Criteria for the Evaluation of the Soil Functions 2020

Methodology

01.11.1 Regional Rarity of Soil Associations

Description

In order to preserve a diverse range of habitats, it is essential to safeguard the existence of as many distinct types of soil as possible.

The ‘rarity’ criterion describes the spatial distribution of soil associations in the State of Berlin. Soils vary in frequency across the Berlin area. The Soil Associations Map provides an overview of the distribution, indicating the rarity or frequency of soil associations.

The smaller the area a soil association occupies, the more endangered it is, i.e. the level of endangerment increases as the area proportion decreases.
Rarity is assessed for soil associations, rather than for individual soil types. Thus, rare soil types may occur within soil associations that are common or less rare, and vice versa.

Methodology

The proportions of each soil association were determined using area size data available in the Urban and Environmental Information System, excluding areas occupied by roads and bodies of water. Subsequently, these area sizes were aggregated for each soil association and compared to the total area under observation. As a result, percentages were generated to indicate the proportion of the total area occupied by each soil association.

The method described by Stasch, Stahr and Sydow (1991) was chosen to assess the rarity of the soils. This evaluation was based on the distribution of soil associations throughout Berlin.

The ‘rarity’ of soil was categorised into five levels, ranging from ‘very rare’ to ‘very common’ (Tab. 1). Combined associations (cf. Map 01.01) received the same rating as the soil association occupying the smallest area within the combined association (3020 –> 1100; 3030 –> 1340, 1350; 3040 –> 1360, 1370). The Concept Soil Association 2471 [49a] was classified as ‘common’, similar to Soil Association 2470 [49].

Tab. 1: Regional rarity of soil associations based on their area proportions (as of 2024)

Tab. 2: Evaluation of the regional rarity of soil associations

01.11.2 Distinctive Landscape Character of Soils

Description

Glacial deposits have sculpted Berlin’s landscape, bestowing upon it a distinctive character that sets it apart from other landscapes in Germany. Noteworthy in this landscape are geomorphological features, such as kettle holes, end and push moraines, dunes, and former glacial meltwater channels.

Kettle holes, remnants of ice blocks from the last ice age that later melted away, now appear as round depressions, sometimes filled with water. They are characterised by soils influenced by groundwater and bog associations. Loamy soils with sand wedges, where drift sand was blown into desiccation cracks during the late ice age, lie on undisturbed boulder marl plateaus, forming a regular network of polygons discernible in aerial views.

End and push moraines are accumulation moraines, shaped by a balance between ice replenishment and melting at its edges. In the landscape, they now appear as ridges and hills.

Late and post-glacial dunes, while still retaining their distinctive shapes, have ceased movement largely due to vegetation covering them.

Some glacial meltwater channels have been preserved, forming chains of lakes and wetlands. Soil development and present soil associations have been significantly influenced by morphology and parent materials. They reflect the unique characteristics and peculiarities of the natural space.

Methodology

The analysis exclusively focused on soil associations linked to geomorphological features shaped by the ice age, which were able to develop from glacial deposits without disturbance. Soils with a distinctive character remain largely untouched by human activity; hence, only near-natural soil associations were included (cf. legend for Map 01.01). Soils consisting of aggraded material or relocated soil material are not classified as having a distinctive character. Table 1 presents an overview of soil associations with a distinctive landscape character, attributed to their parent material, special morphology, and largely undisturbed soil development. These include primarily moraine plateaus with sand wedges, moraine hills, glacial meltwater channels with groundwater soils and bogs, river floodplains with fluvisol, gyttjas and peats, as well as dunes.

The soil associations listed in Table 1 have received a positive rating for their distinctive landscape character. The remaining soil associations do not exhibit such characteristics.

Tab. 1: Soil associations with a distinctive landscape character

01.11.3 Degree of Naturalness of Soils

Description

In the Berlin city area, soils have undergone significant alterations due to human activity. The degree of naturalness, reflects the extent of these changes compared to the original natural state of the soils. Changes here include soil translocation between natural horizons, removal of soil material, or overlaying with foreign materials. Substance inputs and lower groundwater levels are not considered here. Based on the Soil Associations Map and information on land use, an overview is provided on the degree of anthropogenic alteration, thereby indicating the extent to which Berlin’s soils and soil associations maintain natural characteristics.

This aspect is particularly important, as it is presumed that areas with minimal alterations have preserved natural soil characteristics and a diverse range of soil properties, whereas human influence has led to the homogenisation of soil types and their properties. Notably, the legend items on the Soil Associations Map already roughly differentiate between near-natural and anthric soil associations.

Methodology

To assess the degree of naturalness of soils, Blume and Sukopp (1976) introduced a ‘hemeroby index’, drawing on the botanical concept of hemeroby. This index classified various land-use types, based on their impact on ecosystems. Grenzius (1987) utilised this system to describe the anthropogenic influence on soils and soil associations in relation to the 1985 Map of Soil Associations of Berlin (West).

Grenzius (1987) refined the hemeroby index to account for different types of land use (cf. Tab.1). The underlying premise was that it is particularly the diversity of human land use that results in varying degrees and types of soil alteration, and the destruction of natural soils.

Table 1 illustrates the classification of areas based on their respective land use, referring to insights from multiple authors.

Tab. 1: Evaluation of the degree of naturalness based on the hemeroby index

Due to the absence of completely unaltered soils in Berlin, categories for unaltered or very slightly altered soils were excluded. Consequently, the categories for the evaluation of Berlin soils were revised, based on the classification criteria of Blume (1990), Grenzius (1985) and Stasch, Stahr, Sydow (1991).

To gauge the naturalness of the soils, data on soil associations, land use, area type and degree of impervious coverage were analysed. Initially, an automatic classification was carried out to group together specific combinations of soil associations, land uses and degrees of impervious soil coverage. These were then assigned ratings for their ‘degree of naturalness’ (categories 1 to 10 as per Grenzius, as shown in Tab. 1).

Selected land uses, such as green spaces and park facilities, fallow areas etc., required an individual assessment of naturalness. Soils in these areas may have undergone varying degrees of alteration. Typically, soils in the inner city have been significantly altered or newly formed by humans from aggraded material. Near-natural soils that fall into the same land-use category are often found on the outskirts, some of which with minimal alterations. The degree of naturalness of these areas was therefore determined on a case-by-case basis with the aid of topographic maps, protected area maps and expert reports.

Four levels, ranging from ‘very low’ to ‘high’, were devised to rate and aggregate the data to present it on the map (cf. Tab. 2, according to Lahmeyer 2000).

Tab. 2: Evaluation of the near-natural quality based on its classification

01.11.4 Soil Water Exchange Rate

Description

The soil water exchange rate reflects how quickly incoming precipitation water replaces the water within the active soil zone. A lower exchange rate indicates a longer dwell time for water in the soil. Longer dwell times, in turn, may have a compensatory effect on groundwater flow rates, and enable a more powerful decomposition of certain inputs.

Methodology

The soil water exchange rate was determined by calculating the ratio (quotient) between percolation (in mm per annum, long-term means from 1991 to 2020) and the usable field capacity of the effective root zone (in mm). Impervious soil coverage was disregarded here.

Percolation was computed using the ABIMO runoff formation model of the Federal Institute of Hydrology, which calculates the difference between precipitation and evaporation. This model incorporates area-specific data on precipitation, land use, vegetation structure, field capacities (based on soil textures), and depths to groundwater (measured from the surface to the water table) (Glugla et al. 1999) (cf. Map 02.13.4).

When calculating percolation for the evaluation of soil functions, the influence of impervious coverage was not considered, assuming complete permeability of surfaces. However, soils located near impervious surfaces experience increased exchange rates due to runoff precipitation.

The available water capacity in the effective root zone was derived from land use data and the Map of Soil Associations, incorporating soil profile models devised by Grenzius (1987) for individual soil associations.

Since determining the exchange rate of soil water is not common practice, there are no universally applicable evaluation standards. The values determined for Berlin were thus categorised to ensure each level covers a similar proportion of the municipal area.

Tab. 1: Soil water exchange rate levels

01.11.6 Nutrient Storage Capacity/ Pollutant Binding Capacity of Soils (KAKeff)

Description

The storage and binding capacity of soil refers to its ability to retain nutrients or pollutants by binding them to organic substances or clay minerals within the soil. This capacity is influenced by factors such as clay content, types of clay minerals, and the humus content. Organic materials, such as humus or peat, typically exhibit a considerably higher binding capacity compared to clay minerals. This capacity also depends on the pH value, however, decreasing as the pH value decreases. Soils with high clay content and a high proportion of organic matter, coupled with slightly acidic to neutral pH values, therefore have a high capacity for binding nutrients and pollutants.

Methodology

The nutrient storage capacity and pollutant binding capacity of soils are derived from the levels of the previously determined effective cation exchange capacity (cf. Map 01.06.9), which largely encapsulate the aforementioned characteristic values.

Binding capacity is evaluated based on three categories as shown in Table 1, building on the levels of effective cation exchange capacity (KAK_eff). Levels 1 and 2 are grouped as ‘low’, and levels 4 to 6 are combined as ‘high’.

Tab. 1: Evaluation of the nutrient storage capacity/ pollutant binding capacity, based on the levels of mean effective cation exchange capacity (KAKeff)

01.11.7 Nutrient Supply in the Topsoil (S Value)

Description

The nutrient supply of a site depends on both the nutrient stock and the nutrients accessible to plants. The nutrient stock comprises minerals from the parent material, which are released during soil weathering. The soil solution contains nutrients that are accessible to plants as base cations, such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺) and sodium (Na⁺). Base cations present in the soil solution may be derived from the total of exchangeable cations (referred to as S Value) (cf. Map 01.06.08). This only provides a total number, however, and does not indicate their relative proportions. For example, a site may therefore be rich in calcium and magnesium but deficient in potassium.

Phosphorus (P) and nitrogen (N), which may be estimated based on organic matter content, are not included here. Instead, only the proportion of base cations is taken into account.

Methodology

To gain an overview of the current nutrient supply of the soil associations, the levels of the aggregated exchangeable cations in the topsoil were consulted (cf. Map 01.06.8).

Table 1 presents a simplified evaluation of the nutrient supply based on base saturation: levels 1 to 6 indicate nutrient-poor conditions, level 7 suggests medium or fair conditions, and levels 8 to 10 nutrient-rich conditions.

Tab. 1: Nutrient supply levels based on the sum of exchangeable cations (S-Value)

01.11.8 Water Supply of Soils

Description

The water supply available to plants hinges on the soil’s capacity to retain precipitation in the root zone and to release it back to the roots. The volume of water soil can retain is influenced by factors, such as soil texture, humus content, bulk density, and the proportion of coarse soil. Soils connected to the groundwater may benefit significantly from capillary water rising from below, greatly increasing the water available to plants.

The assessment of soil water supply relies on the average available water capacity in the shallow-root zone.

Methodology

The water supply for sites and soil associations is determined by the average available water capacity (nFk) in the shallow-root zone (0 to 30 cm) (cf. Map 01.06.2). This measure is only required for evaluating the yield function for cultivated plants (cf. Map 01.12.2) and the habitat function for near-natural and rare plant communities (cf. Map 01.12.1). The water supply for deep-rooted plants (> 30 to 150 cm), such as trees, is not determined here. The evaluation is based on Table 1. If the depth to groundwater is < 0.8 m, the rating is increased by one level to account for capillary rise (unless already rated as ‘high’).

Tab. 1: nFK levels and evaluation of the water supply

01.11.9 Filtering Capacity of Soils (kf)

Description

The filtering capacity of soil refers to its ability to retain dissolved and suspended substances, preventing them from reaching the groundwater. This capacity is primarily influenced by soil texture and the resulting velocity at which precipitation moves through it under gravity. Soils that are highly water permeable, such as gravelly or sandy soils, have a low filtering capacity. This is the case as water can travel more than two metres per day in water-saturated soil, whereas in boulder marl soils, it travels only about 0.1 to 0.2 metres per day.

The evaluation of the filtering capacity does not consider whether and how much water actually moves toward the groundwater (depending on evaporation/ vegetation). The Exchange Frequency of Soil Water (cf. Map 01.11.4) addresses this matter to some extent.

Methodology

The filtering capacity of soils is determined based on saturated water permeability (kf value) (cf. Map 01.06.10), without considering the thickness of the soil horizons that substances need to traverse to reach the groundwater.
The evaluation is based on three categories, as shown in Table 1. Soils with high saturated water permeability and kf levels between 4 and 6 have a ‘low’ filtering capacity. Less permeable soils with kf levels of 1 to 2 receive a ‘high’ rating.

Tab. 1: Evaluation of the filtering capacity derived from the saturated water permeability (kf)

01.11.10 Heavy Metal Binding Strength of Soils

Description

Heavy metals are bound through adsorption onto humic substances, clay minerals, and sesquioxide in the soil. The solubility of these heavy metals depends on their total content and the pH value of the soil solution. Generally, higher acidity leads to increased solubility of heavy metal compounds. This is the case because metals tend to form stable oxides or precipitates of poorly soluble compounds, such as PbCaCO₃, at higher pH levels.

The relative heavy metal binding strength is used as a criterion for evaluating the Filtration and Buffering Function (cf. Map 01.12.3).

Heavy metals exhibit varied binding patterns. (DVWK, 1988). Cadmium, for example, is particularly soluble and is a common background pollutant in Berlin. It is relevant here due to its harmfulness. Following the method proposed by the Hamburg Ministry for Environment and Health (2003), the binding strength of easily soluble cadmium is used here as a benchmark for heavy metal binding strength.

Methodology

Blume and Brümmer (1987, 1991) developed a concept for assessing soil sensitivity to metal contamination, which is currently being implemented across Berlin. The assessment is based on the relative binding strength of individual metals depending on the pH value of the soil solution, assuming the conditions of a weakly sorptive, humus-poor sandy soil. The values are adjusted for higher humus, clay, and iron hydroxide contents. The calculation is carried out to a depth of 1 metre. For this purpose, characteristic values for the topsoil and subsoil are determined step by step based on pH value, humus content, and clay content. The sum of these yields the BSSM binding strength. This value is adjusted based on the proportion of coarse soil and horizon thickness. It can range from 0 to 5, representing a heavy metal binding strength from ‘none’ to ‘very high’.

Tab. 1: Evaluation of the relative heavy metal binding strength based on pH value, humus and clay content, the proportion of coarse soil, and horizon thickness

01.11.11 Buffering Capacity of Soils in the Organic Carbon Balance

Description

Soil plays a crucial role in the global organic carbon cycle, acting as both a buffer and at times a carbon sink. This function helps reduce CO₂ emissions, contributing to the mitigation of global warming. Soil’s ability to perform this role is closely tied to its humus and peat content, primarily derived from organic inputs by vegetation. Higher levels of humus and peat in soil may lower CO₂ emissions, yet their decomposition releases CO₂ back into the atmosphere. Under natural conditions, a balance between humus formation and decomposition is typically established over time. Increased humus and peat levels are commonly found in developing, relatively young soils and in intact bogs. Destruction of soil structures, intensive agricultural use, and, in the case of bogs, drainage cause the organic substance to decompose and CO₂ and methane (CH₄) to be released. Gentle agricultural and horticultural practices and the spontaneous development of urban (raw) soils lead to an accumulation of organic matter, creating a CO₂ sink.

Regarding the organic carbon balance, two soil types with high buffering capacities may be identified:

• raw soils, which, if allowed to develop undisturbed, can still bind large amounts of organic carbon, and
• soils with currently high humus or peat content, the disruption or destruction of which leads to the release of CO₂.

The binding of organic carbon in young soils is a slow process, while the release of CO₂ after the soil structure has been destroyed occurs relatively quickly. Therefore, this release is considered the primary factor and is the sole criterion assessed here.

The total amount of peat and humus stored in Berlin soils corresponds to approx. 25.8 million tonnes of CO₂. Berlin’s CO₂ emissions amount to approx. 14.6 million tonnes per year (as of 2020, Statistical Office for Berlin-Brandenburg, 2022).

Methodology

The evaluation of the buffering capacity in relation to the organic carbon balance draws on the organic carbon stock levels (cf. Map 01.06.6). Ratings from 1 to 3 indicate a ‘low’ buffering capacity, while ratings from 5 to 6 represent a ‘high’ buffering capacity.

Tab. 1: Evaluation of the buffering capacity in the organic carbon balance based on organic carbon stock levels