The effect of sheep urine on clover performance in a grazed upland sward

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Abstract
Summary: The effect of artificially applied urine on clover performance in a perennial ryegrasswhite clover sward grazed by sheep was examined during summer. Sheep urine or deionized water (51/m2) was applied to areas (2·70 × 1·25 m) which were protected from subsequent excretal return by graze-through cages. Grass and clover populations, nitrogen-fixing activity, soil nitrogen and soil pH were monitored over the following 90-day period.Urine reduced clover population density, stolon length and dry weight but had little effect on number of grass tillers. Nitrogen-fixing activity of clover was reduced initially to less than 30% of control values, perhaps owing to high levels of soil inorganic nitrogen inhibiting fixation, although osmotic effects due to the high salt concentration in urine cannot be discounted. The lower levels of activity at later dates were due largely to reduced clover in urine-treated areas. Soil pH (0–5 cm) was increased by about 0–5 units 3 days after urine application, with a smaller increase in the 5–10 cm horizon. Recovery of urine nitrogen in the soil mineral N pool was at best 27% of the added nitrogen.The results are discussed in terms of the significance of urine-affected areas in determining a patchy distribution of clover in grazed swards; the role of animals in the transfer of fixed N from clover to grass is considered.