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

Abstract Soils are vital for supporting food security and other ecosystem services. Climate change can affect soil functions both directly and indirectly. Direct effects include temperature, precipitation, and moisture regime changes. Indirect effects include those that are induced by adaptations such as irrigation, crop rotation changes, and tillage practices. Although extensive knowledge is available on the direct effects, an understanding of the indirect effects of agricultural adaptation options is less complete. A review of 20 agricultural adaptation case‐studies across Europe was conducted to assess implications to soil threats and soil functions and the link to the Sustainable Development Goals (SDGs). The major findings are as follows: (a) adaptation options reflect local conditions; (b) reduced soil erosion threats and increased soil organic carbon are expected, although compaction may increase in some areas; (c) most adaptation options are anticipated to improve the soil functions of food and biomass production, soil organic carbon storage, and storing, filtering, transforming, and recycling capacities, whereas possible implications for soil biodiversity are largely unknown; and (d) the linkage between soil functions and the SDGs implies improvements to SDG 2 (achieving food security and promoting sustainable agriculture) and SDG 13 (taking action on climate change), whereas the relationship to SDG 15 (using terrestrial ecosystems sustainably) is largely unknown. The conclusion is drawn that agricultural adaptation options, even when focused on increasing yields, have the potential to outweigh the negative direct effects of climate change on soil degradation in many European regions.

temperature, precipitation, and moisture regime changes. Indirect effects include those that are induced by adaptations such as irrigation, crop rotation changes, and tillage practices. Although extensive knowledge is available on the direct effects, an understanding of the indirect effects of agricultural adaptation options is less complete. A review of 20 agricultural adaptation case-studies across Europe was conducted to assess implications to soil threats and soil functions and the link to the Sustainable Development Goals (SDGs). The major findings are as follows: (a) adaptation options reflect local conditions; (b) reduced soil erosion threats and increased soil organic carbon are expected, although compaction may increase in some areas; (c) most adaptation options are anticipated to improve the soil functions of food and biomass production, soil organic carbon storage, and storing, filtering, transforming, and recycling capacities, whereas possible implications for soil biodiversity are largely unknown; and (d) the linkage between soil functions and the SDGs implies improvements to SDG 2 (achieving food security and promoting sustainable agriculture) and SDG 13 (taking action on climate change), whereas the relationship to SDG 15 (using terrestrial ecosystems sustainably) is largely unknown. The conclusion is drawn that agricultural adaptation options, even when focused on increasing yields, have the potential to outweigh the negative direct effects of climate change on soil degradation in many European regions. KEYWORDS agricultural adaptation, DPSIR, regional case-studies, soil degradation, Sustainable Development Goals 1 | INTRODUCTION Soil systems are fundamental to sustainable development due to their multifunctional role in providing services including biomass production (food, feed, fibre, and fuel); habitats for living organisms and gene pools (biodiversity); cleaning of water and air; mitigation of greenhouse gas emissions; contributions to carbon (C) sequestration; buffering of precipitation extremes; and provisions to cultural, recreational, and human health assets (Coyle, Creamer, Schulte, O'Sullivan, & Jordan, 2016;Montanarella, 2015;Tóth et al., 2013). The effects of climate change are associated with increases in temperature (T) and extreme weather events such as heavy rainfall, droughts, frosts, storms, and rising sea levels in coastal areas. These effects may also increase the threats to soil such as soil erosion, soil compaction, reduced soil fertility, and lowered agricultural productivity, which ultimately deteriorate food security and environmental sustainability (Lal et al., 2011). These climate-related risks raise major concerns regarding the future role of soils as a sustainable resource for food production.
Climate change can affect soil functions directly and indirectly. The direct effects include soil process changes in organic carbon transformations and nutrient cycling through altered moisture and T regimes in the soil or increased soil erosion rates due to an increased frequency of high-intensity rainfall events. Blum (1993) was among the first to frame a systematic concept of linking soil processes via soil functions to services for the environment and society in Europe. Climate change and soil management can change the ability of soils to perform soil functions, which, for the sake of simplicity, the study calls changes in soil functions. Several studies have assessed the effects of climate change on soil functions (Coyle et al., 2016;Ostle, Levy, Evans, & Smith, 2009;Xiong et al., 2014). For instance, in organic-rich soils in the UK, increased T and decreased soil moisture linked to warming or drought were observed to reduce the C storage capacity (Ostle et al., 2009).
The indirect effects of climate change on soil functions include those that are induced by climate change adaptation options. Agricultural management can mitigate climate change effects, for example, through increased soil organic carbon (SOC) sequestration (Haddaway et al., 2015). Farmers may implement adaptations as a result of multiple, intertwined driving forces, including market price changes, new technologies, and improved knowledge in combination with climate change (Reidsma et al., 2015b). Regarding European agriculture, several scenario studies have investigated agricultural adaptation options in response to climate change, including the introduction of irrigation regimes in drought-prone areas, crop rotation changes, increased fertilization rates on cropland, amended soil tillage practices, and cultivation of melting permafrost soils (Mandryk, Reidsma, & van Ittersum, 2017;Schönhart, Schauppenlehner, Kuttner, Kirchner, & Schmid, 2016;Ventrella, Charfeddine, Moriondo, Rinaldi, & Bindi, 2012a).
Although ample knowledge is available for the direct effects (although the interactions are not completely understood), evidence of the indirect effects of agricultural adaptation options on soil functions is more scattered and difficult to derive experimentally because it depends on an uncertain future climate and corresponding adaptation. However, the anticipation of development pathway impacts is a precondition for decision-making.
Although farm management concerns the local field level, the multiple soil functions need to be maintained and improved at higher spatial aggregates to achieve the Sustainable Development Goals (SDGs) formulated by the United Nations agenda 2030. Montanarella and Alva (2015) assessed soil functions as being particularly relevant for three of the 17 SDGs, namely, SDGs 2 (achieving food security and promoting sustainable agriculture), 13 (taking actions on climate change), and 15 (using terrestrial ecosystems sustainably, reversing land degradation, and halting biodiversity loss).
The objective of this paper was to review case-studies on future adaptation options in European regions for their information on how adaptations may affect soil functions and what that means in the context of the SDGs. Taking current climate systems and management practices as counterfactuals, the cases were used to assess how future climate change in combination with adaptation options may impact European soils.

| 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

| 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 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).    (Table 3).

| 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).

| RESULTS AND DISCUSSION
The results indicate that all case-studies considered soil degradation, although they all had other primary research objectives (e.g., yields, profitability, and greenhouse gas emissions). This confirms the high awareness of soil degradation issues in agricultural climate change research. In general, the adaptation options under climate change conditions seem to have positive impacts on soils (Table 3)

| 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      (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).

| Impacts of adaptation options on soil functions
In addition to reducing soil threats, most of the adaptation options were 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.

| Progress towards the SDGs
The 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.

| SUMMARY AND CONCLUSIONS
Climate change is a major threat that could lead to a decline in agricultural production in many regions of the world. Adaptation is important to manage the risks and utilize the benefits from climate change. However, when the primary aim is to increase food production, soils and ecosystem services may be adversely affected. Thus, understanding the possible future impacts of agricultural adaptation options for addressing potential risks of soil degradation is vital.
The results of this study provide some clear general insights. They Finally, this study demonstrated that despite the broad range of local contexts and farming systems assessed in the 20 case-studies across Europe, it is possible to identify converging win-win policies that are able to support adaptation options that could, at the same time, minimize soil threats and enhance multiple soil functions. However, more studies are needed in the future to support this ambition given the uncertainties inherent to climate change, its implications for long-term soil process dynamics, interactions with agricultural practices, and the multiple interacting factors affecting the consequences of adaptation options as well as the market, technology, and policy changes for soils.