NITROGEN MANAGEMENT Timing of Nitrogen Fertilization in Wheat under Conventional and No-Tillage System Mariana A. Melaj,* Herna ´n E. Echeverrı´a, Silvia C. Lo ´ pez, Guillermo Studdert, Fernando Andrade, and Ne ´stor O. Ba ´ rbaro ABSTRACT tion rates and greater immobilization generated by sur- face residues (Rice and Smith, 1984). Grain yield, N accumulation, and remobilization in wheat (Triticum aestivum L.) may be modified by fertilization timing, tillage system, Timing of N application can be an adequate strategy and environmental conditions. Little information about the combina- to ensure N availability when crops need it or when tion of tillage effect and fertilization timing on wheat development water is available to enhance N uptake. Generally, appli- is available in the southeastern Humid Pampa Region. The objective cations of fertilizer at sowing increase wheat grain yield, of this work was to study the timing of fertilization effect under two and late fertilizations increase grain protein concentra- tillage systems on wheat grain yield and N accumulation and losses. tion (Fowler and Brydon, 1989). Nitrogen uptake during Two field experiments were performed during 1998 and 1999, at Bal- the postanthesis period can contribute to total grain N carce, Argentina, under no-tillage (NT) and conventional tillage (CT). Nitrogen was applied as 15 N-labeled urea at two rates (0 and 120 kg content (Bauer et al., 1987; Bashir et al., 1997). External N ha -1 ) and at two times (sowing and tillering). Tillage system affected factors such as soil water or soil and air temperature grain yield only in 1998 when NT promoted better soil water availabil- influence N uptake during grain filling (Harper et al., ity conditions. Highest yields were obtained when urea was applied 1987). Nevertheless, both yield and grain protein mostly at tillering. Nitrogen in plant derived from fertilizer (Ndff) at physio- depend on N accumulated in wheat at anthesis and on logical 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. N translocation efficiency to grain (Dalling, 1985; Saran- This effect was more pronounced in NT than in CT. No effect of don et al., 1997). tillage was found along growing season. No dry matter or net total Several studies on fertilization timing or on fertiliza- N losses were detected during grain filling both years, but N accumula- tion under different tillage systems have been per- tion in fertilized plants virtually ceased by ear emergence in 1999. A formed in the southeastern Humid Pampa Region significant N fertilizer loss occurred during grain filling in 1999 (7.8– (Bergh, 1997; Garcı´a and Fabrizzi, 1998), but little infor- 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 mation about the combination of tillage effect and fertil- during grain filling. ization timing on wheat development is available in this area. Furthermore, the effective uptake of N from fertilizer applied at different wheat growth stages was W heat is one of the most important crops in Argen- not measured by a direct method, i.e., isotopic method. tina, with an average global production of 14.9 Isotope 15 N studies have proven to be useful for esti- million tonnes over the last 5 yr. Thirty percent of this mating plant N uptake from the various sources of avail- amount is produced in the southeastern Humid Pampa able N in complex systems. Field experiments with Region. To increase or maintain productivity without 15 N-labeled fertilizer give accurate information on the damaging the soil and the environment, an efficient use quantities of fertilizer N taken up by the crop, N losses of fertilizer and conservation tillage is required. from the soil/crop system, and the amount of unlabeled Fertilization timing, tillage system, and environmen- (i.e., soil-derived) N taken up by the crop (Pilbeam et tal conditions may modify grain N yield, N accumulation al., 1996). Most of the research detecting N losses di- pattern, and N remobilization in wheat (Johnston and rectly from aerial plant parts was performed with 15 N- Fowler, 1991; Wuest and Cassman, 1992b; Sarandon et labeled fertilizer (Harper et al., 1987; Palta and Fillery, al., 1997; Falotico et al., 1999). 1993; Bashir et al., 1997). Losses of N from aboveground The tillage system affects grain yield and fertilization parts of wheat plant during grain filling have been attrib- response through its effects on nutrient and water avail- ability. No-tillage is generally associated with better wa- uted to a variety of factors, including losses of plant ter storage (Brandt, 1992) but also with soil and fertilizer material (Daigger et al., 1976; Echeverrı´a et al., 1992), N unavailability for plants because of lower mineraliza- leaching by rainfall (Wetselaar and Farquhar, 1980), and NH 3 volatilization from senescing leaf tissue (Harper et M.A. Melaj, S.C. Lo ´ pez, and N.O. Ba ´ rbaro, Agronomic Section, Comi- al., 1987; Parton et al., 1988). sio ´ n Nacional de Energı ´a Ato ´ mica, Av. del Libertador 8250, Capital The objective of this work was to study the effect of Federal, Repu ´ blica Argentina; and H.E. Echeverrı ´a, G. Studdert, and F. Andrade, Unidad Integrada INTA-FCA Balcarce, C.C. 276, 7620 timing of 15 N urea fertilization under two tillage systems Balcarce, Repu ´ blica Argentina. Received 18 Sept. 2002. *Correspond- on wheat grain yield and N accumulation and losses. ing author (melaj@cae.cnea.gov.ar). Published in Agron. J. 95:1525–1531 (2003).  American Society of Agronomy Abbreviations: CT, conventional tillage; Ndff, nitrogen in plant de- rived from fertilizer; NT, no-tillage. 677 S. Segoe Rd., Madison, WI 53711 USA 1525 Published online November, 2003