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Published in Agron. J. 95:1525-1531 (2003).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPERS

Timing of Nitrogen Fertilization in Wheat under Conventional and No-Tillage System

Mariana A. Melaj*,a, Hernán E. Echeverríab, Silvia C. Lópeza, Guillermo Studdertb, Fernando Andradeb and Néstor O. Bárbaroa

a Agronomic Section, Comisión Nacional de Energía Atómica, Av. del Libertador 8250, Capital Federal, República Argentina
b Unidad Integrada INTA-FCA Balcarce, C.C. 276, 7620 Balcarce, República Argentina

* Corresponding author (melaj{at}cae.cnea.gov.ar).

Received for publication September 18, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
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.

Abbreviations: CT, conventional tillage • Ndff, nitrogen in plant derived from fertilizer • NT, no-tillage


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
WHEAT IS ONE of the most important crops in Argentina, with an average global production of 14.9 million tonnes over the last 5 yr. Thirty percent of this amount is produced in the southeastern Humid Pampa Region. To increase or maintain productivity without damaging the soil and the environment, an efficient use of fertilizer and conservation tillage is required.

Fertilization timing, tillage system, and environmental conditions may modify grain N yield, N accumulation pattern, and N remobilization in wheat (Johnston and Fowler, 1991; Wuest and Cassman, 1992b; Sarandon et al., 1997; Falotico et al., 1999).

The tillage system affects grain yield and fertilization response through its effects on nutrient and water availability. No-tillage is generally associated with better water storage (Brandt, 1992) but also with soil and fertilizer N unavailability for plants because of lower mineralization rates and greater immobilization generated by surface residues (Rice and Smith, 1984).

Timing of N application can be an adequate strategy to ensure N availability when crops need it or when water is available to enhance N uptake. Generally, applications of fertilizer at sowing increase wheat grain yield, and late fertilizations increase grain protein concentration (Fowler and Brydon, 1989). Nitrogen uptake during the postanthesis period can contribute to total grain N content (Bauer et al., 1987; Bashir et al., 1997). External factors such as soil water or soil and air temperature influence N uptake during grain filling (Harper et al., 1987). Nevertheless, both yield and grain protein mostly depend on N accumulated in wheat at anthesis and on N translocation efficiency to grain (Dalling, 1985; Sarandon et al., 1997).

Several studies on fertilization timing or on fertilization under different tillage systems have been performed in the southeastern Humid Pampa Region (Bergh, 1997; García and Fabrizzi, 1998), but little information about the combination of tillage effect and fertilization timing on wheat development is available in this area. Furthermore, the effective uptake of N from fertilizer applied at different wheat growth stages was not measured by a direct method, i.e., isotopic method.

Isotope 15N studies have proven to be useful for estimating plant N uptake from the various sources of available N in complex systems. Field experiments with 15N-labeled fertilizer give accurate information on the quantities of fertilizer N taken up by the crop, N losses from the soil/crop system, and the amount of unlabeled (i.e., soil-derived) N taken up by the crop (Pilbeam et al., 1996). Most of the research detecting N losses directly from aerial plant parts was performed with 15N-labeled fertilizer (Harper et al., 1987; Palta and Fillery, 1993; Bashir et al., 1997). Losses of N from aboveground parts of wheat plant during grain filling have been attributed to a variety of factors, including losses of plant material (Daigger et al., 1976; Echeverría et al., 1992), leaching by rainfall (Wetselaar and Farquhar, 1980), and NH3 volatilization from senescing leaf tissue (Harper et al., 1987; Parton et al., 1988).

The objective of this work was to study the effect of timing of 15N urea fertilization under two tillage systems on wheat grain yield and N accumulation and losses.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Experimental Design and Treatments
Two field experiments were performed during 1998 and 1999 spring wheat growing seasons at the Balcarce Experiment Station of the Instituto Nacional de Tecnología Agropecuaria, Argentina (37°45' S; 58°18' W; altitude 130 m). Meteorological data, rainfall and air temperature, were collected by the Meteorological Service of the Balcarce Experiment Station. The soil is a complex of a fine, mixed, thermic Typic Argiudoll and a fine, illitic, thermic Petrocalcic Paleudoll (petrocalcic horizon was below 0.7 m) with 2% slope (no erosion). Main properties of the surface horizon are a loam textural class, 65.0 g kg-1 organic C, a pH of 6.0 (1:2.5 in water), 33.1 cmol kg-1 cation exchange capacity, and 5.0 mg kg-1 Bray and Kurtz P.

Red clover (Trifolium pratense L.) had been grown in the area before the beginning of a tillage–rotation study in 1996 when sunflower (Helianthus annuus L.) was cultivated under reduced tillage. In 1997, several tillage treatments were imposed. In 1998, two of them were selected to carry out the present study: NT and CT (moldboard plowing, disking twice, and harrowing). Wheat (‘ProINTA Oasis’, widely grown in the area) was sown in August 1998 and in July 1999 in 12.5-m2 plots with 19.2-cm row spacing and with a sowing density of 370 seeds m-2. Wheat was preceded by corn (Zea mays L.) in 1998 and by sunflower in 1999. Phosphorus fertilizer was drilled with the seed at a rate of 30 kg P ha-1 as superphosphate. In 1998, irrigation was applied on 29 October, 16 November, and 25 November (15 mm each time).

Two rates of N urea (0 or 120 kg N ha-1) were studied at two fertilization timings (sowing or tillering). Consequently, for each tillage system, treatments were 0 (without N fertilization), 120S (120 kg N ha-1 applied at sowing), and 120T (120 kg N ha-1 applied at tillering). The experiment was performed with a randomized complete block design with a split-plot treatment arrangement (tillage system was assigned to the main plots and N level/time of application to the subplots) with three replications. Enriched 15N urea (3.030 and 5.138% 15N a.e. in 1998 and 1999, respectively) was applied in 3.58- by 0.57-m microplots, established in each subplot, which were covered when unlabeled fertilizer was broadcast.

Soil and Plant Analyses
Aboveground wheat biomass samples were collected from each plot at the following Zadoks growth stages (Zadoks et al., 1974): 65 (anthesis) in 1998, 55 (ear emergence) in 1999, and 90 (physiological maturity) in both years. For total N and 15N analysis, all plants from 0.45 m of the two central rows of each microplot were cut at ground level. Total aboveground dry matter and grain yield were determined from larger samples collected outside the microplots. For this purpose, samples were taken at anthesis (1998) or ear emergence (1999) by cutting plants from 0.35 m of five rows at random, and at physiological maturity, by cutting whole plants from 0.5 m (1998) or 1.0 m (1999) of five rows at random. All samples were oven-dried at 65°C until constant weight, weighed, and ground (1 mm mesh).

Total N was determined by Kjeldahl (Axmann et al., 1990) on 0.5 g of oven-dried ground subsamples. To avoid cross-contamination during distillation, the distillator was cleaned with alcoholic solution (1:5 ethanol/water) between samples. Kjeldahl distillates were concentrated and analyzed for N isotope ratios with an optic emission spectrometer NOI 6PC (Fisher ANalysen Instrumente GmbH, Leipzig, Germany). More details for 15N plant analysis procedures are described in López et al. (2002).

Soil samples were collected at sowing; at Zadoks growth stages (Zadoks et al., 1974) 20 (tillering), 73 (early milk), and 85 (soft dough); and at plant sampling to measure NO-3–N and water content. Samples were taken at 40-cm depth, being a composite sample of four subsamples per experimental unit. Nitrate N was determined by microdistillation (Keeney and Nelson, 1982). Water content was determined gravimetrically (López Ritas and López Mélida, 1990).

Standard statistical procedures were used for analysis of variance and calculation of simple correlation coefficients. Means were compared by using the least significant difference (LSD) procedure at the 0.05 probability level (Steel and Torrie, 1980).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Climatic Environmental Conditions
Differences in rainfall and temperature were registered between 1998 and 1999 growing season. During 1998, mean air temperature between May and December (wheat growth season in Argentina) did not differ from the average of the last 20 yr, but rainfall plus irrigation during the same period reached 309 mm, only 54% of the last 20-yr average value for rainfall (570 mm). In 1999, rainfall during the crop growth season (500 mm) was close to the average value, but rainfall was principally accumulated between May and September. During the final grain-filling period, rainfall was scarce, and temperature increased, reaching higher values than in 1998 (Fig. 1a) .



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  		Fig. 1. Mean (a) air temperature and (b) growing degree days (GDD) during wheat 1998 and 1999 growing seasons. EE, ear emergence; A, anthesis; EM, early milk; SD, soft dough; PM, physiological maturity.

 
The differences in temperature and rainfall were reflected in soil water content (Fig. 2) . During 1999, soil water content was high during the vegetative period and declined sharply between ear emergence and early milk stage. In 1998, soil water content decreased early in the growing season. Soil water content was higher under NT at all sampling times. It is generally accepted that soil under NT improves water storage due to lower evaporation losses (Ferreras et al., 1999).



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  		Fig. 2. Soil water content at 0- to 40-cm depth along wheat growing season, under no-tillage (NT) and conventional tillage (CT), in 1998 and 1999. EE 99, ear emergence in 1999; A 98, anthesis in 1998; PM 98 and PM 99, physiological maturity (1998 and 1999, respectively).

 
Grain Yield and Its Components
Considering all of the treatments for each year, grain yields averaged 5102 and 4523 kg ha-1 in 1998 and 1999, respectively. Grain yields were higher under NT than under CT in 1998, even when fertilizer was not applied (Table 1). The effect of tillage system on grain yield could have been related to the soil water content and water use efficiency, as was reported by Brandt (1992) for long-term trials with crop rotations. Greater water content under NT (Fig. 2) was reflected in significantly higher grain yield only in 1998 when water deficiencies were greater throughout the growing season.


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  		Table 1. Wheat grain yield, grain number, and 1000-grain weight under no-tillage (NT) and conventional tillage (CT), as affected by rate and timing of N application in 1998 and 1999. Nitrogen applied as urea: 0 kg N ha-1 (0) and 120 kg N ha-1 at sowing (120S) or at tillering (120T).

 
Applied N increased grain yield and the number of grains per square meter and decreased 1000-grain weight (Table 1). The effect of fertilization on grain yield was more pronounced when N was applied at tillering. The highest yields were obtained when urea N was applied at tillering under NT both years; in spite of that, a significant effect between tillage systems was registered only during 1998. This is the result of high water (Grant and Flaten, 1998) and nutrient availability.

The relationship between grain yield and number of grains per square meter was better in 1998 (r2 = 0.89) than in 1999 (r2 = 0.53). Weight of 1000 kernels was greater in 1998 (34.7 g) than in 1999 (30.4 g). During 1999, grain weight was probably reduced by high temperature and the way that minimum growing degree days needed by wheat was reached. Low temperatures in September and October delayed the accumulation of growing degree days, making grain filling occur during a high-temperature period (Fig. 1a and 1b). Likewise, high temperatures in November and December 1999 would promote an acceleration of senescence, reducing the final weight of growing grains. Number of grains was not affected by temperature differences.

Dry Matter and Nitrogen Accumulation
Dry matter accumulation at physiological maturity was similar for both tillage system and years (Table 2). In 1998, dry matter accumulation at anthesis was greater under CT than NT because of an earlier plant emergence and greater initial growth under CT. Growth rates till tillering (sampling data not shown) were 3.55 and 8.24 kg dry matter ha-1 d-1 under NT and CT, respectively, and significantly differed at P <= 0.05. As the wheat crop grew up and water demand increased, higher water content under NT improved growth rate. Nitrogen accumulation was also associated with water availability.


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  		Table 2. Wheat aboveground accumulated dry matter and total N at anthesis (1998) or ear emergence (1999) and physiological maturity (1998 and 1999), under no-tillage (NT) and conventional tillage (CT), as affected by rate and timing of N application. Nitrogen applied as urea: 0 kg N ha-1 (0) and 120 kg N ha-1 at sowing (120S) or at tillering (120T).

 
Nitrogen application increased aboveground dry matter and N accumulation in the whole plant both years (Table 2). In 1998, N effects were similar for the two timings of fertilization. In 1999, however, N application at tillering enhanced dry matter accumulation in the whole plant and N accumulation in grain. The greater N accumulation was associated with grain yield rather than with grain protein concentration (data not shown). Nitrogen accumulation increase in response to fertilization was higher in 1998 than in 1999, both at anthesis (139 vs. 78%) and at maturity (80 vs. 42%).

The response to fertilization was lower in 1999 when no fertilized plants accumulated more N than in 1998 as a consequence of high soil N availability at sowing (Fig. 3) . Previous crop generated differences in soil N availability. Wheat after corn in 1998 produced greater response to N fertilization than wheat after sunflower in 1999 because corn residues produced N immobilization, just as had been reported by Echeverría et al. (1992).



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  		Fig. 3. Available N as nitrate (NO3–N) in soil at 0- to 40-cm depth in (a) 1998 and (b) 1999 at sowing (S), anthesis (A) or ear emergence (EE), and physiological maturity (PM) under no-tillage (NT) and conventional tillage (CT). 0 = 0 kg N ha-1; 120S = 120 kg N ha-1 at sowing; 120T = 120 kg N ha-1 at tillering.

 
Nitrogen accumulation was generally greater in 1999 at ear emergence than in 1998 at anthesis (Table 2). In 1999, net N accumulation in fertilized plants ceased by ear emergence. Consequently, further growth during grain filling depended on remobilization of N accumulated in the vegetative plant parts.

Preanthesis-accumulated N was better correlated to N concentration and N content in grain at maturity in 1998 (r2 = 0.71 and 0.89, respectively) than in 1999 (r2 = 0.52 and 0.52, respectively). Nitrogen stored in the crop before grain filling during 1999 was not efficiently translocated to grain. Nitrogen harvest index was not affected by year (data not shown) but was better related to harvest index in 1998 than in 1999 (r2 = 0.77 and 0.34, respectively). Partitioning of dry matter and N to the grain are separate processes. The major source of N partitioned to the grain is the remobilization of N resulting from organ senescence whereas the major source of dry matter partitioned to the grain is current photosynthesis (Deckard et al., 1996).

Considering dry matter and total net N content in the whole plant at anthesis (or ear emergency), no losses were detected during grain filling either in 1998 or 1999.

Fertilizer Nitrogen Recovery
Nitrogen in plant derived from fertilizer (Ndff) at physiological maturity ranged from 21.9 to 70.4 kg ha-1 in the whole plant. These values implied a percentage recovery in crop of 18.3 to 58.7% of the 120 kg N ha-1 applied. The lowest values corresponded to N applied at sowing under NT, probably explained by high microbial N immobilization (Rice and Smith, 1984). The lowest recoveries were obtained in 1999 (51.3 and 33.3%, all treatments averages in 1998 and 1999, respectively). Other researchers have reported values of labeled N fertilizer recovery from 18 to 68% (Wuest and Cassman, 1992a; Isfan et al., 1995; Zapata and Hera, 1995; López et al., 2002). Approximately 70% of the fertilizer N recovered in the whole crop was in the grain where Ndff averaged 43.7 kg ha-1 in 1998 and 28.0 kg ha-1 in 1999 (Table 3). Under NT, the recovery of Ndff added at tillering was greater than that added at sowing, independent of the growth stage of wheat (Table 3). An interaction between tillage system and fertilization timing on N fertilizer recovery by the crop was expressed at anthesis in 1998 and at ear emergence in 1999 (Table 3). Under CT, less or equal Ndff was recovered by the crop from tillering application compared with sowing application. In 1998, the initial fast development of wheat under CT (Table 2) would have increased plant N requirements that were satisfied, principally, by N from fertilizer due to low initial soil NO-3–N levels. Greater N immobilization is generally observed under NT during early crop stages (Rice and Smith, 1984; Haugen Kozyra et al., 1993). This would explain the benefit in N recovery when N application was delayed under this tillage system. Timing of fertilization and duration of crop growth following fertilizer application determine the partitioning of labeled urea applied between soil and crop, leading to more Ndff found in crop when the growth period is longer (Pilbeam, 1996). Uptake of fertilizer added at sowing by the crop can be greater than or equal to uptake of fertilizer added at tillering in some locations, especially in dry areas where crop season is short (Pilbeam et al., 1996). Nevertheless, the recovery of N from fertilizer in crop is also conditioned by other factors such as plant requirements and climatic and soil characteristics.


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  		Table 3. Nitrogen derived from fertilizer (Ndff) accumulated at wheat anthesis (1998) or ear emergence (1999) and physiological maturity (1998 and 1999), under no-tillage (NT) and conventional tillage (CT), as affected by rate and time of N application. Nitrogen applied as urea: 120 kg N ha-1 at sowing (120S) or at tillering (120T).

 
Nitrogen uptake is increased around the time of maximum crop growth, so application of fertilization at tillering would increase N fertilizer recovery by the crop. Effectively, the recovery of fertilizer in the whole plant and in the grain was greater when N was applied at tillering under both tillage systems.

In 1999, Ndff was higher in ear emergence than in physiological maturity. Losses of 7.8 to 10.9 kg ha-1 Ndff were registered (Table 3). As it was pointed out before, no losses of dry matter were registered, so the losses of N accumulated at ear emergence can be accountable to physiological processes involving N uptake and N remobilization. While N losses were occurring, N uptake probably continued from recently mineralized soil N as the main source. Bashir et al. (1997) found that fertilizer N losses from plant could be greater in magnitude than total N losses, especially when native soil N uptake was still taking place. Nevertheless, N uptake would have little effect on N losses due to the low rate of postanthesis N accumulation.

Gaseous N losses from aboveground parts of wheat have been related to plant N content (Papakosta and Gagianas, 1991). Between 150 and 200 kg N ha-1 at anthesis, N losses would depend on grain yield. Nitrogen losses from the plants can be avoided with high yields because the sink for accumulated N is large. The low grain yields and the high total N at ear emergence (>150 kg N ha-1) in fertilized crops explains the observed losses in 1999.

High air temperatures and vapor pressure deficits during the anthesis–maturity period may have enhanced volatilization of N, which is an important mechanism of N loss from plant material during maturation (Palta and Fillery, 1993). A large pool of N was available for remobilization to grain, but high temperatures reduced the period of grain filling, accelerated senescence, and reduced the availability of carbohydrates needed to ensure an efficient translocation.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
The better conservation of soil moisture under NT led to a greater grain yield and N use, especially when rainfall was low, as it was in 1998. Nevertheless, adequate water availability by itself did not ensure higher wheat grain yield because above-average air temperature during grain filling shortened grain development period.

Meteorological conditions affected not only N remobilization and postanthesis N uptake but also the proportion of N uptake from soil and fertilizer. When high temperatures are registered during grain filling, a high accumulation of N before that period does not ensure an efficient remobilization because N losses can be promoted. In 1999, N losses were related to the great amount of accumulated N at ear emergence and to high temperature and low air humidity during grain filling, which accelerated senescence.

In conclusion, fertilizer application at tillering increases grain yield and N recovery from labeled urea when soil organic matter and mineralization rates ensure adequate mineral N availability for initial crop development. Urea 15N recovery at physiological maturity, in the whole plant and in grain, was not affected by tillage system even though environmental conditions were different.


   ACKNOWLEDGMENTS
 
The authors thank O. Martín and A. Solís for sample analysis. They also thank the field and laboratory crew at Vegetal Production (INTA Balcarce) for their help with field operations and data collecting. The authors also thank Dr. Fernando Selles for his helpful suggestions and Dr. Felipe Zapata for his friendly collaboration. This work was supported by FAO/IAEA-Arcal XXII, Project ARG/5/008 and Project 15/A107 of the Facultad de Ciencias Agrarias, Universidad Nacional de Mar del Plata.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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