Impacts of climate change adaptation options on soil functions: A review of European case-studies

2.1 Study area and climate

Climate change adaptation options and resulting soil impacts are likely to be diverse across Europe due to heterogeneous biophysical and socio-economic production conditions. Additionally, research design likely determines conclusions on adaptation options and their impacts in a region. To tackle both bio-physical and socio-economic dimensions, 20 case-studies across Europe were assessed at the NUTS 2/3 level (Figure 1). Each case-study undertook an integrated assessment with quantitative tools (e.g., scenario modelling) or qualitative, stakeholder inclusive tools or a combination of both. Published results from case-studies were compiled and further substantiated with information from 23 involved scientists—most of them co-authors of this article—via a semi-structured questionnaire (Appendix S1). This led to a unique data set that reflects the impacts of adaptation options on soils across Europe. The 20 case-studies represent 13 European countries and cover 11 of the 13 major environmental zones of Europe (Metzger, Bunce, Jongman, Mücher, & Watkins, 2005). This classification represents the environmental heterogeneity of Europe and utilizes European ecological data sets for climate, geomorphology, geology and soil, habitats, and vegetation. The two zones not presented in the sample are Anatolia and Lusitania.

To classify the case-studies in terms of soil types, the World Reference Base for Soil Resources (FAO, 2006) was used. The 20 case-study areas cover the 15 most common arable soil types of the 32 World Reference Base types (Table 1). Table 1 also lists the features of climate change scenarios that are relevant to agricultural production, land use and farming systems, methods employed to obtain the results, and key publications for each of the case-studies. Regarding the assessment methods, most studies (17 out of 20) modelled the effects of alternative adaptation management options under climate change on yields and environmental impacts. Such adaptation options were identified by means of stakeholder interaction with regional farmers or extension services in 14 cases and by researchers in the other cases. Therefore, the adaptation options that were regarded as the most suitable by farmers could be identified. Three case-studies simulated changing climatic conditions by employing field experiments at different locations for studying adaptation options (e.g., crop rotation and no tillage).

2.2 Analytical framework

The Driver–Pressure–State–Impact–Response framework was used to study the impacts of climate change adaptation options on the soil functions and SDGs (Figure 2). The framework conceptualizes complex sustainability challenges and provides insight into the relationships between the environment and human beings (Gabrielsen & Bosch, 2003). It links the emergence of climate change (Drivers of change) and its impacts on natural and human systems to decision makers (farmers) who adopt new management practices (Pressures), which can lead to soil threats (State 1) and altered soil functions (State 2). Subsequently, the SDG targets (Impact) can be affected. As a result, policy action (Response) may be required (not covered in the present study). Adaptation options, soil threats, and soil functions are understood as dynamic processes over time, such that the ‘States’ in the Driver–Pressure–State–Impact–Response framework represent dynamic biophysical indicators and human practices.

Adaptation options can be triggered by climate change. However, in reality, this driver is intertwined with other factors such as market conditions, technological development, farmer perceptions, and policy interventions (Mitter, Schönhart, Larcher, & Schmid, 2018; Techen & Helming, 2017). All case-studies assessed climate change adaptation but in different scenario contexts. For the sake of comparability, only those scenarios and adaptation options were included in the review that had been developed from a farming system perspective intended to maintain farm profitability and improve yield level and stability. Other adaptation options focusing primarily on environmental (e.g.,reduced nutrient leaching) and/or social (e.g., employment, health, and culture) objectives (Mandryk, Reidsma, Kanellopoulos, Groot, & van Ittersum, 2014) were not included. The current situation of management practices and climate conditions is the counterfactual to which scenarios of future climate and management situations were assessed. However, in reality, transition is already occurring, and the adoption of adaptation practices can already be observed at individual farms in some cases (e.g., in North Savo, FI).

2.3 Characteristics of soil threats and soil functions

The European Commission's (2002) report lists seven major threats that cause soil degradation in Europe: soil erosion, decline in SOC, compaction, decline in soil biodiversity, salinization, contamination, and sealing. Because the study focuses on agricultural soil management, only the first five soil threats were considered. Soil contamination and soil sealing were excluded because the first is by definition associated with industrial, mainly point-source pollution, whereas the latter refers to taking land out of production (European Commission, 2002).

Soils provide numerous functions to society. The European Commission (2006) lists seven key functions: food and biomass production; storing, filtering, transforming, and recycling water and nutrients; habitat and gene pool; SOC pool; providing raw materials; serving as physical and cultural environment for mankind; and storing the geological and archaeological heritage. In this study, focus was laid on the first four functions (Table 2), which are most relevant to agricultural land use (Schulte et al., 2014). The concept of soil functions was introduced in the Thematic Strategy for Soil Protection (European Commission, 2006), although it has not resulted in a legal implementation of soil conservation measures. Soil functions connect the physical, chemical, and biological processes in the soil system with the provision of benefits to society (Glæsner, Helming, & de Vries, 2014). Agricultural management affects the performance of soil functions in close interaction with geophysical site conditions. The optimization of one of the functions is often to the disadvantage of others. The assessment presents aggregated impacts of one to several adaptation options on soil threats and functions (Table 3).

2.4 Relevance of soil functions for realizing the SDGs

In 2015, the United Nations member countries adopted the agenda 2030 with its 17 SDGs. Although not explicit in the 17 SDG guidelines, the ability of soils to perform their functions plays an important role in meeting specific goals (Keesstra et al., 2016). The review of case-studies was used to examine the potential of supporting the SDGs in the European context through links with soil functions (Montanarella & Alva, 2015; Table 2).

3.1 Impacts of adaptation options on soil threats

The study shows that adaptation options under climate change scenarios reduced SOC losses in 75% of the cases examined (Figure 3). For example, farmers and extension experts in the North Savo case (FI) are already worried about wet conditions in winter and more frequent heavy rains as well as wet conditions during the harvest periods, which affect crop yields, nutrient leaching, and erosion. In response, modified crop rotations, including the use of deep-rooted crops (i.e., clover and oilseed), have been proposed by local scientists (Huttunen et al., 2015; Peltonen-Sainio et al., 2016). An expert from the region anticipates that these changes may maintain or even improve the SOC levels and water retention. For the case-study of Foggia (IT), adopting 2- or 3-year crop rotations (based on winter wheat and tomato) under future conditions similar to a climate model realization of the IPCC A2 climate emission scenario led to an increase in SOC by approximately 10% of the SOC content of the current system that is based on continuous wheat (Ventrella et al., 2012b).

The SOC levels were expected to decrease in only two cases (10%) as a result of implementing adaptation options. For example, using the CLIMSAVE Integrated Assessment Platform, Holman, Harrison, and Metzger (2016) identified adaptation options for NE Scotland (UK). The options included an expansion of the agricultural area and conversion of extensive permanent grassland to ley grassland and arable land due to expected increases in T and reduced summer wetness limitations by 2050. These measures would likely lead to a loss of SOC in the area. Three case-studies (15%) did not analyse SOC changes.

Twelve studies (60%) anticipated a reduced potential risk of soil erosion due to implementation of adaptation measures, including improved crop rotations, permanent soil cover by crop residues, and minimum tillage or zero tillage.

Although adaptation options are anticipated to reduce many soil threats in most cases across Europe, there are concerns regarding the likely increase in soil compaction (approximately 40%). Soil compaction is a common problem worldwide. It affects plant root development and reduces water retention capacity; it can also lower crop yields (D'Or & Destain, 2016). With the increase in total irrigated cropland and more intensive use of agricultural machinery, the risk of soil compaction may increase. For Brandenburg (DE), Gutzler et al. (2015) identified the irrigation of key crops, such as wheat, rye, maize, and sugar beet, as an agricultural adaptation strategy to cope with climate change (e.g., less rainfall in summer and more in winter) and to increase crop productivity. However, irrigation and the use of heavy machinery may increase the risk of soil compaction in the area. Thus, an appropriate use of agricultural machinery (e.g., low pressure and wide tires) is one effective measure against compaction (Prager et al., 2011). In Flevoland (NL), some farmers are concerned about SOC loss and soil compaction and therefore intend to replace root crops with wheat. However, if they were only interested in profits, the area of root crops such as potatoes would likely increase (Mandryk et al., 2017).

The results further show that little knowledge or awareness is currently available among agricultural researchers regarding the influence of climate change and adaptation on soil biodiversity, although the decline in soil biodiversity has been reported as the key future threat (McBratney, Field, & Koch, 2014). Although eight cases anticipated positive and two cases anticipated negative impacts on biodiversity, 10 cases (50%) did not consider soil biodiversity.

Most of the case-studies reported that the risk of salinity is limited, at least in the medium term, due to their locations in northern and western parts of Europe. Salinity issues are more prominent in the southern and eastern parts of Europe, such as in the Mediterranean climate region (Zalidis, Stamatiadis, Takavakoglou, Eskridge, & Misopolinos, 2002), where the annual water balance may become negative. In the case of the Guadalquivir Valley (ES), increased irrigation using reclaimed wastewater might create environmental problems due to increased soil salinity accumulation. Studies carried out in Almería (southern Spain) showed that irrigation with nutrient enriched disinfected urban wastewater can result in low macronutrient absorption efficiency and high soil salinity (Segura, Contreras París, Plaza, & Lao, 2012).

3.2 Impacts of adaptation options on soil functions

In addition to reducing soil threats, most of the adaptation options were found to have positive effects on some soil functions (Figure 4). Adaptation options were expected to increase agricultural food and biomass production in 80% of the case-studies. This finding reveals that the integration of climate change adaptation and yield increase was plausible for the time range of the studies (i.e., the years 2025–2100). In the example of Oristanese (IT), decreased rainfall in the spring and more frequent and extreme droughts are expected as part of climate change. Adaptation of crop varieties/hybrids and improved organic fertilizer use and management have been proposed to offset such climate change challenges when irrigation water is available (Dono et al., 2016), which may result in increased crop and biomass production due to the extended growing season, the CO₂ fertilization, and the effect of milder winters on irrigated autumn–spring hay crops.

Increased biomass production accompanied an expected increase in SOC in 11 of the 16 cases. The results highlight that adaptation options such as reduced tillage, establishment of cover crops, and manuring have the possibility to maintain or even increase the SOC content. For example, Schönhart et al. (2016) illustrated the positive impacts of reduced tillage on the SOC levels for Mostviertel (AUT) based on integrated modelling.

The storing, filtering, transforming, and recycling functions of soils were also found to be positively impacted by the adaptation options in 70% of the case-studies. For example, in the Broye (CH) case-study, increasing irrigation resulted in a denser and more permanent crop cover throughout the year and therefore helped to maintain agricultural productivity and to reduce nutrient losses through leaching or soil loss through water erosion. Furthermore, it was found that both conservation soil management and an increase in the share of winter crops can contribute to a reduction in soil loss by providing soil coverage, particularly during the periods of the year with the most intense rainfall events (Klein, Holzkämper, Calanca, & Fuhrer, 2014).

Similar to the results of soil threats, the impacts on the function of soils as a habitat and gene pool are largely unknown. Of the 20 case-studies, only six (30%) addressed the impacts of agricultural adaptation on soil biodiversity. The obvious ignorance of soil biodiversity issues in most of the case-studies is a mismatch with the emerging knowledge of the important functional role of soil organisms for soil processes (Cluzeau et al., 2012). This is a clear knowledge gap that must be addressed in the future. Among the few cases addressing biodiversity, Odgaard, Bøcher, Dalgaard, and Svenning (2011) proposed adaptation, including changing crop rotations (e.g., reduced maize area) for Norsminde (DK). Increasing drainage and extending buffer zones along water courses (Christen & Dalgaard, 2013) can be responses to more extreme weather events. Local experts in Norsminde expect positive impacts on habitats with larger and perhaps more diverse gene pools. In general, in Denmark, there is a trend towards more organic farming, which will ultimately promote soil biodiversity.

3.3 Progress towards the SDGs

The adaptation options represented in the case-studies potentially support the achievement of SDGs. Adaptation in most of the case studies likely supports SDGs 2 and 13, whereas the impacts on SDG 15 appear uncertain and depend on the regional context and the choice of adaptation options. Most case-studies are largely based on modelling and experts' expectations of possible effects of future management and less on measured empirical evidence, which increases uncertainties of soil biodiversity effects due to climate change adaptation. However, with respect to SDGs 2 and 13, several climate change adaptation options are already practised on farms in order to increase resilience to harmful weather events (e.g., Mitter et al., 2018), which increases confidence. For example, some evidence has been found for effects on crop yields and soil functions under conditions of elevated temperatures, rainfall, or extreme events (Peltonen-Sainio et al., 2016), which are most likely becoming more frequent due to climate change in some European regions. Other adaptation options, such as more diversified land use at the farm level suggested by Peltonen-Sainio et al. (2016), require further empirical evidence.

Although the contribution to SDG 2 through increased food and biomass production in many areas of Europe is in line with other model results on climate change adaptation (Ergon et al., 2018; Gabaldón-Leal et al., 2015; Klein, Holzkämper, Calanca, Seppelt, & Fuhrer, 2013; Klumpp, Tallec, Guix, & Soussana, 2011), less evidence is available to validate findings on the other soil functions, which are more important for SDGs 13 and 15. Further uncertainty results from the huge knowledge gap on the potential and adoption rates of emerging technologies in agriculture and on process interactions between climate change, soil management, and soil functions. Detailed, integrated case-studies of climate and management changes are required to verify which adaptation options perform best to promote sustainable development in a particular regional context and how their adoption can be supported.