Nitrogen Use Efficiency and Nitrogen Fertilizer ...

Nitrogen Use Efficiency and Nitrogen Fertilizer Recovery of Durum Wheat Genotypes as Affected by Interspecific Competition

Abstract

A better understanding of the genotype response to N fertilization under weed competition is necessary to identify varieties that exhibit high N use efficiency even when weeds compete for available N. Such varieties may be more suitable for low input or organic systems. This study assessed the variations in nitrogen use efficiency (NUE) (and its components) and the recovery of 15N-labeled fertilizer in three durum wheat (Triticum durum Desf.) genotypes (one landrace and two varieties that differ in terms of plant growth, grain yield potential, and adaptability to stressful environments) grown in the presence or absence of interspecific competition and varying soil N availability (0 or 80 kg N ha–1 fertilization). The results showed that wheat genotypes had different grain yield potentials and the yields were similar when plants were grown in conditions of low N availability and in presence of interspecific competition. Differences among genotypes in N uptake efficiency were very small, and the low NUE value observed for the landrace seemed to be due to its reduced ability to use absorbed N for increasing grain yield compared with the two varieties. Furthermore, the genotypes showed different competitive abilities against competitor, and seemed to depend on the genotypes’ ability to reduce resource availability (N) for their competitors rather than on their ability to tolerate a reduction in contested resources due to competitors.

O

ver the last four decades, N fertilization has been an essential tool for increasing crop yield and quality, especially for cereals, and for ensuring maximum economic yield (Hirel et al., 2001). However, the energetic cost of synthesizing N fertilizers is very high (Smil, 2001), and N fertilization often represents the most expensive energy input in cereal-based cropping systems (Crews and Peoples, 2004). Moreover, because of its high mobility in the soil–plant–atmosphere system, N greatly contributes to agriculture-related pollution through leaching, volatilization, and denitrification (Drinkwater et al., 1998; Limaux et al., 1999). Indeed, it has been estimated that often 50% or less of the N fertilizer applied to soil is recovered by cereals and that this percentage decreases as the N fertilizer rate increases (Foulkes et al., 1998; Raun and Johnson, 1999; Blankenau et al., 2002). Developing cropping systems and management practices that improve the ability of crops to absorb N could minimize the potential for N losses. Nitrogen use efficiency is generally defined as the grain yield produced per unit of N available from the soil and fertilizer (Moll et al., 1982); it is the product of two physiological factors: (i) N uptake efficiency (NUpE, defined Dipartimento di Agronomia Ambientale e Territoriale, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy. Received 30 Sept. 2009. *Corresponding author ([email protected]). Published in Agron. J. 102:707–715 (2010) Published online 01 Feb. 2010 doi:10.2134/agronj2009.0380 Available freely online through the author-supported open access option. Copyright © 2010 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

as the amount of N uptake by the crop per unit of N available to the crop) and (ii) N utilization efficiency (NUtE, defined as the grain yield per unit of N uptake by the crop). With regard to management practices, the choice of plant variety is particularly important; in fact, several studies have shown that many crop species have genetic variability for NUE (Fageria et al., 2008) and that the use of the best-adapted genotype can contribute to improved efficiency in how cereal crops acquire and use soil N or fertilizer N. Foulkes et al. (1998) found that modern wheat varieties were less efficient at recovering soil N than older varieties, which suggests that old varieties may be the best choice for low input and organic growing systems. In contrast, other researchers (Le Gouis et al., 2000; Brancourt-Hulmel et al., 2003; Guarda et al., 2004) have found that NUpE and NUE have increased with the introduction of improved varieties, and that modern varieties give the best results even under limited N availability. Sylvester-Bradley and Kindred (2009) state that wheat breeding has greatly increased grain yield and that this improvement has been associated with an increase in optimum N rate; the increase in N fertilizer use has counter-acted the improvement in grain yield, resulting in a static NUE at optimum N levels. The varieties suited for low input or organic systems should combine high N use efficiency with superior competitive ability against weeds. To this end, it is also necessary to take into account the fact that N application can significantly affect the competitive interactions between the crop and the weeds and that N application often increases the competitiveness of weeds more than that of the crop (Ampong-Nyarko and De Datta, 1993; Dhima and Eleftherohorinos, 2001, 2005; Blackshaw et al., 2003; Blackshaw, 2005). Abbreviations: NUE, nitrogen use efficiency; NUpE, nitrogen uptake efficiency; NUtE, nitrogen utilization efficiency; 15N REC , labeled-fertilizer nitrogen recovery on an area basis; %15N REC , labeled-fertilizer nitrogen recovery on percentage basis.

A g ro n o my J o u r n a l • Vo l u m e 10 2 , I s s u e 2 • 2 010

707

Nutrient Cycling & Uptake

Dario Giambalvo, Paolo Ruisi, Giuseppe Di Miceli, Alfonso Salvatore Frenda, and Gaetano Amato*

Therefore, development of crop varieties suitable for low input or organic systems (i.e., varieties with a high NUE even when N is contested by weeds) requires knowledge about how different varieties respond to N fertilization under weed competition, yet the currently available information is inadequate. This study assessed variations in NUE and its components and the recovery of 15N-labeled fertilizer among three durum wheat genotypes that differ in terms of plant growth habit, grain yield potential, and adaptability to stressful environments; these plants were grown in the presence or absence of interspecific competition and under conditions of varying soil N availability. Materials and Methods Experimental Site The experiments were conducted during two consecutive growing seasons in 2004–2005 and 2005–2006 at the experimental farm Pietranera, located about 30 km north of Agrigento, Italy (37º30' N, 13º31' E; 178 m). In 2004–2005, the soil was a Chromic Haploxerert with a clay texture (501 g kg–1 clay, 227 g kg–1 silt, and 272 g kg–1 sand; pH 8.1; 14.5 g kg–1 total C and 1.10 g kg–1 total N). In 2005–2006, the experiment was performed on a Vertic Haploxerept with a sandy–clay–loam texture (273 g kg–1 clay, 249 g kg–1 silt, and 488 g kg–1 sand; pH 8.0; 7.4 g kg–1 total C and 0.86 g kg–1 total N). The climate of the experimental site is semiarid Mediterranean with a mean annual rainfall of 551 mm, concentrated mostly during the autumn-winter period (September–February; 74%), followed by spring (March–May; 18%). There is a dry period from May to September; the mean air temperatures is 15.9ºC in autumn, 9.8ºC in winter, and 16.5ºC in spring. Mean minimum and maximum year temperatures are 10.0 and 23.3ºC, respectively. The weather data were collected from a weather station located within 500 m of the experimental site. Experimental Design and Crop Management The experiments were set up in a split-split-plot design with four replications. Each main plot received a different level of N application (0 or 80 kg N ha–1; hereafter referred to as N0 and N80, respectively). Within each main plot, subplots were planted with three different durum wheat genotypes (i.e., Simeto, Valbelice, and Russello). Sub-subplots represented interspecific competition treatments (present or absent). The size of each sub-subplot was 2.4 by 3.5 m (12 rows, each 3.5 m long, spaced at 0.20 m). Two N levels were applied. N0 is an extreme condition, chosen to evaluate the response of each genotype under low N availability; N80 is the rate usually applied to durum wheat in the study area. The three Italian durum wheat genotypes used in the experiment differed in their morphophenological traits. Simeto (released in 1988) is the most widely grown variety in southern Italy and Sicily. It is characterized by short plant stature, early heading and maturity, high yield potential, and good pastamaking quality. Valbelice (released in 1992) is characterized by medium-tall stature, early heading and maturity, high yield potential, and good adaptation to poor environments. Russello is a landrace that was widely grown in Sicily in the first half of the last century; it is now grown in small areas mainly to satisfy local consumers who appreciate the sensorial properties of its products (bread in particular). It is characterized by tall plant stature, high tillering capacity, medium-late heading and maturity, moderate 708

productivity, and good adaptability to environments characterized by scarce water and nutrient resources. A surrogate weed was needed to obtain homogenous weed density across the experimental plots. Barley (Hordeum vulgare L.) was chosen because it has greater competitive ability than wheat due to its greater seedling vigor and tillering capacity, denser growth habit, and higher NUE (Satorre and Snaydon, 1992; Lanning et al., 1997; Delogu et al., 1998). A variety with medium-late heading and maturity (Marado) was used. In both years, the previous crop planted in the fields was wheat, and the soil was plowed in August and harrowed after the first autumn rainfalls. Phosphate fertilizer was applied before sowing at 30 kg P ha–1 as triple superphosphate. Plots were hand-sown during the first 10 d of January using 350 germinable wheat seeds per square meter. In the relevant sub-subplots, barley was seeded in the same row as the wheat to maximize interspecific competition. Barley was planted at 100 germinable seeds per square meter. Spontaneous weeds were controlled at an early growth stage with an application of Diclofop methyl (0.546 g ha–1) and Triasulfuron (7.4 g ha–1). Ammonium sulfate fertilizer was applied at the time of seed emergence. Plots were labeled with 15N fertilizer [80 kg N ha–1 as (NH4)2SO4 with an isotopic enrichment of 1.57 atom%] added to a 2.10 m2 (1.4 × 1.5 m) area in the middle of the plot, following the application procedure described by Høgh-Jensen and Schjoerring (1994); the rest of the plots (outside of the 15N-labeled area) received equivalent amounts of unlabeled fertilizer. Soil samples (0–0.40 m layer) were collected from each plot immediately before sowing and soon after wheat harvest and analyzed for 2 mol L–1 KCl-extractable NH4– and NO3–N using a Bran & Luebbe II AutoAnalyzer. At sowing, soil test N values were 11.3 ± 0.7 mg kg–1 in 2004–2005 and 9.8 ± 0.5 mg kg–1 in 2005–2006. Ideally, soil samples would have been collected from a depth greater than 0 to 0.40 m to obtain a more accurate estimate of the total mineral N in the soil profile; however, the soil strength, which increased as the soil dried out, made it impossible to penetrate beyond a depth of 0.40 m at wheat harvest. At harvest, a 1 m2 portion at the center of each plot was sampled, sorted by species (wheat and barley), oven-dried at 60ºC for 72 h, weighed, ground to a fine powder (sieved using a 0.1-mm mesh size) in a fast running mill, and analyzed for total N and 15N enrichment. The concentrations of total N and 15N were determined with elemental analyzer-isotope ratio mass spectrometry (Carlo Erba NA 1500). Plant height, intensity, and extension of lodging, heading and maturity date, grain yield and its components (spike number per square meter, kernel number per spike, 1000-kernel weight), and N grain content were recorded for both wheat and barley. Calculations and Data Analysis For both wheat and barley, data on 15N enrichment of biomass were used to calculate the labeled-fertilizer N recovery (15NREC) on an area basis (kg N ha−1) and percentage basis, according to Hauck and Bremner (1976): 15 N fp - 15 N nfp and 15 N REC ´100 % N REC = 15 f N fert - N nfp where Nt was the plant N content measured at maturity (kg ha−1); 15N was the atom% 15N in the fertilized plants; 15N was the fp nfp 15

15

N REC = N t ´ 15

Agronomy Journal • Volume 102, Issue 2 • 2010

Fig. 1. Daily air temperature and daily and accumulated rainfall at the experimental site during the two growing seasons (2004– 2005 and 2005–2006); the 30-yr average daily temperatures and accumulated rainfall are also included. Major events are plotted on the x axis (S = sowing; E = emergence; N = 15N application; H = heading; Hst = harvest).

atom% 15N in the nonfertilized plants; 15Nfert was the atom% 15N in the fertilizer, and f was the fertilizer rate (kg N ha−1). Nitrogen efficiency parameters were calculated according to Moll et al. (1982) and Huggins and Pan (1993). Nitrogen use efficiency was defined as the ratio of grain produced (Gw, kg ha–1) to N supply (Ns, kg N ha–1), where Ns was estimated as the amount of applied N (f) plus aboveground plant N (Nt, kg N ha–1) plus residual postharvest N in the soil (Nsph, kg N ha–1), both determined from control plots (no applied N). Nitrogen uptake efficiency (NUpE) was calculated as Nt/Ns; N utilization efficiency (NUtE) was determined as Gw/Nt. All measured variables were assumed to be normally distributed. All variables corresponding to proportions were arcsine transformed before analysis to assure a better fit with the Gaussian law distribution. Data from each year were analyzed separately, and the homogeneity of variances was assessed using Bartlett’s test before combined analyses were performed. Analysis of variance (procedure ANOVA, SAS Institute, 2004) was performed according to the experimental design on the combined 2-yr data set. Years were treated as random factors while all other factors were treated as fixed. Treatment means were compared using Fisher’s protected LSD test at the 5% probability level. Where treatment × year interactions were significant, LSDs were calculated separately for each year. Results Weather Conditions The weather conditions during the experimental period are shown in Fig. 1. Total rainfall in 2004–2005 was 836 mm, 54% higher than the long-term average for the area. About 400 mm

of rainfall was recorded during the winter, frequently causing excessive soil water conditions but without any apparent effect on plant density and root diseases. The mean monthly temperature was lower than average, particularly during the first phase of the growing season (January−March). During 2005–2006, total rainfall was 539 mm, which is very close to the long-term average for the area. Rainfall was adequately distributed and this was reflected in satisfactory plant growth and yield for the trial environment. Mean year temperature was similar to the normal mean temperature, but wide fluctuations were observed on a daily basis, particularly in the last period of the growing season. The seasonal differences in climate conditions between the two growing seasons likely account for the year and year × treatment interaction effects for all measured traits. Wheat Heading Time, Biomass, and Grain Yield As expected, the latest-heading genotype was the landrace Russello, which headed 11 and 13 d later on average than Simeto and Valbelice, respectively. The variety of barley used in the experiment (Marado) headed as late as Russello. All treatments had significant effects on plant height, biomass production, grain yield, and grain yield components (Table 1). In general, the landrace Russello displayed greater plant stature, higher biomass production, and lower grain yield than the varieties Simeto and Valbelice (Table 2), particularly in the second year when favorable climatic conditions induced excessive vegetative growth and, therefore, early lodging of Russello. On average, N application had a significant effect on grain yield. However, different responses were observed among wheat genotypes and


709

Table 1. Results of the ANOVA for the effects of year, genotype, N application, and interspecific competition on plant height, yield and yield components, N uptake, grain protein content, and N efficiency parameters. Source of variation

df

Plant height Biomass Grain

No. of No. of Grain spikes kernels 1000-kernel N protein –2 m per spike weight uptake content NUE†

NUpE

NUtE %15NREC

Year (Yr) N application (NA) Yr × NA Genotype (G) Yr × G NA × G Yr × NA × G

1 1 1 2 2 2 2

** *** *** *** ns ns **

* *** ns *** ns ns ns

ns‡ *** ns *** * *** *

* * ns ** ns * ns

** * ns *** * * ns

*** ** * *** *** * ***

** *** ns ns ns ns ns

* ns * *** ns *** ns

*** *** ns *** ns ** ns

ns *** ns ** ns ns ns

*** ns * *** ns ** ns

** – – ns ** – –

Interspecific competition (IC)

1

ns

***

***

***

***

***

***

**

***

***

ns

***

Yr × IC NA × IC G × IC Yr × NA × IC Yr × G × IC NA × G × IC Yr × NA × G × IC

1 1 2 1 2 2 2

ns ns ns ns ** ns ns

*** ** * ns ns ** ns

*** ** *** ns ** * ns

** ** ns ns ns ns ns

ns ns * ns ns ns ns

ns ns *** ns * ns ns

** ** *** ns *** ns ns

*** ns ns ns ns ns **

*** ns *** ns ns ns ns

*** ns *** ** *** ns ns

ns ns ns ns ns ns ns

ns – ** – ns – –

* Significant at P = 0.05. ** Significant at P = 0.01. *** Significant at P = 0.001. † NUE, nitrogen use efficiency; NUpE, nitrogen uptake efficiency; NutE, nitrogen utilization efficiency; %15 NREC , labeled-fertilizer nitrogen recovery on an percentage basis. ‡ ns, not significant at P = 0.05.

between the 2 yr. The grain yield of Simeto and Valbelice greatly improved with increasing N availability (+43% and +39% on average, respectively). In contrast, for Russello, N application led

to only a small increase in grain yield in the first year (+17%) and a reduction in yield in the second year (−22%) due to the increase in lodging. On average, wheat grain yield decreased by 27% when

Table 2. Means and effects of wheat genotype, N application, and interspecific competition on plant height, biomass production, grain yield, and its components in 2004–2005 and 2005–2006 growing seasons

Genotype

No. of spikes m–2

Plant height

Biomass

Grain

cm

Mg DM ha–1

Mg ha–1

No. of kernels per spike

1000-kernel weight

N Interspecific application competition 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 g

Simeto

N0

absent

54

72

5.21

7.42

2.43

2.98

192

224

20.5

23.6

62.0

56.5

Simeto

N0

present

54

69

4.76

4.02

2.08

1.54

183

204

19.2

15.2

59.3

49.7

Simeto

N80

absent

68

78

8.50

10.50

3.61

4.46

221

263

28.9

30.0

56.6

56.4

Simeto

N80

present

68

75

6.23

5.50

2.68

2.13

210

191

23.9

22.8

53.5

49.0

Valbelice

N0

absent

76

100

5.59

7.24

2.61

2.91

198

265

27.0

24.3

48.7

45.3

Valbelice

N0

present

81

94

5.11

5.42

2.29

2.17

192

213

25.9

23.4

46.0

43.6

Valbelice

N80

absent

104

107

9.33

11.13

3.97

4.25

243

305

36.2

31.0

45.2

44.9

Valbelice

N80

present

103

108

7.18

7.66

3.01

2.66

229

225

29.8

26.8

44.1

44.1

Russello

N0

absent

111

143

7.40

8.95

2.26

2.14

248

258

20.3

18.4

45.0

45.3

Russello

N0

present

104

145

6.49

7.09

2.03

1.74

239

235

19.2

16.4

44.2

45.1

Russello

N80

absent

142

129

9.95

10.11

2.65

1.52

247

244

24.9

14.6

43.0

42.6

Russello

N80

present

142

136

8.96

8.07

2.37

1.50

232

222

24.5

16.4

41.7

41.4

Mean effects genotype Simeto

67c†

6.52c

2.70a

2.78a

211b

23.1b

22.9b

57.9a

52.9a

Valbelice

97b

7.33b

2.97a

3.00a

234a

29.7a

26.4a

46.0b

44.5b

Russello

132a

8.38a

2.3 b

1.73b

241a

22.2b

16.5c

43.5c

43.6b

N application N0

80b

104a

6.23b

2.27b

221b

21.1b

50.9a

47.6a

N80

105a

106a

8.59a

2.90a

236a

25.8a

47.4b

46.4a

Interspecificw competition Absent

99a

7.66a

9.23a

2.92a

3.04a

225a

260a

25.0a

49.3a

Present

98a

6.46b

6.29b

2.41b

1.96b

214a

215b

22.0b

46.8b

† Within-column treatment means followed by the same letter are not different according to Fisher’s protected LSD test at P = 0.05.

710


Fig. 2. Effects of N doses (N0 and N80) on barley biomass at maturity, as affected by wheat genotype (S = Simeto; V = Valbelice; R = Russello) and year (2004–2005 and 2005–2006). Different letters indicate significant differences at P < 0.05.

a competitor was present, but different responses were observed among genotypes. In particular, the decrease in grain yield due to interspecific competition was 38% for Simeto and 26% for Valbelice, whereas it was only 11% for the landrace Russello. The effects of interspecific competition were stronger when N fertilizer was applied (−23% for N0 and −30% for N80, on average). Barley Biomass and Nitrogen Uptake In 2004–2005, wheat genotype and N application significantly affected the biomass of barley. Barley biomass and N uptake at maturity were higher with Simeto and lower with Russello (Fig. 2 and 3), at both N0 and N80 fertilizer rates (2.31 and 0.97 Mg DM ha–1 and 19.7 and 7.4 kg N ha–1, on average for the two genotypes, respectively). Nitrogen application increased barley biomass by 240% and N uptake by 220% compared to unfertilized treatments, and the response was similar across the three wheat genotypes. In 2005–2006, barley biomass and N uptake were significantly affected by wheat genotype (Fig. 2 and 3) with effects similar to those observed in the previous year (Simeto > Valbelice > Russello), but N application did not significantly influence barley biomass production (3.20 and 3.33 Mg DM ha–1 at N0 and N80, respectively) and resulted in only a small increase in N uptake (+7%). In both years, the differences observed among wheat genotypes in N uptake by barley were greater than the differences among wheat genotypes in barley biomass yield. This was due to the difference in N concentration in the barley biomass, which was lower when barley grew in competition with the landrace Russello than when it grew in competition with the other two varieties. In both years, the number of barley spikes per square meter was closely related to barley

Fig. 3. Effect of N doses (N0 and N80) on barley N uptake (bars) and N concentration in barley biomass (lines), as affected by wheat genotype (S = Simeto; V = Valbelice; R = Russello) and year (2004–2005 and 2005–2006). Different letters indicate significant differences at P < 0.05.

biomass and ranged from 33 to 152 spikes m–2 in 2004–2005 and from 58 to 167 spikes m–2 in 2005–2006. Nitrogen Uptake and Grain Protein Content In both years, the total N uptake of wheat varied significantly with N application and interspecific competition. Total wheat N uptake was always significantly greater when N fertilizer was applied compared to unfertilized conditions (Table 3), which is consistent with higher wheat biomass and grain yield. Interspecific competition resulted in decreases in N uptake of 21 and 30% in the first and second years, respectively, compared to the N uptake of competitor-free wheat. No differences in N uptake among the three wheat genotypes were found in either year. The N application × interspecific competition interaction was significant, and the detrimental effect of interspecific competition on N uptake was greater when N fertilizer was applied. In 2005–2006, the detrimental effect of interspecific competition on wheat N uptake was very large for the two varieties (−53% for Simeto and −31% for Valbelice) and almost absent for the landrace Russello (−1%). In both years, significant differences among genotypes were observed for grain protein content, with Russello having the highest value. No significant differences were observed between Simeto and Valbelice. On average, N application had no significant effect on grain protein content in either year. Interspecific competition decreased grain protein content in 2005–2006 only (−11% on average compared to competitorfree wheat). A significant interaction between genotype and N application was detected. Nitrogen fertilization led to an increase in grain protein content only in the landrace Russello and had almost no effect in Simeto and Valbelice.


711

Table 3. Means and effects of wheat genotype, N application, and interspecific competition on N biomass content, grain protein content, N efficiency indices, and recovery of 15N fertilizer in the 2004–2005 and 2005–2006 growing seasons.

Genotype

Grain protein N uptake content NUE† NUpE NUtE %15NREC N Interspecific application competition 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 2004–05 2005–06 kg ha–1

g kg–1

kg kg–1

Simeto

N0

absent

57.1

80.4

130

149

30.9

28.8

0.70

0.77

44.7

37.3

–

Simeto

N0

present

52.8

39.7

127

124

23.6

14.2

0.59

0.36

40.3

39.4

–

– –

Simeto

N80

absent

84.2

123.4

123

145

23.9

24.3

0.55

0.67

43.3

36.3

27.7

23.0

Simeto

N80

present

52.9

56.9

113

131

16.8

11.4

0.33

0.30

50.3

37.8

16.4

7.7

Valbelice

N0

absent

61.6

80.0

131

139

31.5

28.2

0.74

0.78

42.6

36.2

–

–

Valbelice

N0

present

51.6

55.8

131

126

28.1

19.1

0.63

0.49

44.5

39.1

–

–

Valbelice

N80

absent

85.5

115.4

128

147

25.7

23.8

0.55

0.65

47.1

37.0

28.7

22.4

Valbelice

N80

present

64.8

79.4

127

134

19.7

13.8

0.42

0.41

47.0

33.7

17.4

12.3

Russello

N0

absent

63.0

71.0

139

153

27.3

23.3

0.76

0.77

36.2

30.3

–

–

Russello

N0

present

54.0

67.9

150

143

26.4

17.3

0.69

0.65

38.6

27.1

–

–

Russello

N80

absent

84.8

86.4

145

179

17.2

8.9

0.55

0.50

31.4

17.9

30.2

12.7

Russello

N80

present

68.2

88.0

155

165

15.9

8.2

0.46

0.48

34.7

17.6

23.7

10.4

15.4ab

Mean effects Genotype Simeto

68.4a‡

130b

21.7b

0.53c

41.2a

22.1b

Valbelice

74.3a

133b

23.7a

0.58b

40.9a

23.1b

17.4a

Russello

72.9a

154a

18.1c

0.61a

29.2b

27.0a

11.6b

N application N0

61.2b

135a

139a

24.9a

0.66a

41.2a

34.9a

–

–

N80

82.5a

132a

150a

17.5b

0.49b

42.3a

30.1a

24.0

14.8

Interspecific competition Absent

72.7a

92.8a

133a

152a

26.1a

22.9a

0.64a

0.69a

36.7a

24.1a

Present

57.4b

64.6b

134a

137b

21.8b

14.0b

0.52b

0.45b

37.5a

14.7b

† NUE, nitrogen use efficiency; NUpE, nitrogen uptake efficiency; NutE, nitrogen utilization efficiency; %15 NREC , labeled-fertilizer nitrogen recovery on percentage basis. ‡ Within-column treatment means followed by the same letter are not different according to Fisher’s protected LSD test at P = 0.05.

Nitrogen Efficiency Nitrogen use efficiency was significantly lower for Russello (Table 3) than for Valbelice and Simeto (−13% in 2004–2005 and −30% in 2005–2006). In both years, mean value of NUE for the control treatment (N0) was 43% higher than that of the treatment using 80 kg N per ha. The detrimental effect of N application on NUE was greater in Russello than in Valbelice and Simeto (interaction genotype × N application significant at P < 0.001). Interspecific competition always reduced NUE as compared to competitor-free wheat (−17% in 2004–2005 and −39% in 2005–2006). The effects of competition were stronger in the two varieties (interaction genotype × interspecific competition significant at P < 0.001). Nitrogen fertilization led to a strong reduction in NUpE (−30% in the first year and −22% in the second year) compared to control plots (N0). Interspecific competition had an appreciable effect on NUpE; values decreased more with decreasing plant stature (Simeto > Valbelice > Russello). On average, the shortest variety, Simeto, had a lower NUpE than Valbelice and Russello; this result was related to the low N uptake efficiency of Simeto when it competed with barley for N. No differences were observed among wheat genotypes for NUpE when the wheat was grown without N application and without interspecific competition. In both years, NUtE varied significantly with wheat genotype, with lower values for the landrace Russello (29.2 on average) than for Valbelice (40.9) or Simeto (41.2). In Russello, the increase in N uptake with increased N fertilization was not 712

associated with an increase in grain yield. Neither N application nor interspecific competition had a significant effect on NUtE. Recovery of Nitrogen-15 Fertilizer In 2004–2005, the percentage recovery of labeled N fertilizer from different wheat genotypes in the absence of interspecific competition was 27.7 and 30.2% for Simeto and Russello, respectively (Table 3). On average, interspecific competition resulted in a 34% reduction of %15NREC , with the decrease being greater for the two modern varieties than for the landrace (interaction genotype × interspecific competition significant at P < 0.001). In the second year, the %15NREC of competitorfree wheat was much lower in Russello than in Simeto and Valbelice. This result was probably due to the lodging phenomena that occurred during the early reproductive phase of the Russello landrace. Even during the second year, interspecific competition caused a large decrease in %15NREC (−48% on average). This reduction differed across wheat genotypes (−67% for Simeto, −45% for Valbelice, and −18% for Russello). In both years, the highest %15NREC for barley was observed when barley grew in competition with Simeto (12.5% of the applied fertilizer on average); the lowest was observed with Russello (3.6% on average), which was statistically similar to the %15NREC observed when barley grew in competition with Valbelice (6.2% on average) (Fig. 4). Agronomy Journal • Volume 102, Issue 2 • 2010

Discussion As expected, in the absence of competition, grain yield was significantly lower in the landrace Russello than in the other two varieties; these differences increased when N was applied mainly because of Russello’s high susceptibility to lodging. These results are consistent with those of other authors who have compared old and modern varieties under different N soil levels (Wall et al., 1984; Guarda et al., 2004). It is noteworthy that modern wheat varieties (generally dwarf or semi-dwarf) have been selected because of their ability to respond to N fertilizer application by increasing their grain yield rather than by growing taller (Jensen, 1978). Decreases in wheat grain yield due to interspecific competition were larger in N fertilized treatments than in unfertilized treatments, indicating that N application increased the competitive ability of barley (used in this experiment as a surrogate weed) more than that of wheat, as has been observed for many agricultural weeds (Dhima and Eleftherohorinos, 2001; Blackshaw et al., 2003; Blackshaw, 2004). This was confirmed by the response of barley biomass at maturity, which was markedly higher when N was applied than in unfertilized treatments. Many studies have highlighted an association between grain yield reduction in wheat and weed biomass at maturity (Lemerle et al., 1996, 2000; Gill and Coleman, 1999). The detrimental effect of interspecific competition on wheat grain yield varied significantly among genotypes. In both years, yield reductions were smallest in the tallest genotype (Russello) and largest in the shortest genotype (Simeto). Many authors have found a positive correlation between wheat plant height and competitive ability against weeds (Lemerle et al., 1996; Ogg and Seefeldt, 1999; Gonzales Ponce and Santin, 2001). Notably, no significant differences among wheat genotypes were observed for grain yield when the crops grew in the presence of competition and under limited N availability (N0 treatment). In contrast, Vandeleur and Gill (2004) found that high yielding modern wheat varieties had yields superior to those of older varieties in both weedy and weed-free environments, and Guarda et al. (2004) achieved higher yields with modern varieties than with old populations or varieties in both N-poor and N-rich environments. To explain the differences between these findings and our results, one must considered that we analyzed the two factors (weed competition and soil N availability) simultaneously and that the competitive ability of wheat genotypes was differently affected by varying soil N availability. In both years, when wheat was grown in the absence of the competitor, N application led to an increase in total N uptake of about 40%. This result was expected because wheat generally responds positively to increasing soil N availability (Raun and Johnson, 1999). In 2004–2005, no differences in N uptake were observed among wheat genotypes grown without interspecific competition. In 2005–2006, when N supply was 25% higher on average than in 2004–2005, N uptake was lower for the landrace Russello than for Simeto and Valbelice, particularly in plots where N fertilizer was applied. These differences were probably due to the lodging that occurred in the early reproductive phase of Russello and indicate an inability of this old tall genotype (Russello) to use N when it is available at high levels. However, when N is a limiting factor, the ability of Russello to explore the soil and absorb available N did not differ from that of the other two varieties. There are conflicting reports on the relationships between plant height or year of variety release and total N uptake for wheat. Austin et al. (1977) found a general reduction in total wheat N uptake for semi-dwarf

Fig. 4. Recovery of 15N fertilizer by barley, as affected by wheat genotype (S = Simeto; V = Valbelice; R = Russello) and year (2004–2005 and 2005–2006). Different letters indicate significant differences at P < 0.05.

lines compared with taller lines. Ortiz-Monasterio et al. (1997) and Guarda et al. (2004) reported that wheat breeding has led to progressive improvement of N assimilation capacity, regardless of N supply. On the other hand, other authors have found no relationship between N uptake and year of release of the variety or plant height (Fischer and Wall, 1976; Paccaud et al., 1985; Feil and Geisler, 1989; Calderini et al., 1995; Motzo et al., 2004). Possible explanations for this disagreement include the different genotypes used, the different crop management schemes applied (e.g., timing and application method of N fertilizer), and the different climatic and soil conditions (i.e., water availability, soil fertility, and N availability) in which these studies were performed. The presence of interspecific competition led to a consistent decrease in wheat N uptake. On average, this reduction was larger when N fertilizer was applied than in unfertilized plots. This indicates that barley was more efficient than wheat in taking up N at high soil N levels; this ability was not due to earlier soil N use since barley was as late as the latest genotype (Russello). Similar results have been reported for many agricultural weeds, such as wild mustard (Sinapis arvensis L.), wild oat (Avena fatua L.), littleseed canarygrass (Phalaris minor Retz), and blackgrass (Alopecurus myosuroides Huds.) grown with winter cereals (Henson and Jordan, 1982; Satorre and Snaydon, 1992; Dhima and Eleftherohorinos, 2001, 2003, 2005). Wheat genotypes showed different levels of ability to take up N in the presence of the surrogate weed. The reduction in N uptake due to the presences of the competitor was greater in Simeto (even though it was earlier heading than the competitor) and lower in Russello, and the differences between wheat genotypes increased when N was applied. Therefore, the landrace Russello showed a greater ability to reduce the availability of N for barley, as demonstrated by the extremely low N uptake of barley observed at maturity. In both years, the landrace Russello produced higher grain protein content than the other two varieties. Such differences most likely depended on the strong variations in grain yield among genotypes and therefore on a concentration effect. A negative relationship between grain yield and grain N concentration has been observed by many authors (Calderini et al., 1995; Guarda et al., 2004; Motzo et al., 2004; De Vita et al., 2007). Nitrogen fertilization resulted in an increase in grain protein content only in Russello and had almost no effect in Simeto and Valbelice. Therefore, the latter two varieties used the greater N availability due to N fertilization to increase grain yield, but not to increase protein content. Because wheat plants tend to partition N into grain yield


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in preference to grain protein, it is likely that the N rate adopted in this study (80 kg ha–1) was insufficient to result in the maximum yield and thus the plants used N to increase yield instead of protein. In both years, without interspecific competition, the landrace Russello showed a NUE significantly lower than the two varieties (with no differences between Simeto and Valbelice). Many authors (Le Gouis et al., 2000; Brancourt-Hulmel et al., 2003; Guarda et al., 2004) found that levels of NUE were greater for the modern varieties than for the old varieties. However, Sylvester-Bradley and Kindred (2009) stated that the introduction of new wheat varieties led to no significant change in NUE if the comparison between new and old genotypes was done with the optimum amount of applied N specific to each genotype. Our study, similarly to other studies (Latiri-Souki et al., 1998; Raun and Johnson, 1999; López-Bellido and López-Bellido, 2001; Giambalvo et al., 2004; López-Bellido et al., 2005, 2006; Cabrera-Bosquet et al., 2007), found that N fertilizer application led to a general decrease in NUE (−30%, on average), but these detrimental effects were significantly different across wheat genotypes, ranging from −17% for Valbelice to −50% for Russello (on average over the 2 yr). Nitrogen application had no significant effect on NUtE; therefore, the NUE decrease in high N soil levels can be attributed to the reductions in NUpE when N was applied. Sylvester-Bradley and Kindred (2009), in the UK, observed that there was a progressive decrease in wheat NUE with increasing N application due to decreases in both NUpE and NUtE. Differences between genotypes for NUE mainly depended on the genotypes’ different ability to use the assimilated N in increasing grain yield, as highlighted by the fact that the NUtE value was lower in Russello than in the other genotypes, particularly at high soil N levels. The correlation between NUE and NUpE was small; therefore, the variation in NUpE contributed very little to the variation in NUE among genotypes. Muurinen et al. (2006), in a study aimed at evaluating the achievements of plant breeding on NUE of cereals under northern conditions, found that NUE values were lower in wheat landraces and old varieties than in modern varieties, but in contrast with our results, their research revealed that differences among genotypes have mostly been associated with changes in NUpE. Dhugga and Waines (1989) showed that in wheat, NUpE became more important than NUtE in determining NUE as soil N supply was increased. Other authors (Ortiz-Monasterio et al., 1997; Le Gouis et al., 2000) found that with low N supply, differences in NUE were largely due to variation in NUpE, whereas with high N fertilization, the differences were due to variation in NUtE. More research should be performed to clarify the factors and mechanisms are involved in determining the NUE. Interspecific competition has always reduced NUE in comparison with wheat grown under competitor-free conditions. The effects of competition were significantly smaller in the landrace Russello than in the other genotypes, and these differences increased when N was applied. Nitrogen use efficiency reductions due to competitor are uniquely attributable to variations in the amount of plant N at maturity per unit of potentially available soil N (NUpE), since no variation in the amount of grain production per unit of plant N at maturity (NUtE) was observed due to competition. The percentage recovery of 15N fertilizer of competitor-free wheat was 28.9% on average in 2004–2005 and 19.4% in 2005– 2006. The decrease observed in the second year may be due to 714

the different characteristics of the soils in which the experiments were performed: clay soil with a high cation exchange capacity in the first year and sandy clay loam soil in the second year. It is likely that the characteristics of the latter soil may have favored the nutrient leaching process, negatively affecting N fertilizer uptake. The values recorded in this experiment for recovery of labeled 15N fertilizer distributed on wheat were comparable to values obtained in Tunisia by Sanaa et al. (1992), in Syria by Pilbeam et al. (1997), and in Italy by Giambalvo et al. (2009). López-Bellido et al. (2006), in a study performed in Spain on durum wheat, reported values of labeled 15N-fertilizer recovery ranging from 12.7% when applied at sowing to 41.6% when applied at top dressing. In our study, no differences were found in %15NREC across the three genotypes of wheat when the wheat grown without interspecific competition in the first year, whereas in the second year, Russello showed a lower value than the other two varieties (similar to that observed for N uptake); dependence on lodging affected this genotype. Foulkes et al. (1998) found that the N recovery fraction was greater for newer varieties; the authors ascribed this fact to the capacity of the modern varieties to extend the period of N uptake and, ultimately, increase fertilizer recovery. However, in their trial, they applied N only at top dressing, whereas in our experiment, the N fertilizer was distributed at crop emergence; this could explain the different results between the two studies. The three genotypes of wheat showed different competitive abilities when N was contested by the competitor; the highest reduction in %15NREC was observed for Simeto (average across the 2 yr: −52%), and the lowest for Russello (−20%). These values were strictly related to barley biomass at maturity and confirmed the competitive ability of the landrace Russello against the surrogate weed. In conclusion, this study showed that, although the studied wheat genotypes had different grain yield potentials (Russello < Simeto = Valbelice), the differences between them were very small or absent when they were grown in the presence of interspecific competition and under low N soil availability. Differences in N uptake efficiency between the landrace and the varieties were very small, and the low value of NUE observed for the landrace Russello appears to be due to its reduced ability to use absorbed N for increasing grain yield compared to the other two varieties. Furthermore, the studied genotypes showed different abilities to compete against the surrogate weed; competitive ability increased with increasing plant stature. The different competitive abilities seem to depend on different capacities to reduce the availability of resources (e.g., N, as resulted in this study) to competitor rather than different abilities to tolerate reductions in contested resources. In fact, NUE reductions due to interspecific competition were attributable to variation in N uptake efficiency, which decreased consistently with smaller plant stature. These results could have practical implications for variety choice, particularly in low input or organic systems where weed control and soil N availability are often serious problems. Our results suggest that in weedy and low-N conditions the old, tall varieties are to be preferred. Under such conditions, these varieties can produce similar yields to those of modern dwarf varieties and, due to their greater ability to compete with weeds, they may also have positive effects on subsequent crops due to reductions in the soil weed seed-bank. In addition their greater straw production could be an important source of soil organic matter. Agronomy Journal • Volume 102, Issue 2 • 2010

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