Transformations of nitrogen following the application of 15N-labelled cattle urine to an established grass sward.
Cattle urine, labelled with 15N urea, was applied in August 1987 at a rate equivalent to 74 g N/m2 to a series of confined microplots, at Hurley, UK, inserted to a depth of 30 cm in a 16-year-old grass sward consisting mainly of perennial ryegrass (Lolium perenne). Contents of total N and 15N were determined at intervals between 2 and 321 days after the application in a range of soil and plant components of the sward, and assessments were made of ammonia volatilization and denitrification. About 37% of the applied 15N was rapidly lost from the microplots, apparently by drainage through macropores and/or cracks resulting from the insertion of the microplots. The 15N remaining in the sward after the initial loss by drainage was converted from urea to ammonium within a few days, and about 18% was volatilized as ammonia within 16 days. Most of the remaining 15N was nitrified to nitrate within this period. Uptake of 15N into plant components of the sward (herbage, stubble, leaf litter and roots) continued until early October (day 51), by which time the amount taken up represented about 21% of that present after the initial loss by drainage. Subsequently there was little further uptake. Measurements indicated that N losses through denitrification amounted to only about 0.2% (adjusted for the initial loss by drainage) by early October, but had increased to about 6.7% by the following April. About 25% of the 15N present in the sward in early October was lost by early December, apparently by the leaching of nitrate. A substantial leaching loss would be consistent with rainfall during October and November 1987 amounting to 230 mm, almost twice the long-term average.