The diagnostic continua of the soils of Europe 313Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323.DOI: 10.15201/hungeobull.71.4.1 Hungarian Geographical Bulletin 71 2022 (4) 313–323. Introduction Sustainable land management is based on appropriate soil information (European Commission 2006, p. 231) and on the under- standing of the functional capacity of differ- ent soils (Bouma, J. et al. 2012). The concept of soil functions builds on the soil-based ecosys- tems services (Haygarth, P.M. and Ritz, K. 2009; Bouma, J. et al. 2012). Schulte, R.P. et al. (2014) and van Leeuwen, J.P. et al. (2017) re- lated five major soil functions to agricultural land use. These include (1) primary produc- tivity, (2) water purification and regulation, (3) climate regulation and carbon sequestra- tion, (4) soil biodiversity and habitat provi- sioning, and (5) recycling of nutrients. The assessment or estimation of the capacity of a soil to perform these functions depends on complex interaction of soil properties with environment (climate) and management. In the World Reference Base (WRB) (FAO 1998; IUSS Working Group WRB 2006, 2015) diagnostic horizons, properties and materi- als are used to define the highest taxonomic level – the Reference Soil Group (RSG) – while qualifiers provide supplementary information and serve to further define the soil type. To be 1 Department of Soil Science, Institute of Environmental Sciences, Hungarian University of Agriculture and Life Sciences. Páter Károly u. 1. H-2100 Gödöllő, Hungary. Corresponding author’s e-mail: csorba.adam@uni-mate.hu 2 European Commission, Joint Research Centre (JRC), Ispra, Lombardy, Italy. 3 Institute of Geography and Geoinformatics, University of Miskolc, Egyetemváros, H-3515 Miskolc, Hungary. The diagnostic continua of the soils of Europe Ádám CSORBA1, Arwyn JONES 2, Tamás SZEGI1, Endre DOBOS1,3 and Erika MICHÉLI 1 Abstract Diagnostic horizons, properties and materials are commonly applied building units of national and inter- national soil classification systems. The presence, depth or absence of diagnostic information supports the process of objective soil classification, such as the World Reference Base (WRB). While the diagnostic units and associated descriptive qualifiers convey information that reflect pedogenesis, they also indicate important, and often complex properties that are related to soil fertility and other soil functions. The spatial extent or the continuum of diagnostic information is often different from the spatial extent of the mapping units in general soil maps (mostly reflecting soil types). This paper presents the spatial distribution of selected diagnostic units and qualifiers for the European Union and describes their significance for key soil functions. The derivation of selected diagnostics was performed based on the information provided in the European Soil Database and by taking into consideration the definitions, rules and allocation procedure of soils to the appropriate Reference Soil Group (RSG) defined by the WRB key. The definition of the presence/absence of the diagnostic units were performed by extracting information related to the first level of the WRB classification and to the quali- fiers provided by the ESDB on the Soil Taxonomic Units (STU) level. The areal percentage of the STUs (thus, the derived diagnostics) within Soil Mapping Units (SMUs) was calculated and was visualized on separate maps. The study demonstrated the importance of the spatial information that the diagnostic elements convey, especially related to soil functions. Keywords: diagnostic units, World Reference Base, European Soil Database, soil functions Received October 2022, accepted December 2022. Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323.314 considered “diagnostic”, these elements must reach a certain degree of expression, which is determined by appearance, measurability, im- portance, relevance and quantitative criteria. For example, a surface, organic matter rich ho- rizon requires a minimum thickness (20 cm), a minimum level of base saturation (50%) and organic carbon content (0.6%), together with structure and colour criteria to be considered as a mollic horizon. The diagnostic units and the qualifiers convey information by themselves on com- plex soil properties, that are the result of soil forming processes, which in turn are related to a range of functions, such as the capacity of a soil to cycle nutrients (Michéli, E. et al. 2019). Most soils, belonging to a particular soil type, carry properties that might be char- acteristic for different other soil types, and the principles and rules of the applied soil classification system support the decision on the eventual soil type (RSGs in the WRB) (Dobos, E. et al. 2019). However, the spatial extent or the continua of the diagnostic units may be, and often is, different from the soil mapping units in gen- eral soil maps. Therefore, in studies where the functional capacities of soils are a considera- tion, it is important to define the spatial extent of the constituent diagnostic units. For exam- ple, several RSGs might have hydromorphic properties related to groundwater caused by excess water (gleyic properties), but not all of those soils are classified as Gleysols because of the principles, priorities and construction of the classification key. However, as gleyic properties influence several soil functions, it could be important to define their overall spatial extent in all soils which are affected by hydromorphic properties, regardless of the taxonomic class (soil type). The most complete and uniform soil map and database for the European Union is the 1:1 Million Soil Geographical Database of Eurasia (EC ESBN 2004; Panagos, P. 2006), hosted by the European Soil Data Center (ESDAC) at the JRC (https://esdac. jrc.ec.europa.eu/). The database provides the percentage of the dominant and all as- sociated soil types per map unit while the visualized units reflect only the dominant soil types (RSGs with one qualifier) of the soil mapping units. The objective of this pa- per was to derive the spatial extent (in area percentage of the map units and in area per- centage of the territory of the EU) of selected diagnostic units which may influence the ca- pacity of the five soil functions in conjunction with environment and management. Materials The European Soil Database (ESDB) The derivation of the selected diagnostics was performed based on the Soil Geographi- cal Database of Eurasia (SGDBE), which is a part of the European Soil Database (ESDB) v2.0 (EC ESBN 2004; Panagos, P. 2006), and covers the interest area of the study, the terri- tory of the European Union and Switzerland. This product is the result of a collaborative project involving all European Union mem- ber states and neighbouring countries. The ESDB is a simplified representation of diver- sity and spatial variability of soil coverage. The database consists of Soil Typological Units (STU), which represent soil names and are described by attributes specifying the na- ture and properties of the soils. As the origi- nal geographical representation (1:1 Million scale) did not allow the spatial delineation of STUs, they were grouped into Soil Mapping Units (SMU) to form soil associations. The associations refer to areal percentage of STUs and represented by one or more polygons in the geometrical dataset. The visualized maps generally present the polygons of the SMUs by representing the dominant STU (Figure 1). The digital database includes further ana- lytical and environmental information for the semantic units. Each dominant STU is also sup- plemented with a representative soil profile with basic horizon data. The SGDBE consists of a geometrical and a semantic dataset linking attribute values to the polygons. Besides the wide range of attributes defined for the STUs, https://esdac.jrc.ec.europa.eu/ https://esdac.jrc.ec.europa.eu/ 315Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323. the database defines the WRB (1998) classifica- tion as well. The “WRB-FULL” attribute refers to the group code of the STU from the WRB. The attribute consists of the Soil reference group code (WRB-LEV1) and the first qualifier adjective code (WRB-ADJ1) of the STU. Figure 1. shows the distribution of the major soil types for the European Union. The colours on the map refer to the RSG which is dominant in the particular SMU according to the ESDB. The World Reference Base for Soil Resources The World Reference Base for Soil Resources (WRB) is an international soil classification and correlation system endorsed by the Interna- tional Union Soil Sciences (IUSS) (Deckers, J. et al. 2005). While most countries in the EU developed their own soil classification and mapping systems, the WRB enables the harmo- nization of soil databases and soil maps from Fig. 1. The soils of the European Union. The distribution of the dominant Reference Soil Groups (WRB 1998) in the European Union and Switzerland according to the European Soil Database. Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323.316 different sources (Jones, A. et al. 2005). The 1st and 2nd editions of the WRB (FAO 1998; IUSS Working Group WRB 2006) served as a tool for the correlation of national soil classification units to the ESDB. The WRB consists of two cat- egorical levels. The first level, the Reference Soil Groups (RSG) is defined by the classification key. The RSGs share an assemblage of defined diagnostic features and serve as reference for correlation of national classification units. The second level, the qualifiers, provides additional specific information. The diagnostic informa- tion of the RSGs and qualifiers were designed to provide practical considerations and expres- sions on ecological functions and management strategies. Table 1. summarizes the selected WRB units that were used in this study. All diagnostic features are important for some aspects of soil functions. The selection was based on the expert judgement of the au- thors to select the ones that are mostly influ- encing agricultural land management. The in- formation available at the time of the database construction was also considered and was of- ten a limiting factor (eg., mollic horizon was not diagnostic for Umbrisols, or Stagnosols was not among the defined RSGs). Although the cambic horizon is the most common diag- nostic horizon (with 26.14% area in the EU) it was not selected. The cambic horizon and the Cambisols are distinguished by moder- ate subsurface development without distinct features. For the Cambisols, the most informa- tive indications are the associated qualifiers providing more specifics on function related properties, however, the structure of the da- tabase allowed only one qualifier. The spodic horizons are also common, occu- pying 11.61 percent of the studied territory and carry important information on the soil forming environment. They generally develop in sandy material and represent subsurface ac- cumulation of organic matter and iron oxides under leached, acidic conditions, determining limited choices of land use. Since the area per- centage of the RSG Podzols for which the spo- dic horizon is diagnostic are identical no calcu- lations were needed and Figure 1. provides the information on the spatial distribution. The argic horizon is a clay enriched subsur- face horizon with higher clay content than the overlying layer. The texture differentia- tion may be caused by illuvial accumulation of the clay, by destruction or selective erosion of the clay in the surface horizon, by biologi- cal activities or combination of causes. The argic horizon is diagnostic for the Acrisols, Albeluvisols, Alisols, Luvisols and Lixisols but may occur in several other RSGs. The calcic horizon is an accumulation of sec- ondary carbonates, mostly in the subsurface. In humid areas it is related to the leaching of the carbonates to deeper depth, while in dry areas the calcic horizon occurs closer to the surface and is often associated with carbon- ate rich parent material. The calcic horizon is diagnostic for the Calcisols, Chernozems and Kastanozems RSGs. Gleyic properties are related to reduc- ing conditions caused by saturation with groundwater at a shallow depth for long pe- riods (FAO 2001a, b.) The gleyic properties are diagnostic for the Gleysols. The histic horizon is related to the accumula- tion of organic material, consisting of partially decomposed plant biomass under wet condi- tions (Jobbágy, E.G. and Jackson, R.B. 2000; FAO 2001a, b). The slow decomposition is of- ten associated with low temperature as well. The histic horizon is diagnostic for Histosols. Table 1. Selected WRB units (“Diagnostics”) and the RSGs (WRB-LEV1 and qualifiers (WRB-ADJ1) in the ESDB from which they were derived Diagnostics WRB-LEV1 (RSG) WRB-ADJ1 (Qualifier) Argic horizon Calcic horizon Gleyic properties Histic horizon Mollic horizon Luvisols, Acrisols, Albeluvisols Calcisols, Kastenozems, Chernozems Gleysols Histosols Chernozems, Kastanozems, Phaeozems Luvic (presence of argic horizon) Calcic (indicating calcic horizon) Gleyic Histic Mollic 317Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323. The mollic horizon is the result of the accumu- lation of well humified, stable organic carbon in the topsoil, mostly under ancient grassland vegetation (Lal, R. 2000). The mollic horizon is diagnostic for the Chernozems, Kastanozems and Phaeozems RSGs but occurs in other RSGs. Methods The derivation of the diagnostics and the qualifiers was performed by taking into consideration the definitions, rules and al- location procedure of soils to the appropriate RSG defined by the WRB key. By having a WRB RSG code (WRB-LVL1), and the first adjacent codes (WRB-ADJ1 (qualifier) for each STU in the ESDB, the definition of the diagnostic horizons, materials and proper- ties is possible. While the areal percentage of the STUs within the SMUs is provided, the same attributes (in percent) were calcu- lated for each of the diagnostics. An example of this approach is presented in Figure 2. In this example, SMU1 consist of three STUs. STU1 (Stagnic Luvisol) represents 50 percent, STU2 (Cutanic Luvisol) represents 30 per- cent and STU3 (Luvic Phaeozem) represents 20 percent of the SMUs’ area (making a total of 100%). Luvisols, by definition, have an argic horizon, while the Stagnic qualifier refers to the presence of Stagnic properties, hence STU1 has an argic horizon and stagnic properties. In STU2, the argic horizon is again present while the Cutanic qualifier indicates the presence of clay skins in the argic horizon (no additional information). Similarly, for STU3, Phaeozems by definition have a mollic horizon, while the Luvic quali- fier indicates the presence of the argic horizon, hence STU3 has both mollic and argic horizons. On the visualized map, the entire SMU would be represented by the dominant Stagnic Luvisol STU. The database provides the information on the areal share of the Luvisols (80%) of the SMU. Based on the derived diagnostic infor- mation, the argic horizon occurs in the entire SMU and is combined with the mollic horizon in 20 percent of the SMU area and with Stagnic properties in 20 percent of the SMU area. Fig. 2. Exemplified derivation procedure of the selected diagnostics based on the Reference Soil Group and qualifier provided information in the ESDB. Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323.318 Results and discussions The map series of this study (Figures 3 to 7) provide (left) maps of the spatial distribution of the selected diagnostics together with the indication of their percentage area within the SMUs, and (right) the spatial distribution of the RSGs for which they are diagnostic, also with the indication of their percentage area within the SMUs. The areal percentages are given for the territory of the EU plus Switzerland. The argic horizon occupies 17.81 percent of the examined territory (Figure 3). This is most- ly affiliated to the Luvisols (14.74%), followed by the Acrisols (1.85%) and Albeluvisols (0.26%). The higher clay content in the sub- soil influences infiltration and storage of water, nutrient movements and adsorption processes (Avery, B.W. 1983; Bockheim, J.G. and Hartemink, A.E. 2013). It should be em- phasized that sampling of only the topsoil for nutrient management, monitoring or other purposes often misses this important infor- mation. The presence of a clay accumulation horizon also influences the depth distribution of the stable fraction of soil organic carbon (Torres-Sallan, G. et al. 2017), and so can be attributed to climate regulation. Only 0.96 percent of the argic horizons occur in other RGS (Chernozems, Phaeozems, Kastanozems, Planosols, Andosols and Anthrosols), there- fore the maps in Figure 3, seem very similar. However, in the limited represented area the argic horizon has the same importance on the discussed processes. The calcic horizon occupies 13.36 percent of the examined territory of Europe (Figure 4), how- ever, 11.16 percent of that does not occur in the RSGs for which the calcic horizon is diagnos- tic (Calcisols, Chernozems, Kastanozems) but in other RSGs (Gleysols, Luvisols, Gypsisols, Planosols, Solonchaks, Solonetz, Vertisols). The accumulated carbonates represent a significant, stable carbon reservoir that has implications on climate regulation (Monger, H.C. and Gallegos, R.A. 2000; Nordt, L.C. et al. 2000; Lal, R. 2004). At the same time, the presence of the calcic horizon, especially at shallow depth, is influencing (often limiting) the reaction of soil processes, the availability and cycling of nutrients and also biodiversity (Richter, A. et al. 2018). Fig. 3. The spatial distribution of the argic horizon with the indication of their area percentage within the SMUs (left), and the spatial distribution of the RSGs for which the argic horizon is diagnostic (AB, AC, LV), with the indication of their combined area percentage within the SMUs (right). 319Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323. Gleyic properties occur in 14.25 percent of the presented territory (Figure 5), however, only 5.30 percent occur in the Gleysols for which the gleyic properties are diagnos- tic. The rest (8.95%) occur in many other RSGs (Acrisols, Albeluvisols, Cambisols, Chernozems, Fluvisols, Luvisols, Phaeozems, Planosols, Podzols, Regosols, Solonchaks and Umbrisols). Soils with Gleyic properties and the related reducing conditions, often suffer nutrient availability problems, which causes significant changes in soil biodiversity as well (Richter, A. et al. 2018). The presence of the gleyic properties may limit the rooting depth of several plants as well. While excess water has favourable influence on carbon sequestra- tion and hence on part of the climate regula- tion function (Lal, R. 2004), we must also con- sider the other aspects of climate regulation, such as nitrous oxide and methane emissions which are strongly positively influenced by excess soil water for prolonged periods of time, as defined by the Gleyic properties (Anthony, T.L. and Silver, W.L. 2021). Histic horizons occur in 6.66 percent of the territory of the EU and Switzerland (Figure 6). Most of them (6.48%) occur in the Histosols, for which the histic horizon is diagnostic, while only 0.18 percent occur in other RSGs (Albeluvisols, Andosols, Fluviosls, Gleysols, Podzols, Planosols). The spatial distribution of Histosols well represents the distribution of the important diagnostic horizon. The histic horizon stores a significant portion of the organic matter of the world soils (Batjes, N.H. 1996; Lal, R. 2004; Jones, A. et al. 2005) and plays an important role in climate regu- lation. Their moisture and nutrient holding capacity are also important in the water and nutrient cycles. Therefore, management de- cisions should consider the preservation of histic horizons. The mollic horizon occurs in 5.63 percent of the examined territory (Figure 7), of which 3.62 percent occurs in the Chernozems, Kastanozems and Phaeozems, for which it is diagnostic according to the applied version of the WRB 1998. These soils are regarded as highly fertile soils, however, the mollic horizon serves the same important role in the many other RSGs (Andosols, Cambisols, Fluvisols, Gleysols, Leptosols, Planosols, Fig. 4. The spatial distribution of the calcic horizons with the indication of their area percentage within the SMUs (left), and the spatial distribution of the RSGs for which calcic horizon is diagnostic (CL, CH, KS), with the indication of their combined area percentage within the SMUs (right). Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323.320 Solonetz, Solonchaks) which occurs in 2.01 percent of the area. The mollic horizon has favourable physical and chemical proper- ties. It is an important factor in relation to the water and nutrient holding capacity of soils (Sauerbeck, D.R. 2001) and is also a fa- vourable habitat for biodiversity. The organic carbon that is stored and preserved in mollic horizons are important for climate regulation as the oxidation of the accumulated organic Fig. 5. The spatial distribution of the gleyic properties with the indication of their area percentage within the SMUs (left), and the spatial distribution of the RSG, the Gleysols for which is gleyic properties are diagnostic, with the indication of the area percentage within the SMUs (right). Fig. 6. The spatial distribution of the histic horizon with the indication of their area percentage within the SMUs (left), and the spatial distribution of the RSG, the Histosols for which the histic horizon is diagnostic, with the indication the area percentage within the SMUs (right). 321Csorba, Á. et al. Hungarian Geographical Bulletin 71 (2022) (4) 313–323. Fig. 7. The spatial distribution of the mollic horizon with the indication of their area percentage within the SMUs (left), and the spatial distribution of the RSGs (CH, KS, PH) for which the mollic horizon is diagnostic, with the indication the area percentage within the SMUs (right). matter in cultivated soils may contribute to greenhouse gas emissions (Schlesinger, W.H. 2000; Lal, R. 2004). Conclusions and limitations In this paper we demonstrated the impor- tance of the spatial definition of the diag- nostic units and qualifiers, regardless of the RSGs, as they carry important information by themselves. With the described methodol- ogy, it was possible to extract all diagnostic information and define their areal percentage within the SMUs, thus, providing important information on the functional capacities of the areas covered. In addition to the units selected for this exercise, other diagnos- tic units and qualifiers could be presented. It must be stated that only one qualifier is provided to the RSGs in ESDB, while sev- eral more might be relevant to certain STUs, which might influence the percentage of the extent. The other limitation is that the exact spatial definition with the currently available map and database is not possible. Beside the identified presence of the diagnostics and qualifiers, their depth distribution is often an important issue. The ESDB was released in 2001 and updated in 2006, Since that time national databases and maps were improved or developed, while the 2nd and 3rd editions (2006, 2014) of the WRB were published. However, the small scale of ESDB does not support field-scale management planning the result of this research can be the start- ing point to understand the diagnostic con- tinuum of soils across Europe. 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