Short-term N dynamics following application of 15N-labeled urine to a sandy soil in summer

The short-term fate of 20.5 g N m−2 of sheep urine, spiked with 99 at% 15N urea was followed for 28 d after surface application to intact cores of a deep yellow sand (0–40 cm) in dry summer conditions in Western Australia. Soil N variables and soil pH were compared in cores receiving urine, an equivalent volume of water, or kept dry. A simulated 20-mm rainfall event was imposed 14 d after the urine application. ils dried rapidly after urine addition and after a simulated rainfall. Urine-N hydrolysed rapidly to NH4+, as concentrations of NH4+ of 200 μg N g−1 were found in the 0–2.5 surface soil layer. The dry, windy conditions following urine hydrolysis favored loss of urine-N as NH3 gas, but severely limited nitrification of NH4+. Using the 15N balance method, it was estimated that one-third of the urine-N applied was lost as NH3 during the first 3 d. Soil pH (10 mM CaCl2) increased immediately after urine addition. Emissions of N2O and NO were similar from cores receiving urine or water, and both increased after the simulated rainfall. Nitric oxide fluxes were about 10 times higher than N2O emissions. As a loss-pathway of urine-N, however, the magnitude of the N oxide emissions were insignificant compared to the NH3 loss estimate. table organic-N (total soluble N minus NH4+-N) accounted for 36% of the urine-N initially, and decreased to about 12% by 28 d. Nonextractable organic-N or immobilized urine-N increased slowly over time to make up 16% of the balance by d 28. This study showed that besides a significant early loss as NH3, urine-N transformations proceed slowly in summer conditions in a sand. Ammonium-N that originated as urine is highly conserved in the summer and should be largely available for the following winter wheat crop.