Timing of nitrogen fertilization in wheat under conventional and no-tillage system

Grain yield, N accumulation, and remobilization in wheat (Triticum aestivum L.) may be modified by fertilization timing, tillage system, and environmental conditions. Little information about the combination of tillage effect and fertilization timing on wheat development is available in the southeastern Humid Pampa Region. The objective of this work was to study the timing of fertilization effect under two tillage systems on wheat grain yield and N accumulation and losses. Two field experiments were performed during 1998 and 1999, at Balcarce, Argentina, under no-tillage (NT) and conventional tillage (CT). Nitrogen was applied as 15N-labeled urea at two rates (0 and 120 kg N ha(-1)) and at two times (sowing and tillering). Tillage system affected grain yield only in 1998 when NT promoted better soil water availability conditions. Highest yields were obtained when urea was applied at tillering. Nitrogen in plant derived from fertilizer (Ndff) at physiological maturity ranged from 21.9 to 70.4 kg ha(-1) in the whole plant. Late fertilization increased Ndff recovery in whole plant and in grain. This effect was more pronounced in NT than in CT. No effect of tillage was found along growing season. No dry matter or net total N losses were detected during grain filling both years, but N accumulation in fertilized plants virtually ceased by ear emergence in 1999. A significant N fertilizer loss occurred during grain filling in 1999 (7.8-10.9 kg ha(-1) Ndff). Nitrogen losses were related to low grain yield, high amount of stored N at ear emergence, and environmental conditions during grain filling.