Adaptation: Field of Action Soils

Arable and crop measures against water erosion are aimed at maintaining and building up a stable soil structure, preventing or at least severely restricting the mobilisation of soil particles during heavy rainfall and surface silting. This includes minimising the periods without soil cover through the cultivation of catch crops, subseeds and the leaving or application of a soil-protecting mulch layer (e.g. straw, crop residues, manure, green cuttings, and compost). A soil cover not only protects the soil from the direct impact of raindrops, but also maintains or builds up stable soil aggregates that reduce siltation through the activity of soil organisms. An area-wide mulch layer has a particularly erosion-reducing effect, as it also slows down runoff.

The stability of the soil can be improved by reducing the tillage depth and intensity. In this way, permanent no-till conservation tillage preserves the natural soil structure and reduces the risk of erosion. The use of heavy vehicles and machines can lead to soil compaction and thus limit the infiltration of rainwater into the soil. Soil cultivation methods can be adapted in such a way that the total mass and the specific surface pressure are better distributed and thus the load-bearing capacity of the soils is less stressed. This can be achieved by using wide tyres with low internal tyre pressure and a large contact area or by using lighter machines.

On sloping sites, the following adaptation options are available: cultivation across the slope, the creation of green strips, hedges and roadside ditches running across the slope to slow down runoff, the creation of small terraces, the creation of retention areas as sedimentation space in the slope area, professional water drainage from the upstream area, and the permanent planting of partial areas particularly at risk of erosion.

In addition to these technical measures, legal, political and management measures may contribute to the prevention of water erosion. Soil protection policy should focus on soil-related adaptation measures. Soil functions relevant to climate adaptation should be given greater consideration in laws and in planning and approval procedures. Spatial planning (e.g. regional planning, land consolidation procedures) could contribute to reducing water erosion risks by designating priority areas (e.g. green strips, hedge planting) for soil protection. In the settlement area, areas with no or only little vegetation can be converted into green spaces as compensation areas for construction projects. Concrete specifications regarding the reduction of land consumption and land unsealing in settlement and transport development may also require a political decision. Active protection against water erosion may consist of abandoning particularly endangered areas in favour of other, less erosion-sensitive uses (e.g. establishment of permanent grassland, forest or woodland areas).

Erosion assessment is an important management measure for planning concrete adaptation measures. It allows an assessment of potential erosion damage and the spatial impact of climate change. Also of importance is the establishment of a climate change-related soil monitoring system that bundles meaningful information on soils, land uses and regional climate changes in order to better assess climate impacts on soil functions. The 2nd Progress Report on the German Adaptation Strategy (DAS) states that a climate impact soil monitoring network should be established in order to provide users in administration and science with easy access to soil-related measurement data.

Indicator from the monitoring on the DAS: Permanent grassland

The determination of the risk of wind erosion at a site represents an important planning basis for concrete measures against soil erosion by wind. These measures may be based on factors that influence wind erosion: Landscape structure, soil roughness, vegetation and land cover. In addition, land use changes are conceivable.

Linear landscape structures, such as hedges, hedgerows and stone walls, may act as flow obstacles in sparsely forested regions and thus protect the soil from wind erosion. It should be noted that, in addition to the one-off investment, long-term costs may arise for the maintenance and upkeep of such corridor elements. In addition, the creation of new corridors entails a certain loss of agricultural land. However, the positive effects outweigh this. In addition to erosion control, hedges contribute to an improved microclimate and soil water balance and are important for biodiversity and biotope connectivity.

Dense soil cover with vegetation, subseeds or mulch as well as a high roughness of the topsoil reduces the wind speed directly at the soil surface. A soil cover of > 25 % already provides effective wind erosion protection. The cultivation of alternating crops with different growth heights on smaller, neighbouring areas also leads to a reduction of wind speeds near the soil surface. The soil surface should be left as rough as possible after tillage.

In areas with a high risk of wind erosion, it is conceivable to convert arable land to extensive permanent grassland use or to take it out of use altogether and leave it to natural succession. The conversion of land use also includes the afforestation of formerly agricultural land, the creation of biotopes (e.g. establishment of permanent flowering strips in fields). Another objective can be land consolidation, which reduces the length of the field perpendicular to the main wind direction by re-cutting the agricultural land in order to minimise the area exposed to the wind. Such long-term land use changes are usually only possible with political support and the provision of financial resources.

Targeted adaptation measures to a decreasing water content in the soil are of concern in agriculture, in the landscape, but also in settlement areas.

In agriculture, sufficient humus supply to the soil is of primary importance, as this improves the water retention capacity. The following measures, among others, are important for increasing the humus content: a site-appropriate, diverse crop rotation with a balanced relationship between humus-consuming (e.g. maize, sugar beet, potato) and humus-productive catch crops and subseeds (e.g. clover grass), a periodic use of grassland, a sufficient supply of organic matter to the soil through the crop residues (e.g. straw, roots) remaining on the field at harvest and through organic farm manure (e.g. farmyard manure, liquid manure, compost), as well as conservation/non-turning tillage. A permanent soil cover together with the crops protects the soil from drying out. Soil compaction should be avoided as far as possible, as a good soil structure is the prerequisite for oxygen and water supply and thus for optimal microbial activity. The practices mentioned here are implemented in particular within the framework of organic farming, which is why its promotion may also counteract the occurrence of water deficiency situations in soils.

In principle, irrigation is possible in agriculture if plant development is inhibited by soil water content in critical phases. If irrigation is used, it should be carried out according to need, efficiently and with as little evaporation loss as possible. Crops may also be adapted to low soil water contents and thus to drought stress by cultivating drought-tolerant plant cultures and varieties. This can minimise yield losses.

Where land use allows, reducing drainage, re-wetting and allowing flooding help to retain water more firmly in the landscape. This prepares for and could help to survive dry periods. If land can be made available for the re-wetting of peatlands, this will help the regional and water balance.

In the settlement area, the aim of urban planning should be to achieve an approximation to the natural water balance. To this end, with the help of nature-based adaptation measures, precipitation water is no longer exclusively drained into the urban sewage system, but is instead discharged into open and green spaces; it percolates and thus remains in the city. Nature-based elements, such as swale systems, strengthen decentralised rainwater infiltration and help to increase soil moisture and groundwater recharge in urban areas. In hot and dry periods, this can improve the water supply for plants and improve the urban climate through evaporative cooling of the soil and plants ("Sponge City principle"). An important measure in this context is the unsealing of surfaces. On the part of urban land use planning, the regulation of land use by settlement and infrastructure, the keeping of areas free for precipitation infiltration and the securing of green spaces contribute to the adaptation to water shortage risks. During dry periods, irrigation measures may be created, such as using service water to irrigate urban green spaces, attaching water bags to newly planted urban trees or organising watering partnerships in neighbourhoods. Such measures should in any case be efficient, water-saving and hygienically safe.

Indicator from the monitoring on the DAS: Humus content of arable land – case study