BD2304 Scoping study to assess soil compaction affecting upland and lowland grassland in England and Wales APPENDICES TO SID5 The appendices give more detail about each part of the project and include all relevant references at the end of each section. APPENDIX 1 Mapping the extent of soil compaction (Work Package 1) APPENDIX 2 The causes of soil compaction (Work Package 2a) APPENDIX 3 The impacts of soil compaction (Work Package 2b) APPENDIX 4 Conflicts and synergies within existing and potential ES options, between objectives relating to soil compaction and its remediation and other scheme objectives (Work Package 3) APPENDIX 5 Responses received at the Stakeholder workshop (Work Package 5) APPENDIX 6 Glossary of terms Appendix to SID5 1 Soil compaction in England and Wales January 2008 APPENDIX3:Impacts of soil compaction 3.4 Impact of soil compaction on grassland systems on water resources and flood risk 3.4.1 Background Livestock numbers have generally risen over the past century. Between 1866 and 1980, the number of cattle increased threefold to 13,426,000, but this number declined to 10,345,000 in 2002. In 1922, 75% of UK cattle were in England & Wales, 15% in Scotland and 10% in N. Ireland. Little change took place in the 1930s and World War II. Cattle have become relatively less important in England & Wales since the mid 1950s (as land was more suitable for profitable crop production) while in Scotland and Northern Ireland the opposite has occurred. Sheep and lamb numbers rose rapidly in the 1980s (a 60% increase above 1960 levels). Typically there were 25-30 m during the 1930s and World War II but this then fell to 17m in 1947. From 1956, profitability began to improve and the numbers rose to 30 m. Following CAP for sheep meat, numbers rose further in the 1980s to just under 40 m. The increases in recent years have also been due to a relative decline in profitability of suckler calf production in uplands and milk and beef production in other regions, resulting in a switch to sheep meat production. In some parts of the country, particularly Yorkshire and Cumbria, this increase was halted by the foot and mouth disease epidemic of 2001, but restocking of many farms took place soon after the outbreak was declared over. Pig numbers increased from 4,500,000 in 1937 to 5,588,000 in 2002, down from a peak of 8,100,000 in 1970. There has been a movement to outdoor pig production, a trend which may increase as a consequence of legislation banning sow stalls (an indoor farming practice) in 1999 and other animal welfare issues. 3.4.2 Impacts of livestock Introduction The following are extracts from Trimble and Mendel’s (1995) review paper on the cow as a geomorphic agent: “Cows are important agents of geomorphological change. On the uplands, heavy grazing compacts the soil, reduces infiltration, increases runoff, and increases erosion and sediment yield. However, light and moderate grazing have effects that are much less significant. In riparian zones, grazing decreases erosional resistance by reducing vegetation and exposing more vulnerable substrate. Trampling directly erodes banks, thus increasing turbulence and consequent erosion. Most landscapes are composed of mostly upland slopes and it is here that cattle have perhaps collectively their greatest effects. They directly reshape the earth, compact the soil and cause increased runoff, sometimes transforming the runoff regime from variable source area to unsaturated (Hortonian) overland flow. They further weaken biological resistance and trample and loosen soil, changing its susceptibility to both water and wind erosion. The direct force of cattle hoofs reshapes the land. That force is often conceptually underestimated because it is conceived as static, i.e. the mass of the cow (typically 400-500 kg) divided by a few cm2 of basal hoof area. But in the movement of a cow, that mass is often transferred to one or two hooves and there is acceleration in the movement. Using a mechanical simulator, Scholefield and Hall (1986) calculated that a 530 kg cow would exert 250 kPa of vertical stress while walking on level ground. However, the process is best seen and most effective when a cow is climbing a steep slope. Then, the mass is often concentrated on the downslope rear leg which propels the animal some distance upslope. The most common Appendix to SID5 70 Soil compaction in England and Wales September 2007 manifestation of direct force is the path or trail. Because the trails are less permeable (from compaction and crusting: Rostagno, 1989) and because they conduct water, they may erode to larger proportions (Hole, 1981) even under “light” grazing (Naeth et al., 1990). Cooke and Reeves (1976) speculate that concentration of runoff along such trails could help initiate downslope gully development and the work of Rostagno (1989) would appear to support such a suggestion. Removal of phytomass by grazing and lessened phytomass production can reduce fertility and organic matter content of the soil. Soil aggregate stability is decreased and the surface sometimes becomes crusted. Proportion of bare soil appears to correlate well with surface runoff (and sediment yield) (Copeland, 1965; Lusby, 1970; Branson et al., 1981; Thurow et al., 1986; Warren et al., 1986; Takar et al., 1990; Bari et al., 1993). Although the literature is very sketchy, it appears that fauna ranging from earthworms to moles have more difficulty surviving in the impacted soil condition resulting from heavy grazing (Hole, 1981; Abbott et al., 1979). Lusby (1970), working in western Colorado, found that runoff from a grazed watershed was 30% greater than that from an ungrazed watershed. Rauzi and Smith (1973) report that infiltration rates varied with grazing intensity on pastures in northeastern Colorado. Under “light” to “moderate” grazing, infiltration rates were 5.6 and 5.9 cm hr-1, respectively, of which about 30% of the total water infiltrated within the first 15 minutes. Under “heavy” grazing, the infiltration rate was 4.8 cm hr-1. Usman (1994) also found that infiltration rates decreased substantially under “moderate” and “heavy” grazing and he attributed these reductions to changes in soil structure. It will be observed that there is a general decrease of infiltration capacity with grazing intensity, but there is also large variance about all of the means. Branson et al. (1981) cite several studies which investigate the recovery of soil infiltration rates with the cessation of grazing. Hydrologic recovery was evident within 3 years on pastures in southwestern Wisconsin, within 4 years on sandy loam soil in Utah, within 6 years on ponderosa pine-grassland and within 13 years on grassland locations in Colorado. Overgrazing alters streambank morphology by creating false, setback banks (Kauffman and Krueger, 1984). A hoof can actually shear off slices of bank material ≤ 10 cm thick, pushing them toward the stream. The net results of grazing riparian areas, can be both (1) direct modification of stream channels and banks and (2) reduction of resistance to erosion by higher flows which promotes channel erosion. Grazing on riverine and upland areas usually go hand-in-hand so that riverine erosion is increased by the enhanced runoff regime from grazed upland areas.” Soil compaction and infiltration Livestock exert a significant pressure on soil. Godwin and Dresser (2003) noted that it is no coincidence that sheep were used to compact soils in canal beds and are used in compacting soils for earth dams and roadways. It is estimated that for a sheep with a body mass of 40 kg and area per foot of 0.0006 m2, the pressure under the sheep’s foot, when static, is approximately 160 kPa, but this could easily rise to 320 kPa when walking and to 480 kPa under dynamic conditions (Godwin and Dresser, 2003). Figure 16 shows the relationship between water table levels and the surface strength of a grassland soil, indicating that, under the shallow watertables common in many upland or poorly drained soils under grassland, compaction by livestock is likely. Appendix to SID5 71 Soil compaction in England and Wales September 2007 Figure 16 The relationship between water table levels and the surface strength of a grassland (redrawn from Massey et al., 1974) The problem is compounded by the fact that many of our grass growing regions are in areas of high rainfall. There is considerable economic pressure on livestock farmers to extend the grazing season so as to increase the proportion of grazed grass in animal diets and to increase grass utilisation by heavier grazing - grazed grass is much cheaper as a feed than the use of concentrates and less costly than using silage or hay. Headage payments have historically encouraged high stocking rates, but the replacement of these by area payments may help the situation. These trends for increases in grazing pressure and extending the length of the grazing season result in more treading of the soil surface which leads to greater sealing of the soil (Figure 17). Overgrazing also leads to the appearance of more bare patches which tend to shed rainwater quickly rather than allowing it to infiltrate (Harris et al., 2004). Figure 17 Topsoil compaction caused by poaching due to high stock density in cattle holding area (from Holman et al., 2001) Dense, compacted surface layer with lack of soil structure Improving structure with depth Fieldwork in the Yorkshire Ouse, Severn, Uck and Bourne catchments in the winter of 2000/01, following the severe flooding of that autumn, showed significant proportions of grassland soils having soil structural degradation (Table 13) (Holman et al., 2003). In many case, this was Appendix to SID5 72 Soil compaction in England and Wales September 2007 manifest in extensively poached soil surfaces and topsoil compaction leading to extensive areas of standing water and marked vertical wetness gradients (topsoil water content being significantly greater than in the subsoil).