Impact of defoliation on the distribution of 15N-labelled synthetic sheep urine between shoots and roots of Agrostis capillarisand soil N pools

An experiment was carried out to test the hypothesis that defoliation of a semi-natural grassland species influenced the fate of the urea nitrogen (N) from sheep urine. The distribution of the 15N added as urea in a synthetic sheep urine (SSU) at 50 g N m^–2, to Agrostis capillarisand fallow soil was followed over a period of 56 days in the plant and soil in a glasshouse microplot experiment. The grass was subject to three regimes; regular twice weekly defoliation, a single defoliation (both to 40 mm), and uncut. Regular cutting increased the total N concentration of shoots and reduced the biomass of roots, though 15N recovery in the shoots was not appreciably different from the uncut grass. A single defoliation immediately before SSU addition decreased 15N recovery. More 15N was recovered in the surface 50 mm of soil beneath the regularly cut than in the single and uncut treatments. The soil mineral N pool was the largest sink for SSU 15N. Microbial N increased threefold with the addition of SSU, but effects of the grass treatments on the proportion of SSU 15N in the microbial biomass could not be detected. It was concluded that regular defoliation of grass compared with no cutting could lead to greater losses of NO3 – by leaching and denitrification.