|Working Group II: Impacts, Adaptation and Vulnerability|
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220.127.116.11. Soils and Land Resources
Climate change will impact directly and, through land-use change, indirectly on a wide range of soil processes and properties that will determine the future ability of land to fulfill key functions that are important for all terrestrial ecosystems, as well as several socioeconomic activities, that underpin the well-being of society. The following subsections summarize key effects. Further information is summarized in the section on soils in the European ACACIA report (Rounsevell and Imeson, 2000).
Soil water contents respond rapidly to variability in the amounts and distribution of precipitation or the addition of irrigation. Temperature changes affect soil water by influencing evapotranspiration, and plant water use is further influenced by elevated CO2 concentrations, leading to lower stomatal conductance and increased leaf photosynthetic rates (Kirschbaum et al., 1996). Soil water contents are highly variable in space (Rounsevell et al., 1999), so it is difficult to generalize about specific climate impacts.
Climate change can be expected to modify soil structure through the physical processes of shrink-swell (caused by wetting and drying) and freeze-thaw, as well as through changes in soil organic matter (SOM) contents (Carter and Stewart, 1996). Compaction of soils results from inappropriate timing of tillage operations during periods when the soil is too wet to be workable. Soil workability has a strong influence on the distribution and management of arable crops in temperate parts of Europe (Rounsevell, 1993; Rounsevell and Jones, 1993). Therefore, wet areas with heavy soils could benefit from climate change (MacDonald et al., 1994; Rounsevell and Brignall, 1994). In a similar way, grassland systems can suffer poaching by grazing livestock (i.e., damage caused by animal hooves) (Harrod, 1979). Thus, drier soil conditions for longer periods of the year would affect the distribution of intensive agricultural grassland in temperate Europe (Rounsevell et al., 1996a) and may result in intensification of currently wet upland grazing areas.
Soils with large clay contents shrink as they dry and swell when they become wet again, forming large cracks and fissures. Drier climatic conditions will increase the frequency and size of crack formation in soils, especially those in temperate regions of Europe, which currently do not reach their full shrinkage potential (Climate Change Impacts Review Group, 1991, 1996). Soils that shrink and swell cause damage to building foundations through subsidence, creating a problem for householders and the housing insurance industry (Building Research Establishment, 1990). Crack formation also results in more rapid and direct movement of water and solutes from surface soil to permeable substrata or drainage installations through bypass flow (Armstrong et al., 1994; Flurry et al., 1994). This will decrease the filtering function of soil and increase the possibility of nutrient losses and water pollution (Rounsevell et al., 1999).
Accumulation of salts in soils (salinization) results from capillary movement and dispersion of saline water because evapotranspiration is greater than precipitation and irrigation (Vàrallyay, 1994). Such conditions, which are widespread throughout the warmer and drier regions of southern Europe, will be exacerbated by temperature rise coupled with reduced rainfall. Climate change also will increase flood incidence and salinity along coastal regions, through the influence of sea-level rise (Nicholls, 2000).
A decrease in precipitation and/or increase in temperature increases oxidation and loss of volume in lowland peat soils that are used for agriculture. It has been suggested that under climate change, the volume of peats in agricultural use will shrink by 40% (Kuntze, 1993). Some peat soils in western Europe are associated with acid sulfate conditions (Dent, 1986); strong acidity largely precludes agricultural use (Beek et al., 1980). Soil acidification also can result from depletion of basic cations through leaching (Brinkman, 1990) where the soil is well drained and structurally stable and experiences high rainfall amounts and intensity—as in many upland areas of Europe. In wetter climate, soil acidification could increase if buffering pools become exhausted, although for most soils this will take a very long time.
Climate change is likely to increase wind and water erosion rates (Rosenberg and Tutwiler, 1988; Dregne, 1990; Botterweg, 1994), especially where the frequency and intensity of precipitation events grows (Phillips et al., 1993). Erosion rates also will be affected by climate-induced changes in land use (Boardman et al., 1990) and soil organic carbon contents (Bullock et al., 1996). Relatively small changes in climate may push many Mediterranean areas into a more arid and eroded landscape (Lavee et al., 1998) featuring decreases in organic matter content, aggregate size, and stability and increases in sodium adsorption ratio and runoff coefficient. However, increased erosion in response to climate change cannot be assumed for all parts of Europe. For example, in upland grazed areas, erosion rates will be reduced as a result of better soil surface cover and topsoil stability arising from higher temperatures that extend the duration of the growing season and reduce the number of frosts (Boardman et al., 1990).
Climate change will impact directly on SOM through temperature and precipitation (Tinker and Ineson, 1990; Cole et al., 1993; Pregitzer and Atkinson, 1993) and indirectly (and possibly more importantly) through changing land use (e.g., Hall and Scurlock, 1991). SOM contents increase with soil water content and decrease with temperature (Post et al., 1982, 1985; Robinson et al., 1995), although rates of decomposition vary widely between different soil carbon pools (van Veen and Paul, 1981; Parton et al., 1987; Jenkinson, 1990). Changes in SOM contents depend on the balance between carbon inputs from vegetation and carbon losses through decomposition (Lloyd and Taylor, 1994); most SOM is respired by soil organisms within a few years. Net primary productivity (NPP) usually increases with increasing temperature and elevated atmospheric CO2, leading to greater returns of carbon to soils (Loiseau et al., 1994). However, increasing temperature strongly stimulates decomposition (Berg et al., 1993; Lloyd and Taylor, 1994; Kirschbaum, 1995) at rates that are likely to outstrip NPP and lead to reduced SOM contents (Kirshbaum et al., 1996). This effect will be strongest in cooler regions of Europe, where decomposition rates currently are slow (Jenny, 1980; Post et al., 1982; Kirschbaum, 1995). Conversely, excess soil water resulting from increased precipitation will reduce decomposition rates (Kirschbaum, 1995) and thus increase SOM contents.
Plant growth and soil water use are strongly influenced by the availability of nutrients. Where climatic conditions are favorable for plant growth, the shortage of soil nutrients will have a more pronounced effect (Shaver et al., 1992). Increased plant growth in a CO2-enriched atmosphere may rapidly deplete soil nutrients; consequently, the positive effects of CO2 increase may not persist as soil fertility decreases (Bhattacharya and Geyer, 1993). Increased SOM turnover rates over the long term are likely to cause a decline in soil organic nitrogen in temperate European arable systems (Bradbury and Powlson, 1994), although, in the short term, increased returns of carbon to soils would maintain soil organic nitrogen contents (Pregitzer and Atkinson, 1993; Bradbury and Powlson, 1994). Greater mineralization may cause an increase in nitrogen losses from the soil profile (e.g., Kolb and Rehfuess, 1997; Lukewille and Wright, 1997), although there is evidence to suggest that temperature-driven, increased nitrogen uptake by vegetation may reduce these losses (Ineson et al., 1998).
There is great uncertainty surrounding the response of soil community function to global change and the potential effects of these responses at the ecosystem level (Smith et al., 1998). Most soil biota have relatively large temperature optima and therefore are unlikely to be adversely affected by climate change (Tinker and Ineson, 1990), although some evidence exists to support changes in the balance between soil functional types (Swift et al., 1998). Soil organisms will be affected by elevated atmospheric CO2 concentrations where this changes litter supply to and fine roots in soils, as well as by changes in the soil moisture regime (Rounsevell et al., 1996b). Furthermore, the distribution of individual species of soil biota will be affected by climate change where species are associated with specific vegetation and are unable to adapt at the rate of land-cover change (Kirschbaum et al., 1996).
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