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

Abstract

Sheep urine labelled with ¹⁵N-urea was applied toconfined micro-plots at different times of the year to follow the fate of ureaN in urine in the grazed pasture-wheat rotation in south Western Australia.Three field experiments were conducted on the same site on a loamy sand.Applications were made either to pasture residues (Expts 1 and 2) which weresubsequently sown to wheat, orto growing pasture in winter-spring, (Expt 3).In Expt 1, urine was applied in November 1990 (9·8 gN/m²) and April 1991 (46·1 gN/m²). From both applications, losses of¹⁵N attributed to NH₃volatilisation were c. 50% within 2 weeks of application. Another10% loss was attributed toNO^-₃ leaching during the followinggrowing season and 15% was recovered by the wheat crop. In Expt 2,urine was applied in October 1991 (4·6 gN/m²), January 1992 (15·6 gN/m²), and March 1992 (13·6 gN/m²). Attributed NH₃ losseswithin 2 weeks, in terms of ¹⁵N-urea applied, were40% (October and January urine) and 30% (March urine) andNO-₃ leaching losses were estimated to be 20% forthe 3 applications. Recoveries in wheat (November 1992) were 4, 7, and12% of ¹⁵N applied in the October, January, andMarch urine applications. In Expt 3, urine was applied in August 1992(12·3 g N/m²) and September 1992 (25·9g N/m²). Attributed NH₃ losseswere 10% of applied ¹⁵N for the August and30% for the September application. Plant uptake of¹⁵N was rapid and by mid October was 42% from theAugust application and 47% from the September application. Recovery of¹⁵N in soil organic N was generally 17-25% whenurine was applied to pasture residues and bare soil,and 21-37% whenurine was applied to growing pasture. It is suggested thatNH₃ volatilisation was the predominant N loss mechanism.The amount of NO^-₃ leached wasprimarily influenced by summer rainfall, the length of time urine-N was insoil before the onset of winter rainfall, and the distributionof winterrainfall. Little of the ¹⁵N-labelled urine was eitherrecovered by, or available for, subsequent wheat crops, suggesting thatcalculations for estimating the N supply from pastures to cereal cropsmustdiscount most N returned in urine by grazing animals.