Application of anhydrous ammonia or urea during the fallow period for winter cereals on the Darling Downs, Queensland .II. The recovery of ¹⁵N by wheat and sorghum in soil and plant at harvest

Abstract

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-serve. 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 15N-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-serve treatment, as were used in field experiments, were evaluated in terms of crop recovery of 15N and total 15N recovery in plant and soil at harvest. Recovery of 15N by wheat, sown at two sites in June, showed that neither fertilizer management practice, application depth nor N-serve affected 15N recovery. At only one site did wheat recover less February-applied N than May-applied N. N-serve had no effect on 15N recovery by sorghum sown in October, of N applied in February or May, but 15N recovery was increased by deeper fertilizer placement. Total recovery of 15N in soil and plant after wheat harvest was lower (-74%) for February-application than for May-application (>94%). Similarly, total 15N recovery after sorghum was lower the earlier the fertilizer was applied. Unrecovered 15N 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 15N 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 15N at the three sites.