FATE OF UREA NITROGEN IN SHEEP URINE APPLIED TO SOIL AT DIFFERENT TIMES OF THE YEAR IN THE PASTURE-WHEAT ROTATION IN SOUTH WESTERN-AUSTRALIA

Sheep urine labelled with N-15-urea was applied to confined micro-plot s at different times of the year to follow the fate of urea N in urine in the grazed pasture-wheat rotation in south Western Australia. Thre e field experiments were conducted on the same site on a loamy sand. A pplications were made either to pasture residues (Expts 1 and 2) which were subsequently sown to wheat, or to growing pasture in winter-spri ng, (Expt 3). In Expt 1, urine was applied in November 1990 (9.8 g N/m (2)) and April 1991 (46.1 g N/m(2)). From both applications, losses of N-15 attributed to NH3 volatilisation were c. 50% within 2 weeks of a pplication. Another 10% loss was attributed to NO3- leaching during th e following growing season and 15% was recovered by the wheat crop. In Expt 2, urine was applied in October 1991 (4.6 g N/m(2)), January 199 2 (15.6g N/m(2)), and March 1992 (13.6 g N/m(2)). Attributed NH3 losse s within 2 weeks, in terms of N-15-urea applied, were 40% (October and January urine) and 30% (March urine) and NO3- leaching losses were es timated to be 20% for the 3 applications. Recoveries in wheat (Novembe r 1992) were 4, 7, and 12% of N-15 applied in the October, January, an d March urine applications. In Expt 3, urine was applied in August 199 2 (12.3 g N/m(2)) and September 1992 (25.9 g N/m(2)). Attributed NH3 l osses were 10% of applied N-15 for the August and 30% for the Septembe r application. Plant uptake of N-15 was rapid and by mid October was 4 2% from the August application and 47% from the September application. Recovery of N-15 in soil organic N was generally 17-25% when urine wa s applied to pasture residues and bare soil, and 21-37% when urine was applied to growing pasture. It is suggested that NH3 volatilisation w as the predominant N loss mechanism. The amount of NO3- leached was pr imarily influenced by summer rainfall, the length of time urine-N was in soil before the onset of winter rainfall, and the distribution of w inter rainfall. Little of the N-15-labelled urine was either recovered by, or available for, subsequent wheat crops, suggesting that calcula tions for estimating the N supply from pastures to cereal crops must d iscount most N returned in urine by grazing animals.