Wm. Strong et al., NITROGEN-FERTILIZER RESIDUES FOR WHEAT CROPPING IN SUBTROPICAL AUSTRALIA, Australian Journal of Agricultural Research, 47(5), 1996, pp. 695-703
Applied nitrogen (N) recovered by fertilised wheat and by successive w
heat crops in a 4-crop sequence (1987-90) was studied by applying N-15
-depleted ammonium nitrate (0, 2.5, and 7.5 g/m(2)) to a Vertisol in t
he summer-dominant rainfall region of northern Australia. Recoveries o
f applied N by each of the 4 crops in order of cropping sequence were
60.3+/-4.2, 4.4+/-2.3, 1.3+/-0.49, and 0.8+/-0.56%, there being no eff
ect of 2 tillage treatments, conventional tillage (CT) and no till (NT
), on uptake of applied N. There was very low recovery of residual fer
tiliser N after the first wheat crop was harvested; usually <10% of th
e applied N was recovered. There was evidence of a substantial N carry
over benefit where fertiliser N (7.5 g/m(2)) was applied in 1987, but
not when applied at the same rate in 1988 or 1989. Carryover effect wa
s shown only when fertiliser N was applied after a long fallow when an
tecedent NO3--N was already high (100-150 v. 30-55 kg/ha with a normal
summer fallow). Carryover of subsoil NO3--N from a single N fertilise
r application to the crop, as occurred with application in 1987, will
provide useful buffer for declining N supplies of soil N in seasons of
good crop response. Routine application of N at moderate rates (<75 k
g/ha) provides an effective means of supplementing declining soil N re
serves for winter cereals in this region of unreliable rainfall.