Impacts of livestock on soil fall into two broad categories: firstly the physical impact of the animal on soil as it moves around and secondly the chemical and biological impact of the faeces and urine that the animal deposits to soil. Physically damaged soil can be even more susceptible to the chemical and biological impact of faeces and urine.
Heavy livestock such as cattle compact soil structure and destroy vegetation on parts of a field that they tread most often. This is visually apparent around drinking water troughs, entrances to fields and other parts of the land where the animals congregate. Destruction of soil structure in this way is known as 'poaching' and can be seen to be harmful because restoration of vegetation does not always occur spontaneously once the grazing animal is withdrawn. Sheath et al. (1998) found losses of 5-10 kg dry matter ha-1 d-1 where up to 50% of an area was affected by cattle treading but recovery occurred within a few months. Compacted soil becomes strong making it difficult for new shoots to penetrate the soil and emerge; structureless soil is unlikely to drain well and will pond after moderate rainfall. Soil particles from these zones will be susceptible to erosion carrying particles, organic matter and phosphorus to surface waters (Warren et al., 1986). Anaerobic zones in waterlogged soils will encourage denitrification which implies a loss of nitrogen and pollution of the atmosphere with N2O if conditions for denitrification are sub-optimal in the compacted zone (see below).
Problems with soil structure are not limited to cattle farming. Pig production is notorious for its destructive effects on vegetation. Part of pig behaviour is to dig into soil with the snout, the effect on soil and vegetation is obvious but without the protective effect of plant roots that confer strength to the rooting zone and without a plant withdrawing water from a field, the soil become weak and the structure collapses under the regular passage of the animal. Soil becomes compacted and the same problems listed above ensue. High stocking rates on pig farms exacerbate the problem. Sheep grazing, particularly in the UK is not normally thought of in these same terms because production is largely extensive on upland rough grazing. In some farms, however, sheep are used to graze root cover crops (such as turnips) in the late winter and all but sandy soils are likely to be susceptible to damage. At equivalent (i.e. metabolic weight) stocking densities on wet soils, short-term treading by sheep was, however, found to be less damaging than treading by cattle (Betteridge et al., 1999).
Chemical and biological impacts of manure and urine
Although many of the impacts of animal wastes on the environment concern losses to water or the atmosphere, soil is an intermediary and as such these impacts deserve space here. The amount of urine delivered to soil by a grazing cow is of the order of 2 litre applied to an area of about 0.4 m2 (e.g. Addiscott et al, 1991). This represents an instantaneous application of 400-1200 kg N ha-1. Such an amount burns vegetation and is often toxic to plant roots which cannot immediately recover to take up the N (full recovery can take up to 12 months and the problem is obviously worst in areas where animals congregate). Urea in soil is quickly hydrolysed and given that grass can take up perhaps 400 kg N ha-1 annually without loss, pollution of groundwater or the atmosphere is almost inevitable whenever urine is applied to soil. Both Calcium and Magnesium are also lost in substantial amounts from urine patches on pasture soils (Early et al., 1998).
Losses of N from urine and manure will normally be as ammonia, dinitrogen and nitrous oxide (during denitrification) or as nitrate leaching. Two key processes deserve mention. The first is that during dentrification (of nitrate to N2 or N2O) the major product is almost always N2. If conditions for this process are in anyway sub-optimal, especially if there is a deficiency of organic carbon relative to nitrate such as might occur under a urine patch, N2O production increases (e.g. Swerts, 1996). Since N2O is a potent greenhouse gas its emission from soil is clearly undesirable. Secondly nitrate is produced from urine and manure during nitrification which is itself a multi-stage process. Where organic matter levels are high such as in or around manure not all the N is converted to the end product, nitrate (NO3-), and some remains as nitrite (NO2-). Nitrite is equally susceptible to leaching as nitrate but is far more toxic. Debate in recent years has focussed wrongly on nitrate, which is in fact a precursor to the production of NO (nitric oxide) that is one of the first lines of the body's defence against pathogenic organisms. Most instances of damage to health that have been attributed to nitrate are in fact the result of nitrite such as methaemoglobinaemia from well water contaminated with nitrate but also nitrite. Incidence of stomach cancers have been found to be negatively correlated with nitrate in the diet but a theoretical link assumed that nitrate could be reduced in situ to nitrite in the stomach. Fortunately nitrite in the wider environment is generally short-lived, but arises during sub-optimal nitrification of ammonia to nitrate, for example where ammonium is washed directly into surface waters either from the soil or because the animal urinates close-by. Nitrite is nonetheless occasionally found in natural waters at levels that exceed EU limits.
Compaction of and damage to soil also limits the growth and use pasture can make of available nutrients. Douglas and Crawford (1998) found between 1.7 and 2.1 t ha-1 reduction in dry matter production in a compacted sward and reduction in recovery of N from 71% to 55% of that applied in the uncompacted and compacted swards respectively.
Cattle sometime spread pathogenic organisms by picking them up from a point source but urinating or defecating elsewhere. Weeds, plant diseases, e-coli O157 are all thought to be spread in this way.
The amounts of nutrients in manure are equally a source of waste, a missed opportunity and potentially of pollution. Manure is partly microbial in composition derived from fermentation during digestion and partly composed of recalcitrant components of the feed. As such it is rather less decomposable than fresh plant material and does not supply N to soil as rapidly or damagingly as urine. It does, however, block light and grass growth underneath manure will be temporarily retarded. Some regrowth occurs with penetration where the pasture is well enough established, some with reseeding directly into the manure.
Application of manures is not necessarily harmful. As implied in much of what has been said above, manure and urine contain nutrients that grass or crops can use. Because manure is relatively long-lived in soil it releases its nutrients slowly and can continue to benefit crop production for many years. Whitmore and Schroder (1996) estimate that applications of slurry to maize during the 1970s and 80's has increased the N-supplying power of Dutch soils by about 70 kg N ha-1. Because the extra fertility is long-lived this extra N-supply is expected to take 10 years to decline to half its current level. This is beneficial, however, only so long as a pasture or crop recovers the N. The N can also mineralise during winter or at some other time when the crop is not growing at its full potential. Under these circumstances losses to the environment are inevitable. The fertility is only maintained as long as the pasture remains in place. Ploughing a grassland soil results in a burst of nutrient availability that slowly declines. Whitmore et al. (1992) showed that the intensive ploughing of grassland during the 1940's and 50's in the UK is a probable cause of the increases in nitrate found in aquifers in the 1970's onwards. Watts et al. (1996) have shown that increased levels of organic C in soil confers desirable resilience to soils in relation to tillage. Mineral pasture soils almost certainly resist hoof damage in proportion to their organic matter content.
The impact of manure and urine on soil from livestock is not simply one of perturbing nutrient cycles. Additives such as copper, zinc, antihelminthics and antibiotics or other veterinary treatments are given to animals. The presence of Cu and Zn can make manure unsuitable for use as a fertilizer on other farms and metals such as these pose a long-term risk in pasture soils because they can accumulate and are only slowly removed by leaching or offtake in vegetation. Heavy metals have been shown to reduce the microbial life and diversity in soil (Griffiths, 2000) and the activity of N-fixers in particular (Giller, 1999).
One rough and ready way of assessing the impact of livestock farming has been to consider the balance between inputs and measured outputs of the nutrients used in livestock farming. The difference is usually large and positive implying enormous loss of nutrients to the wider environment or retention in soil. Given that in the majority of the loss pathways nutrients pass through the soil, impacts on soil is an appropriate place to consider this imbalance. As a very rough rule of thumb worries about surpluses of N are immediate in that more N is lost than retained by soil; worries about P concern the gradual build-up over many years that leads to subsequent but sustained losses. On one Dutch dairy farm in the 1980's about 400 kg N, 23 kg P and 56 kg K ha-1 annually of 467, 35, and 73 kg ha-1 applied respectively was unaccounted for. More generally 75% of the 1.1 x 109 kg N applied annually throughout the whole of the Netherlands is thought to be wasted (Whitmore and Van Noordwijk, 1995). Surpluses of N on UK dairy farms were recently reported to range from 63-667 g N ha-1 with a mean of 257 kg N ha-1 (Jarvis 2000) and exports in produce were estimated to be only 20% of the N applied (Jarvis 1993). Haygarth (1998) estimated gains of P by soil in a typical UK dairy farm to be 26 kg P ha-1 annually with a stocking density of 2.26 animals ha-1 on average. On an upland sheep farm the gain was 0.24 kg P ha-1 only. Strategies to reduce the impact of animal manure and slurry on the environment usually focus on limiting spreading according to the amount of P (e.g. Van der Molen et al., 1998). This is because the relative amounts of NPK required by pasture and arable crops differs from the rate these elements are found in manure; manure is too enriched in P relative to N.
Grazing systems can have an effect on soil and more particularly water courses if manure or silage is not stored properly and leaks out. The resultant point source contamination can affect soil for many years, destroy aquatic life and make water unfit for consumption.
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