Impact of cattle on soil physical properties and nutrient concentrations in overland flow from pasture in Ireland
Introduction
Grassland managed with the aim of supporting intensive animal husbandry can, under some circumstances, become a source of nutrients in water bodies (Kurz et al., 2005a, Sharpley and Syers, 1979). Stock can impact on stream water quality directly by entering and excreting into the streams (Sharpley and Syers, 1979), and by trampling and eroding the stream banks (Line et al., 2000). Grazing animals can alter the hydrology and the drainage pathways at a site by compacting the topsoil, which is indicated by increased bulk density (BD) and decreased macroporosity (MP) (Singleton et al., 2000). This can result in a decrease of the infiltration capacity of the soil (Pietola et al., 2005) and, consequently, in an increased occurrence of overland flow (Heathwaite et al., 1990), which is considered to be an important nutrient loss pathway (Kurz et al., 2005b). Grazing animals can also change the characteristics of grassland as a nutrient source. They may alter the type and amount of nutrients that can be mobilised and lost to water by effecting a spatial and chemical re-distribution of nutrients and, sometimes, by causing enough soil physical damage to reduce grass growth (Drewry and Paton, 2000).
The effects of grazing animals on nutrient losses to water are reported to range from not measurable (Owens et al., 1989) to considerable (Heathwaite and Johnes, 1996). This variation is probably due to the great number of variables involved in the nutrient loss process, and to the considerable effect the relative timing of management and weather factors can have on nutrient movement.
The aims of this project were (1) to measure the impact of rotationally grazing cattle on bulk density, macroporosity and resistance to penetration; (2) to investigate whether rotationally grazing cattle influence the nutrient concentrations in overland flow produced at pasture sites; (3) to assess the likely impact of rotationally grazing cattle on nutrient concentrations in overland flow.
Section snippets
Overview of study sites and measurements taken
Four fields of pasture were used as study sites for the soil physical investigations. Two of the sites (JC1 and JC2) were located at Johnstown Castle, Wexford, Ireland (302404/11fc UTM) and two (G1 and G2) at Grange, Co. Meath, Ireland (297959/254978 UTM). At each of the four field sites two treatments were applied to small (1.5 m × 15 m) delineated plots. Treatment 1 (UnAc) gave cattle unrestricted access to the five plots assigned to that treatment. Treatment 2 (NoTrEx) allowed cattle to eat the
Soil nutrient concentrations
The analyses of variance of the soil nutrient concentration data are summarised in Table 3. At the end of the experiments, the potassium concentrations were significantly higher in plots of the UnAc treatment than in plots of the NoTrEx treatment at all study sites. There was no statistically significant effect of treatment on soil P levels at site JC1 and the two sites at Grange. At site JC2, soil P levels were significantly higher in the UnAc than the NoTrEx treatment.
Resistance to penetration and soil moisture
The repeated measures
Soil hydrology
Bulk density and MP are known to represent an important influence on the infiltration characteristics of soils (Free et al., 1940), and they have been found to be the most useful indicators of topsoil compaction caused by cattle treading (Drewry et al., 2000). Soil moisture and BD are the main parameters influencing RP at sites of equal soil type (Vaz and Hopmans, 2001).
The treatment effects of about 30–50% on RP show that either BD or SM or both differed greatly between UnAc and NoTrEx areas
Conclusions
At the sites examined in this study, the presence of cattle led to physical changes in the topsoil. Bulk density and RP were increased and MP decreased in areas to which cattle had access. These changes favoured the occurrence of overland flow and altered the natural drainage characteristics of soil. They persisted over the winter period when the animals were housed. Soil physical recovery did, however, occur when cattle were excluded from areas over the growing season.
The effect of cattle on
Acknowledgements
This report has been prepared as part of the Environmental Research Technological Development and Innovation Programme 2000–2006. The programme is financed by the Irish Government under the National Development Plan 2000–2006.
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