - | - |