Three field experiments were conducted on the Darling Downs (Queensland) to evaluate fertilizer management practices such as application depth and addition of nitrification inhibitor (N-serve), for nitrogen (N) applied in the February-May fallow period for winter cereals. Anhydrous ammonia or urea was applied in February, March or May at two depths (7 or 17 cm), with or without N-ser-ve. Soil fertilized in February generally had a lower mineral-N content at sowing than soil fertilized in May. Deeper application (17 cm) in February did not increase soil mineral-N content to 0 - 2 m depth in May but addition of N-serve did at one site where it appeared to slow the movement of mineral N into the subsoil (0.2-0.4 m). A companion experiment was conducted at each site in which N-15-enriched urea was applied to a small (1 m2) area at the centre of a 4 m2 fertilized plot. Effects of fertilizer placement and N-ser-ve treatment, as were used in field experiments, were evaluated in terms of crop recovery of N-15 and total N-15 recovery in plant and soil at harvest. Recovery of N-15 by wheat, sown at two sites in June, showed that neither fertilizer management practice, application depth nor N-serve affected N-15 recovery. At only one site did wheat recover less February-applied N than May-applied N. N-ser-ve had no effect on N-15 recovery by sorghum sown in October, of N applied in February or May, but N-15 recovery was increased by deeper fertilizer placement. Total recovery of N-15 in soil and plant after wheat harvest was lower (approximately 74%) for February-application than for May-application (>94%). Similarly, total N-15 recovery after sorghum was lower the earlier the fertilizer was applied. Unrecovered N-15 was presumed lost due to denitrification during periods of temporary waterlogging of surface soil. Use of N-serve with the fertilizer application had no effect in conserving N-15 applied for wheat or sorghum. However, deeper (17 cm) placement of N than normal (7 cm) promoted higher total recoveries, and therefore reduced losses, of applied N-15 at the three sites.
