The effects of grazing with and without excretal returns on the accumulation of nitrogen by ryegrass in a continuously grazed upland sward

Rates of N accumulation were studied on sheep grazed grass swards maintained at a constant height of 5 cm for two growing seasons (1985 and 1986) and receiving no N fertilizer using a tiller tissue turnover technique. Grazing with normal excretal returns resulted in an 85-105% increase in the estimated rates of N accumulation by laminae compared with similarly grazed swards where excretal returns were prevented. Generally, increases in tiller numbers in plots receiving excreta were mainly responsible for the increased rates of N accumulation, rather than increased rates of N accumulation per tiller. Rates of N remobilization from laminae and senescence (flux of N to standing-dead litter pool) per tiller, and rates of leaf extension and leaf appearance/disappearance were unaffected by excretal returns, in urine patches increased tiller numbers and increased rates of accumulation per tiller both resulted in greater estimates of N accumulation compared with tillers not visibly affected by excreta. However, at any one time only about 11% of the plot area was affected by excreta and increases in N accumulation in excreta patches could not account for the overall plot increases in rates of N accumulation.
The increased tillering in plots receiving excreta may have increased exploitation of the soil N. With an average stocking rate of 15 sheep ha^?1, soil nitrate-N, but not total N (Kjeldahl) nor bulk density, was increased during the winter of the second year as a result of excretal returns.