Global climate regulation (Carbon storage in the soil)

Description

The ecosystem service of soil carbon storage is the result of the storage of non-mineralized carbon from dead plant material to the soil, where it is stored in the long term. The more atmospheric CO₂ is captured in the soil in this way, the less it can contribute to global warming. The benefits of this service are, on the one hand, the preservation of existing carbon stocks and, on the other hand, the storage of additional carbon in the soil.

Soils under natural ecosystems generally exhibit higher carbon stocks than those under intensive land use (due to regular cracking of the soil). Carbon stocks are therefore greater in forest soils and permanent grassland than in temporary grassland or cropland soils. Wetlands and historic peat soils in particular contain large amounts of carbon.

Required information:

• Land use
• Measured carbon stock current situation or if not available
• % clay and sand in the top 100 cm (see table)
• Average highest groundwater level (GHG) and average lowest groundwater level (GLG) current situation and estimate of the future situation
• The presence of certain soil textures from the WRB soil map
• The presence of peat can be deduced from the soil map
• Slope derived from the Digital Height Model Flanders
• For agricultural land, fertilizer input in C equivalent based on agricultural region from Wesemael et al. (2010)
• For forests, the presence of the forest on the Ferraris map, the slope of the terrain and the tree species.

GHG is the average of this year's 25% highest groundwater levels in the area. This average is smaller (read less deep) than the average lowest groundwater level. GLG is the average of the 25% lowest groundwater levels in the area this year. This average is larger (read deeper) than the average highest groundwater level.

The groundwater levels can be obtained from measurements or via simple groundwater models. Interpolated maps based on the drainage classes are also available within the web tool of the Nature Value Explorer. Soil texture, drainage class and WRB class can be found in the information sheet on the soil map of Flanders https://www.dov.vlaanderen.be/page/bodemkaarten.

The required maps can also be found in the Nature Value Explorer web tool.

Qualitative valuation

The amount of organic carbon in the soil depends on land use, soil texture and groundwater level (Meersmans, 2008). Changes in land use or groundwater level can lead to an increase in carbon storage in the soil or to the breakdown and emission of CO₂.

Land use

Almost all forms of soil cultivation have a negative impact on carbon stocks. The more biomass that remains on site in managed systems (crop residues, clippings, crown wood), the more carbon can be stored in the soil. Land disturbances such as plowing lead to reduced physical protection of the organic material, making it easier to mineralize and reducing carbon storage. As a result, soils under natural ecosystems will exhibit larger stocks than intensively farmed soils.

Soil texture and groundwater level

Independent of land use, the moisture status and clay content of the soil determine the capacity for carbon storage. The wetter the soil and the higher the clay content, the more carbon can be captured. Management interventions such as drainage reduce storage, while rewetting processes increase the stock of soil carbon.

In addition, time also plays an important role in potential carbon storage, especially under permanently wet soils. During the development of ecosystems, the content of organic matter increases. Soils that have been under a natural (wetland) forest for years have accumulated large amounts of carbon over time. As long as the hydrological conditions and land use do not change, these stocks can evolve to a maximum and remain more or less stable (equilibrium situation). The carbon stock may be at its maximum, but the storage potential itself has decreased. For example, wetlands reach their equilibrium state after about 60 years (depending on wetland type) and carbon is only captured in (anaerobic) raised bog situations. On the other hand, changes in land use and hydrology can cause the carbon stock to decline again.

A qualitative assessment can be given based on these characteristics.

In the web tool, based on the above characteristics and the ECOPLAN map for potential carbon stoackage in Flanders, a score is developed from 1 minor carbon stock to 10 very important potential carbon stock in the soil.

Quantitative valuation

The calculation is done on the basis of 4 different regression equations that were drawn up within the ECOPLAN project (Ottoy, Beckers et al. 2015; Ottoy, Elsen et al. 2016). Together, these equations allow the calculation to be done for most land uses. carbon storage in the soil can be calculated up to a depth of 1 meter. The regression equations are created on the basis of the most reliable databases available in Flanders. The formula for cropland and meadow was created on the basis of soil fertility data from the Belgian Soil Survey and the Aardewerk-Vlaanderen-2010 database. The formula for forest is based on the INBO ForSite database. The comparison for nature types is based on a database that was created by the KULeuven during a study into LIHD systems (Van Meerbeek, Van Beek et al. 2014).

The study focused only on the mineral soil. This ignores the importance of forest land for the underlying carbon stock. For Flanders, Lettens et al. 2005 calculated a carbon stock in the forest soil of 1 kg/m² under deciduous forest, 2 kg/m² under mixed forest and 3.5 kg/m² under coniferous forest. We add this to the calculated carbon stock under forest using the regression equation.

The equations calculate the potential maximum carbon stock. If land use or hydrology changes, the potential maximum carbon stock will change. We assume that this maximum (new equilibrium situation) will be reached after 100 years. The annual increase/decrease in the carbon stock is approximately proportional (2.5%) to the remaining difference between the equilibrium state to be achieved and the current carbon stock. The annual net decrease/increase of carbon therefore slows down as one approaches the new equilibrium state.

The most accurate method to quantify the current carbon stock is to take a soil sample and see how much carbon is present. The analysis of the organic material content costs approximately €15 per sample and the results are usually available within a week. In ecosystems with microrelief and heterogeneous vegetation, this carbon stock can vary greatly spatially. At least 15 samples per hectare are needed to obtain a representative picture. If there have been no major changes in the water balance and vegetation in the last 30 years, historical soil samples (1960-present) can be used additionally, which are accessible via the Pottery Database (Van Orshoven J. et al. 1993).

If there are no resources or time to take soil samples, the maximum potential carbon stock of the current situation can be estimated based on the formulas. The annual increase/decrease in the carbon stock is then approximately 2.5% compared to the remaining difference between the equilibrium state to be achieved and the calculated maximum potential carbon stock of the current situation.

If an area undergoes major changes due to an infrastructure project (deforestation, drainage) or if excavation works take place, the carbon stock in the soil may be lost. The carbon stock can be released proportionately from the moment the soil is excavated. In the beginning it goes much faster than after decades. If the soil is covered by, for example, sprayed soil or a pavement, without the need for excavation work, then there is probably no loss of carbon stock. However, little is known about this. Some studies do warn of an increased risk of CO₂ emissions, e.g. Pataki et al. 2006. For the time being we are not taking this into account in the calculations.

Monetary valuation

To value carbon storage monetarily, we can use key figures from De Nocker et al. 2010. These numbers are based on the method of avoided reduction costs: if more carbon is stored in nature reserves, emission reduction costs can be avoided in other places in order to achieve the given environmental objectives. These key figures are based on the costs of emission reduction measures necessary to ensure that the global average temperature only increases by a maximum of 2°C compared to pre-industrial levels (1780). The figures are derived from a meta-analysis of results from various climate model studies (Kuik et al. 2009).

A point of attention is that new and more expensive measures must be continuously taken over the years to remain on an emission path that is consistent with the 2°C target. The marginal costs increase over time and range from 20 euros/ton CO₂-eq. in 2010 to 220 euro/ton CO₂-eq. in 2050 (see table).

Assumptions

• The functions do not take stock stability into account. It is much less stable under agricultural land than under, for example, forest land.
• We assume that the new equilibrium situation will be reached after 100 years.
• Every year there is a proportional change in the carbon stock of 2.5% compared to the remaining difference between the equilibrium state to be achieved and the current carbon stock. This figure is based on expert opinion after reviewing literature, including Wei X, M. Shao et al. (2014).
• For the current situation we use the maximum carbon stock of the current land use calculated by the regression model. We then compare the two equilibrium states of current and future land use. The difference between these equilibrium states is what we include in the valuation.
• We assume that the stored carbon is released during excavation due to infrastructure works, but we cannot determine any annual evolution here.
• The rating figures are derived from a thorough literature review (see table). For intermediate years, the key figures are extrapolated linearly. After 2050, the value in 2050 applies. We calculate with a minimum value of 100 and a maximum value of €366.

Numbers to use

Table: score for qualitative valuation

score	lower limit ton C/ha
1	0
2	108
3	216
4	323
5	431
6	539
7	647
8	755
9	862
10	970

source: based on ECOPLAN (2016) map with potential carbon stock

Quantitative valuation of the potential total carbon stock in soil

Ecosystem average estimate (ton C/ha)

Cropland (ton/ha): (4,4118 + 0.2293 x %clay + 5.1805 x fertilization - 0.0047 x GLG + 3.3852 x Podzol + 6.1161 x AnthroSol + 0,0001 x clay x GHG - 0.2460 x clay x fertilization + 0.2027 x peat) x 10

Meadow (ton/ha): (8.6475 + 0.0290 x %Sand - 0.0041 x GLG + 2.2362 x Fertilization + 0.9863 x Podzol + 4.1541 x Anthrosol + 7.3375 x Peat - 0.00004 x GLG x %Sand) x 10

Forests (ton/ha) : (13.6456 + 0.2451 x Clay - 0.0021 x GHG + 13.8138 x Alnus - 2.1068 x Pinus - 1.4378 x Fagus + 1.5349 x Populus + 4.7563 x Anthrosol - 3.7087 x Arenosol + 21.5834 x Gleysol + 55.7464 x Histosol + 3.9704 x OtherWRB - 2.6497 x Podzol - 1.5441 x Retisol + 0.6699 x Ferraris) x 10

Heathland and shrubs, grasslands and tall herbs, sparsely vegetated land, wet nature (ton/ha): (13.8572 + 0.2006 x %Clay - 0.0126 x GLG + 13.4339 x Peat + 4.2009 x Podzol - 3.5461 x Heath + 1.9306 x tall herbs and pioneer vegetation + 2.1491 x Reedland) x 10

source: Ottoy, Beckers et al. 2015; Ottoy, Elsen et al. 2016. To arrive at a tonne/ha we multiply the functions by 10, GLG/GHG in m

Table: sand, loam and clay% by soil texture

soil texture	sand %	loam %	clay %
Z (X)	90	8	2
S	75	20	5
P	60	35	5
L	30	60	10
A (G)	5	85	10
E	35	35	30
You	15	35	50
V		35	30
Other	45	41	14

Source: Meersmans et al. 2008 and adjustment VITO

Note: no value could be calculated for texture classes of the soil map that do not appear in this list, with the exception of texture class G (stony loam) which was added to A and texture class X (dunes) which was added to Z (source: NARA reports on ecosystem services ). For the other texture classes (often combination), an average of the known classes was taken.

Table: amount of carbon in the forest layer additional forest parameters to be added to the forest calculation

Forest type	tons/ha
Deciduous forest	10 tons/ha
Mixed forest	20 tons/ha
coniferous forest	35 tons/ha

The values obtained from these formulas are the potential total stock in the area. To convert this to an annual stockage, we assume that the difference between the current and potential maximum stockage decreases by 2.5% annually.

Table: monetary valuation: key figures for external costs of greenhouse gases for C storage in the period 2010-2050.

Ref year (1)	euro/ton CO2-eq.	euro/ton C (2)
2010	20	73
2020	60	220
2030	100	366
2040	160	586
2050	220	805

(1) Ref year = year of emission or storage of greenhouse gas (2) 1 ton C = 3.66 ton C02 Source: based on De Nocker et al. 2010

Translation to an indicator

To report the service, we use 3 indicators for which we add up the ecosystem services carbon storage in the soil and carbon storage in biomass:

• The costs avoided for measures to mitigate carbon emissions. This is equal to the sum of the average monetary valuation of the carbon storage service in the soil and the carbon storage service in biomass.
• The annual carbon emissions of an average Flemish person: 3.55 tons/year
• The carbon emissions of an average car km: 48 g/km (COPERT)

An example

For the example, we refer to the Dutch version of the manual.