Nitrogen availability in a Darling Downs soil following cereal, oilseed and grain legume crops. 2. Effects of residual soil nitrogen and fertiliser nitrogen on subsequent wheat crops

Abstract

Two dryland wheat crops were grown in 1977 and 1978 following each of 18 cereal, oilseed, or grain legume crops grown in 1976 on a black earth soil on the Darling Downs of Queensland. Combined grain yields of the two crops following the grain legumes fieldpea, lathyrus, lentil and lupin cv. Ultra were higher (P< 0.05) than those following all cereal and oilseed crops except canary seed, safflower and rapeseed cv. Torch. Urea (0-90 kg/ha N), applied to wheat in 1977 on a site adjacent to the crop comparison experiment, had little effect on grain yield in that year. However, in 1978, wheat responded to residues of these applications up to the 50 kg/ha N rate. Variation in wheat yields following the 18 crops appeared to be related to nitrogen (N) supply. The quantity of N assimilated into wheat grain was directly related to the quantity of soil mineral N to a depth of 1.2 m when the 1977 crop was planted. This varied from 37 kg/ha N after oats to 160 kg/ha N after lathyrus. Efficiency of recovery of soil mineral N by the 1977 wheat crop was very low, due probably to the unavailability of N in top soil during the dry winter. Poor availability of N in the top soil was also the most likely cause of a similarly low recovery of fertiliser N applied to the 1977 wheat crop. There was a better apparent recovery of N by the second wheat crop; soil mineral N levels for all treatments had declined to between 17 and 28 kg/ha N after crop harvest. In spite of a low overall efficiency of N uptake by wheat in 1977, more N was assimilated into the grain following legumes (27-39 kg/ha) than following cereals (1 6-2 1 kg/ha), even when fertilised with up to 90 kg/ha N (19 kg/ha). The presence of mineral N in subsoil layers (0.3-0.9 m) following legumes was considered responsible for relatively high grain yields and N uptakes of the following wheat crop in this season of limited growing-season rainfall. Protein concentration of wheat grain was generally higher following grain legumes than following all cereals or oilseeds except safflower. Grain protein concentration was increased by the application of N fertiliser, but fertilised wheat in 1977 generally showed a lower protein content than wheat following grain legumes. However, the second wheat crop following most legumes (except lathyrus) showed a protein content similar to the second wheat crop following N fertiliser application.