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  • Open Access

Effect of split application of different N rates on productivity and nitrogen use efficiency of bread wheat (Triticum aestivum L.)

Agriculture & Food Security20187:92

https://doi.org/10.1186/s40066-018-0242-9

  • Received: 25 September 2018
  • Accepted: 7 December 2018
  • Published:

Abstract

Background

Bread wheat is an important staple and cash crop grown by smallholder farmers in the central highlands of Ethiopia. However, the productivity of the crop is constrained by low soil fertility and poor nitrogen fertilizer management in the area. For example, there is limited information on optimum rates and timing of nitrogen fertilizer application in the area. Therefore, a field experiment was conducted for two consecutive years (2014 and 2015) under rain-fed condition to determine the effect of N fertilizer rate and timing of application on grain yield and nitrogen use efficiency of bread wheat. Factorial combinations of three N levels and five application times plus one control were laid out in a randomized complete block design with four replications.

Results

The optimum grain yield (6060.04 kg ha−1) was recorded when 240 kg N ha−1 was applied ¼ at sowing, ½ at tillering and ¼ at booting, and it showed no significant additional response to N fertilizer above this rate. Higher N level (360 kg N ha−1) always increased N content in the grain and nitrogen uptake by wheat crop. The best recovery of nitrogen (59.74%) by wheat was found when 120 kg of nitrogen was applied (¼ at sowing, ½ at tillering and ¼ at booting). The nitrogen use efficiency traits decreased with increased N rate (120–360 kg N ha−1) indicating poor N utilization. The split application of nitrogen (¼ at sowing, ½ at tillering and ¼ at booting) produced the highest nitrogen use efficiency traits.

Conclusion

The application of 240 kg N ha−1 in three split doses (T5) was required to obtain optimum wheat yield. In addition, increasing the rate of nitrogen beyond 120 kg N ha−1 decreased nitrogen use efficiency traits.

Keywords

  • Efficiency
  • Recovery
  • Split
  • Uptake
  • Yield

Background

Bread wheat (Triticum aestivum L.) is one of the most important cereal crops in the world in terms of area coverage and production. It is a major source of nutrition for humans and livestock, estimated to contribute as much as 60 million tonnes of protein per year [1]. The total worldwide production of wheat in 2012 was around 671 million tonnes on an area of 215 million ha [2]. In Ethiopia, wheat is grown approximately by 4.8 million farmers on 1.6 million hectares representing 13.33% of total crop area [3]. Data aggregated at a worldwide level over several decades have shown a strong link between agriculture production and fertilizer use [4]. Of the nutrients, nitrogen (N) is frequently regarded as the single most important mineral nutrient limiting crop production in many agricultural crops worldwide, and it is needed in large amount, as it constitutes 1–4% of the plant dry matter [5]. However, the average yield of wheat in Ethiopia is very low; it is about 2.5 ton/ha as compared to the world’s average of about 3.4 ton/ha [2]. The low mean national yield of wheat is mainly the result of depleted soil fertility, especially nitrogen (N) deficiency, which is often encountered in cool wet areas or in soils that are frequently water logged such as the highland Vertisols. Therefore, greater usage of chemical fertilizer has been advocated as a primary means of increasing wheat grain yield in Ethiopia [6].

Although N is the key element in increasing productivity and the increase of agricultural food production worldwide over the past four decades, a small fraction of this fertilizer is taken up by the plant [7], being 33% for wheat [8]. Poor N recovery is a function of N flows to competing pathways such as gaseous N losses, leaching and biological immobilization and in-efficiencies in crop N uptake and utilization [9, 10]. However, adoption of appropriate N fertilizer management practices is reported to increase N recovery up to 70–80% [11]. Split application of N is one of the methods to improve N use by the crop while reducing the nutrient loss through leaching, denitrification, runoff and volatilization [12]. Some research findings indicated that late season N application as dry fertilizer material was effective in attaining higher N recovery and use efficiency [13]. In addition, determining the right N fertilizer rates and timing of application is decisive factor in obtaining higher yields [14].

In many parts of the world, limited research has been done on the effect of split application of N for wheat and its association with grain yield and NUE [15], which is also true in Ethiopia where information on the subject is meager. Besides, matching crop N demand with available soil N has been challenging for wheat producers in Enewari due to the susceptibility of Vertisols to water logging, which leads to denitrification, leaching and runoff losses during heavy rainfall [12]. According to Molla [16], this forced farmers of Enewari to apply as large as 256 kg N ha−1 (some even apply more) which is by far higher than the blanket recommendation (87 kg N ha−1). However, the optimum rate of nitrogen fertilizer for wheat production in the study area and its time of application are not yet investigated. This study was, therefore, conducted to determine the effect of N levels and time of application on yield and nitrogen use efficiency of bread wheat.

Methods

The study site

The study was conducted for two consecutive years during 2014 and 2015 main cropping seasons in the district of Moretina Jiru at the Enewari experimental field station in the central highlands of Ethiopia. Enewari is located at 9° 52′ N latitude, 39° 10′ E longitude at an altitude of 2680 meters above sea level. This area is typical of the rain-fed wheat-growing regions of Ethiopia with average annual rainfall of 1153.69 mm. The dominant soil type of the area is Vertisols which are known for their high water logging and drainage problems. Figure 1 shows monthly total rainfall and monthly mean temperatures at the experimental site over the 2-year study period.
Fig. 1
Fig. 1

Monthly total rainfall and average maximum and minimum temperatures in 2014 and 2015 growing seasons at Enewari, central highlands of Ethiopia

Prior to planting, the surface (0–20 cm) soil samples from ten spots across the experimental field were collected, composited and analyzed for determining selected soil physicochemical properties at Debre Berhan Agricultural Research Center following the procedure outlined by [17]. Values for the selected physicochemical properties are presented in Table 1.
Table 1

Soil physicochemical properties at the depth of 0–20 cm during the years of 2014 and 2015 before sowing of bread wheat

Parameter

Value

Rating

References

Year 2014

Year 2015

Sand

15

12

Silt

18

17

Clay

67

71

Texture class

Clay

Clay

pH

7.02

7

Neutral

Tekalign Tadesse [42]

Organic carbon (%)

1.08

1.15

Low

Tekalign Tadesse [42]

Total N (%)

0.08

0.06

Low

Tekalign Tadesse [42]

Av. P (ppm)

6.54

7.82

Low

Olsen et al. [43]

CEC [cmol(+)/kg soil]

48.75

45.25

Very high

Metson [44]

Exchangeable K

0.23

0.2

Low

Metson [44]

Description of the study materials

Fertilizer sources were urea (46% N) for nitrogen fertilizer and triple superphosphate (46% P2O5) for phosphorus fertilizer. A wheat variety called Menze (HAR-3008) was used as a test crop which was developed and released by DBARC (Debre Berhan Agriculture Research Center) in 2007. It has been widely promoted for its resistance to yellow rust and with a yield potential of 1900–3300 kg ha−1 high yielder in Moretina Jiru district, Enewari area. The variety is medium in maturity (154 days), with a medium stature of 64 cm [18].

Treatments and experimental design

The treatments consisted of complete factorial combinations of three N fertilizer rates and five split N applications, plus one unfertilized control. The three N-fertilization levels were 120, 240 and 360 kg N ha−1. The five N split application timings were adjusted according to Zadoks decimal growth stage for wheat [19] at the time when the moisture is available for nutrient dissolution and absorption. These application timings were: T1 = N applied ½ at sowing and ½ at tillering (Zadok scale 21–22); T2 = all N applied at tillering (Zadok scale 21–22); T3= N applied ½ at tillering (Zadok scale 21–22) and 1/2 at booting (Zadok scale 41–45); T4= N applied 1/3 at sowing, 1/3 at tillering (Zadok scale 21–22) and 1/3 at booting (Zadok scale 41–45); and T5 = N applied 1/4 at sowing, ½ at tillering (Zadok scale 21–22) and 1/4 at booting (Zadok scale 41–45).

These treatments were laid out in a Randomized Complete Block Design (RCBD) with four replications. The gross plot size of each treatment was 2 m × 3 m (6 m2) accommodating eight rows spaced 20 cm apart. The plot size for planting was 1.6 m × 3.0 m (4.8 m2), and four central rows were used for data collection and measurement. The distance between the plots and replications was kept at 0.5 m and 1 m apart, respectively.

Crop management

Wheat seed was sown by drilling in rows at the recommended rate of 150 kg ha−1 on July 24 in both years. Each year, all the wheat plots were supplied with triple superphosphate (TSP) at a recommended rate of 138 kg P2O5 ha−1 [20]. Similarly, the N was applied in the form of urea (as per the treatment) at planting and the later stage splits were applied by side dressing at the specified Zadoks growth stages. Plots were kept free of weeds by hand weeding. No insecticide or fungicide was applied since there was no outbreak of any insect or disease incidence. Harvesting was done manually using hand sickle in late December.

Data collection and measurements

In both years, gain yield (kg ha−1) was determined from the harvested net plot area of 2.4 m2 and was adjusted to 12.5% moisture content. At crop maturity, a subsample from each net plot was harvested at ground level, oven-dried at 70 °C until constant weight was reached for dry weight determination and partitioned into straw and grain. The dried samples were milled and the grain and straw N content of the plant samples was determined using the micro-Kjeldahl method as stated by American Association of Cereal Chemists (AACC) [21]. Total grain N uptake (GNUP) in kg ha−1 was calculated by multiplying grain yields by N content percentage. Total nitrogen uptake (TNUP) was calculated as the sum of the respective GNUP and SNUP values.

Nitrogen use efficiency traits

The following N-efficiency parameters were calculated for each treatment following Fageria [22]:
  1. 1.

    Agronomic efficiency (AE, kg kg−1) = \(\frac{{G_{\text{f}} - G_{\text{u}} }}{{N_{\text{a}} }},\) where Gf is the grain yield of the fertilized plot (kg), Gu is the grain yield in the unfertilized plot (kg) and Na is the quantity of N applied.

     
  2. 2.

    Agro-physiological efficiency (APE, kg kg−1) = \(\frac{{G_{\text{f}} - G_{\text{u}} }}{{N_{\text{f}} - N_{\text{u}} }},\) where Gf is the grain yield of the fertilized plot (kg), Gu is the grain yield in the unfertilized plot (kg), Nf is the N accumulation in the fertilized plot (kg) and Nu is the N accumulation in the unfertilized plot (kg).

     
  3. 3.

    Apparent recovery efficiency (ARE, %) = \(\frac{{N_{\text{f}} - N_{\text{u}} }}{{N_{\text{a}} }}*100,\) where Nf is the N accumulation by straw and grain in the fertilized plot (kg), Nu is the N accumulation by the straw and grains in the unfertilized plot (kg) and Na is the quantity of N applied (kg).

     
  4. 4.

    The nitrogen harvest index (NHI) was determined as the ratio of nitrogen uptake by grain and nitrogen uptake by grain plus straw as described by [22].

     

Data analysis

After verifying the homogeneity of error variances, combined analysis of variance was done using the procedure of SAS [23], and to facilitate factorial analysis, the control was excluded. Mean comparisons were done by Duncan’s multiple range tests Gomez and Gomez [24] at the 5% level.

Results

Grain yield

Grain yield was significantly affected by the main effects of year, N rate, time of application as well as the interaction effect of N rate × time of application. What is more, the interaction effect of year × N rate, year × time of application and year × N rate × time of application did not affect this parameter (Table 2). The split application of the different N fertilizer rates significantly (P < 0.01) affected grain yield. The highest grain yield was obtained in response to the application of 360 kg N ha−1 in three splits of ¼ at sowing, ½ at tillering and ¼ at booting, which was in statistical parity with the grain yield obtained in response to the application of 240 kg N ha−1 ¼ at sowing, ½ at tillering and ¼ at booting (Table 3).
Table 2

Mean squares of analysis of variance for year, N fertilizer rate and time of N application, and their interaction

Source

DF

GY

GNUP

TNUP

AE

RE

APE

NHI

Y

1

3,386,971**

789.77**

6813.22**

506.38**

4492.67**

270.07ns

839.08**

Rep (Y)

6

61,018

30.39

120.53

18.85

35.95

51.62

18.28

N

2

12,053,826**

22,875.82**

43,818**

2090.92**

2817.12**

3064.21**

120.10**

T

4

7,906,065**

4969.21**

10,914**

127.93**

1622.94**

287.29**

108.04**

N × T

8

1,127,802**

720.28**

1632.7**

6.12ns

68.09*

11.19ns

26.01*

Y × N

2

233836ns

211.25ns

552.37*

43.59**

142.47*

0.26ns

31.07*

Y × T

4

311205ns

254.56*

59.89ns

4.32ns

24.27ns

167.89**

43.21**

Y × N × T

8

231,387ns

70.27ns

115.94ns

4.89ns

32.98ns

6.73ns

11.28ns

Error

84

130,052

94.9

124.13

3.07

27.57

11.52

9.13

Y year, Rep replication, N N rate, T timing of N application, Df degree of freedom, GY grain yield, GNUP grain nitrogen uptake, TNUP total nitrogen uptake, AE agronomic efficiency, RE recovery efficiency, APE agro-physiological efficiency, NHI nitrogen harvest index

*Significant at the 0.05 probability level; **Significant at P < 0.01 probability level

Table 3

Grain yield (GY), grain nitrogen uptake (GNUP) and total nitrogen uptake (TNUP) as influenced by the interaction effect of N fertilizer rate and time of N application

N timing

N rate (kg ha−1)

N rate (kg ha−1)

N rate (kg ha−1)

120

240

360

120

240

360

120

240

360

GY (kg ha−1)

GNUP (kg ha−1)

TNUP (kg ha−1)

T 1

4436.40efg

4948.16def

5468.85cd

78.04fg

106.19de

127.5bc

96.27fg

131.45de

161.02bc

T 2

4076.89g

4356.21fg

4406.88efg

71.09g

91.56ef

100.24de

85.18g

114.12ef

124.20e

T 3

4307.19fg

5050.70de

4688.13efg

68.08g

91.22ef

94.79def

83.94g

116.11ef

121.24e

T 4

4362.17fg

5538.53bcd

6189.36ab

70.96g

110.15cd

134.17ab

92.97g

144.62cd

175.63b

T 5

4756.73ef

6060.04abc

6436.00a

82.49fg

128.96b

148.55a

100.05fg

170.04b

201.47a

Treated mean

5005.48a

100.27

127.76a

Control mean

1307.96b

24.89

28.36b

  

NR × NT

Treated versus control

 

NR × NT

Treated versus control

 

NR × NT

Treated versus control

CV (%)

 

7.2

3.84

 

9.72

2.32

 

8.72

1.51

Means followed by the same letters for the same parameter are not significantly different at P ≤ 0.05

CV Coefficient of variation, NR nitrogen rate, NT time of nitrogen application

T1 = N application of ½ at sowing and ½ at tillering; T2 = N application at tillering; T3 = N application of ½ at tillering and ½ booting; T4 = N application 1/3 at sowing, 1/3 at tillering and 1/3 at booting; and T5 = N application ¼ at sowing, ½ at tillering and ¼ at booting

Nitrogen uptake

Grain N uptake

Grain N uptake (GNUP) was significantly influenced by the rate and timing of N application. The interaction effect of N rate × time of application and year × time of application also revealed a significant effect on nitrogen uptake by the grain. However, the effect of year, year × N rate and year × N rate × time of application on grain nitrogen uptake was nonsignificant (Table 2). Nitrogen uptake by the grain tended to increase in response to the level of N as it rises from 120 to 360 kg ha−1 in both growing years. The maximum grain N uptake value (148.55 kg ha−1) was obtained when 360 kg N ha−1 was applied in three splits (¼ at sowing, ½ at tillering and ¼ at booting) while the lowest value (68.0 kg ha−1) was recorded when 120 kg N ha−1 was applied equally at tillering and booting (T3) (Table 3). With regard to the interaction effect of year × time of N application, split application of N three times at sowing, tillering and booting (T5) produced the highest N uptake value in both growing years while the lowest grain N uptake (78.61 kg ha−1) was recorded when N was applied equally at tillering and booting (T3) in the year 2014 (Table 4).
Table 4

Interaction effect of the year × N rate and year × N timing on GNUP and TNUP of bread wheat

N timing

Year

2014

2015

GNUP (kg ha−1)

T 1

101.66c–e

106.15b–d

T 2

82.1fg

93.15def

T 3

78.61g

90.78efg

T 4

106.66bc

103.52c–e

T 5

119.45ab

120.55a

Treated mean

100.27a

Control mean

24.89b

CV (%)

9.72

N rate (kg ha−1)

Year

2014

2015

TNUP (kg ha−1)

120

88.29d

95.07d

240

124.33c

145.46b

360

148.07b

165.36a

Treated mean

127.76a

Control mean

28.36b

CV (%)

8.72

Means followed by the same letters for the same parameter are not significantly different at P ≤ 0.05

CV Coefficient of variation, NR nitrogen rate, NT time of nitrogen application

T1 = N application of ½ at sowing and ½ at tillering; T2 = N application at tillering; T3 = N application of ½ at tillering and ½ booting; T4 = N application 1/3 at sowing, 1/3 at tillering and 1/3 at booting; and T5 = N application ¼ at sowing, ½ at tillering and ¼ at booting

Total N uptake

The analysis of variance indicated that year, N rate, time of N application had highly significant effect on total nitrogen uptake of wheat. Likewise, the interaction of N rate × time of N application, year × N rate also revealed a significant effect on total nitrogen uptake. But, the interaction effect of year × N rate × time of application (Table 3) was not significant. The highest total N uptake value (201.47 kg ha−1) was attained when 360 kg N ha−1 was applied three times at sowing, tillering and booting (T5) while the lowest (83.94 kg ha−1) was recorded when 120 kg N ha−1 was applied equally at tillering and booting (T3) (Table 3). The year × N rate interaction shows that wheat N uptake had the highest value (165.4 kg N ha−1) in the year 2015 at the highest N rate while the lowest value (88.3 kg N ha−1) was recorded in 2014 at a rate of 120 kg N ha−1 which was statistically similar to that of 2015 under the same N rate (Table 4).

Nitrogen use efficiency traits

Agronomic efficiency

Nitrogen agronomic efficiency (AE) represents the ability of the plant to increase yield in response to N applied [25]. AE varied significantly according to year, N rates and timing of application, as well as by the interaction of year × N rate. The interaction between year × time of application, N rate × time of application and year × N rate × time of application did not show a significant effect on this parameter (Table 2). In 2015, the year with the highest grain yield, the value recorded for AE was significantly higher than 2014 under all N rates. The application of 120 kg N ha−1 produced the highest AE value (28.8 kg ha−1) in 2015. The lowest (10.47 kg kg−1) value was recorded when 360 kg N ha−1 was applied in 2014 (Table 5).
Table 5

Interaction effect of year × N rate on agronomic efficiency (AE), apparent recovery efficiency (RE) and nitrogen harvest index (NHI) of bread wheat

N rate (kg ha−1)

Year

Year

Year

2014

2015

2014

2015

2014

2015

AE (kg kg−1)

RE (%)

NHI (%)

120

22.58b

28.75a

44.69c

59.85a

84.13a

78.07cd

240

14.1d

18.25c

37.36d

50.92b

82.13ab

75.60d

360

10.47e

12.47d

31.5e

39.48d

79.32bc

76.05d

CV (%)

9.86

11.94

3.82

Means followed by the same letters for the same parameter are not significantly different at P ≤ 0.05

CV Coefficient of variation

Nitrogen agro-physiological efficiency

Nitrogen agro-physiological efficiency (APE) represents the ability of a plant to transform N acquired from fertilizer into economic yield (grain) [26]. APE was also influenced by the main effects of N rate and time of application and by the interaction of year × time of N application. However, the effect of year, the interaction of N rate × time of application and year × N rate × time of N application had no significant effect on this index (Table 2). As to the interaction of year × time of N application, the highest APE (49.75 kg kg−1) was obtained when N was applied in equal split at sowing and tillering in the year 2014 while the lowest value (35.4 kg kg−1) was recorded in response to the application of nitrogen only once at tillering (T2) in 2015 (Table 6).
Table 6

Interaction effect of year × N timing on agro-physiological efficiency (APE) and nitrogen harvest index (NHI) of bread wheat

 

Year

Year

2014

2015

2014

2015

N timing

APE (kg kg−1)

NHI (%)

T 1

37.21d

38.43cd

82.8a

78.05bc

T 2

45.23ab

35.4d

83.35a

80.06ab

T 3

49.75a

42.2bc

81.43ab

78.34bc

T 4

39.62cd

39.35cd

81.43ab

71.72d

T 5

36.61d

38.02cd

80.31ab

74.69cd

CV (%)

8.45

3.82

Means followed by the same letters for the same parameters are not significantly different at P ≤ 0.05

CV Coefficient of variation

T1 = N application of ½ at sowing and ½ at tillering; T2 = N application at tillering; T3 = N application of ½ at tillering and ½ booting; T4 = N application 1/3 at sowing, 1/3 at tillering and 1/3 at booting; and T5 = N application ¼ at sowing, ½ at tillering and ¼ at booting

Nitrogen apparent recovery efficiency

Nitrogen apparent recovery efficiency (RE) depends on the congruence between plant N demand and the quantity of N released from applied N [27]. RE varied significantly according to year and treatment (N application timing and N fertilizer rates) as well as by the interaction of N rate × time of N application (P < 0.05) and year × N rate (P < 0.05). But, the interaction effect of year × time of N application and year × N rate × time of N application did not affect recovery efficiency of nitrogen (Table 2). With regard to the interaction effect of year × N rate, the highest RE (59.85%) was recorded in the year 2015 with the application of 120 kg N ha−1. The lowest RE (31.5%) was recorded with the application of 360 kg N ha−1 in 2014 (Table 5). As to the interaction of N rate × application timing, the highest RE (59.7%) was obtained from the application of 120 kg N ha−1 three times in split (¼ at sowing, ½ at tillering and ¼ at booting). However, the lowest value (25.64%) was obtained from the application of 360 kg N ha−1 two times equally at tillering and booting (T3) which is statistically similar to the recovery efficiency recorded when the highest level of nitrogen was applied only once at tillering (Table 7).
Table 7

Apparent nitrogen recovery efficiency (RE) and nitrogen harvest index (NHI) as influenced by the interaction effect of N fertilizer rate and time of N application

N timing

N rate (kg ha−1)

N rate (kg ha−1)

120

240

360

120

240

360

RE %

NHI (%)

T 1

56.1ab

42.71d–f

36.68ef

80.99a–c

81.08a–c

79.21a–d

T 2

46.85b–d

35.48fg

26.46gh

83.75a

80.55a–c

80.81a–c

T 3

45.81c–e

35.53fg

25.64h

81.45a–c

80.28a–c

78.22b–d

T 4

53.34a–c

48.19b–d

40.74d–f

76.66cd

76.68cd

76.39cd

T 5

59.24a

58.78a

47.92b–d

82.65ab

76.04cd

73.82d

CV (%)

11.94

3.82

Means followed by the same letters for the same parameter are not significantly different at P ≤ 0.05

CV Coefficient of variance

T1 = N application ½ at sowing and ½ at tillering; T2 = N application at tillering; T3 = N application ½ at tillering and ½ booting; T4 = N application 1/3 at sowing, 1/3 at tillering and 1/3 at booting; and T5 = N application ¼ at sowing, ½ at tillering and ¼ at booting

Nitrogen harvest index

Nitrogen harvest index is a measure of N partitioning in the crop, which provides an indication of how efficiently the plant utilized the acquired N for grain production [26]. Nitrogen harvest index (NHI) was significantly influenced by year, N rate and timing of N application. A generally significant effect of two-way interactions was also observed. However, the interaction effect of year × N rate × time of application was not significant (Table 2). With regard to the interaction effect of year × N rate, the highest value of NHI (84.13%) during the first growing year (2014) and the lowest value (75.6%) of NHI in the second growing year (2015) for wheat were recorded with the application of 120 kg N ha−1 and 360 kg N ha−1, respectively (Table 5). In general, application of nitrogen beyond 120 kg N ha−1 did not significantly affect NHI in the second growing season while the application of 360 kg N ha−1 significantly produced lower NHI as compared to the application of 120 kg N ha−1. With regard to the year × time of N application, the highest NHI (83.35%) was recorded when the whole nitrogen was applied only once at tillering (T2) which was statistically similar to the rest timing treatments in 2014, while the lowest NHI (71.72%) was recorded with the split application of nitrogen three times (T4) in 2015 (Table 6). As to the interaction effect of N rate × time of application, the highest value (83.75%) was recorded due to the application of 120 kg N ha−1 only once at tillering (T2), while the lowest nitrogen harvest index (73.82%) was produced from the application of 360 kg N ha−1 three times in split (¼ at sowing, ½ at tillering and ¼ at booting) (Table 7).

Discussion

Variations in climatic conditions registered during the cropping periods (Fig. 1) induced large variations in grain yield and the efficiency of N use by wheat. This agrees with Lopez-Bellido [28], who found a relationship between nitrogen fertilizer, wheat yield and seasonal trend, where there is a decline in yield during the wet years while little or no effect of N fertilizer during the dry years.

Grain yield

In the current experiment, increase in the N rate up to 240 kg N ha−1 and splitting it three times (T5) had a positive effect on grain yield of wheat and were not significantly different with the application of 360 kg N ha−1 with the same timing averaged over years. In general, the highest grain yield obtained in this experiment exceeds the yield obtained in response to the application of 120 kg N ha−1 all at once at tillering by 57.8% (Table 3). Compared to the grain yield obtained from the control plot, the grain yields obtained from the aforementioned most productive treatments were superior by 392.1% and 372%, respectively (Table 3). The optimum wheat grain yield was obtained in response to applying 240 kg N ha−1 applied in three splits ¼ at sowing, ½ at tillering stage of growth and ¼ at booting. This optimum yield exceeds the national average wheat yield of the country by about 152.5%, which is about 2.4 ton ha−1 [3]. It also exceeds the world’s average yield of 3.4 t ha−1 by about 78% [29]. This indicates that evaluated N rates positively affected grain yield. This dramatic yield increase with N fertilizer application is the reason why farmers in the study area use higher rates of nitrogen (256 kg ha−1) than the blanket recommendation (87 kg ha−1). The increased grain yield due to the increased application of nitrogen might be attributed to the high concentration of N in the leaves which increased and prolonged the photosynthesis ability of the plant which leads to an increase in grain yield. In agreement with the present result, Abedi [30] reported that different N rates (120, 240 and 360 kg ha−1) had a significant effect on grain yield increment in wheat (46% at N = 120, 72% at N = 240, and 78% at N = 360) compared to control.

The result of the current experiment also revealed that the application of N three splits yielded more grain than application of nitrogen only once at tillering or just in two splits. The increase in grain yield due to the trice split application of nitrogen might be the better matching of N availability with the crop needs during the growing period. Similarly, higher grain yield of wheat was reported when N was applied in three splits (at planting, tillering and post-anthesis) compared with two splits (at planting and tillering) and one-time application (at planting) Brian [31]. Contrary to the current result, Chen and Neil [32] reported that split application of N did not affect wheat grain yield significantly. Similarly, there was a report where applications of all N rates at planting and twice split application timing showed the same significance effect on grain yield (each 5.4 t ha−1) with 8% higher yield over trice split N timing [33]. The lowest value of grain yield in this experiment was recorded with the full application of N only at tillering, where the applied N was likely susceptible to leaching, denitrification and runoff loss as the amount of rainfall was higher during this period.

Nitrogen uptake

In this study, the highest GNUP was 118% higher than the lowest value which was obtained with the application of 120 kg N ha−1 equally at tillering and booting (Table 3). The overall higher grain N uptake due to the split application of the highest N rate (360 kg N ha−1) at sowing, tillering and booting (T5) can be explained by the more efficient N mobilization to the grain at grain filling stage. This is in conformity with Jan [34] who reported higher efficiency of N partitioning to the grain when N was supplied in splits (at planting, tillering and stem elongation). Similarly, Fageria and Baligar [35] reported that split applications of nitrogen fertilizer cause high amount of nitrogen content to be taken by the grain rather than by straw of wheat. The present experiment also revealed nitrogen fertilizer rates significantly increased wheat N uptake. Uptake values were similar in both growing years at lower N rate, whereas significant differences were recorded between all fertilizer rates, rising as fertilizer rates increased. This might be because application of extra N through increased levels increased the concentration of N in the soil and led to greater absorption of nutrients, which ultimately resulted in vigorous growth of bread wheat in terms of higher dry matter accumulation and enhanced the total uptake of nitrogen. The result also revealed that the split application of the highest dose (360 kg N ha−1) and applying it three times (T5) increased wheat N uptake. The increased N uptake of wheat due to the split application of nitrogen (T5) could be ascribed to the continuous supply of N which may have increased the synchrony between plant N demand and supply from the soil coupled with the reduction of N losses via denitrification, leaching or runoff [4]. This proposition is consistent with the report of N uptake by wheat crop which is significantly enhanced when application of the highest dose of N fertilizer was done and synchronized with the time of high demand of the plant for uptake of the nutrient [36].

Nitrogen use efficiency traits

The present study demonstrated that a significant variation existed in the nitrogen use efficiency traits for year, rate and timing of N applications. In 2015, the year with the highest grain yield had the highest AE and RE of wheat under the rate of 120 kg ha−1 which were notably higher than the year 2014 under the same N rate. The increase in AE and RE in the second growing season might be due to the absence of water logging which reduces the availability of nitrogen which is the problem of the first growing season. In general, AE diminished as the N fertilizer rates increased in both growing seasons, with significant differences among all the levels of nitrogen. This result is in agreement with the finding of Roberts [37] who reported that increase in N fertilizer rates resulted in a decline in agronomic efficiency. Higher AE could be obtained if the yield increment per unit N applied is high because of reduced losses and increased uptake of N [25]. Nitrogen agronomic efficiency value ranging from 10 to 30 is common, and values higher than 30 indicate efficiently managed systems [26]. Consistent with this suggestion, in this study N application resulted in AE between 10.47 and 28.75 kg kg−1 in both the growing seasons, showing the importance of appropriate management system in wheat production.

The highest APE recorded in this study during the first wet growing season (2014) as a result of splitting nitrogen equally at tillering and booting implies that there was a higher loss of nitrogen in treatments where N was applied during sowing time. However, in the second growing year (2015), time of application had less impact on APE since the loss of nitrogen was minimized as a result of reduction in waterlogging problem due to a lower amount of rainfall. In addition, the higher APE due to the split application of nitrogen in two splits (at tillering and booting) in both growing seasons might be attributed to adequacy of available nitrogen during grain development stage that might have increased the assimilation and redistribution of N from the vegetative plant component to wheat grain. In contrast to the present finding, lower nitrogen utilization efficiency was reported in the early N applications at planting and tillering compared with additional split application at anthesis [34].

The current experiment also revealed that the highest value of 59.8% for recovery efficiency (RE) was obtained with the triplicate application of 120 kg N ha−1 (T5) and it is 131% higher than the lowest value of 25.64% which was obtained with the application of the highest dose in two equal splits at tillering and booting. In line with the present result, Haile [36] reported 13.7% rise in recovery efficiency of nitrogen as a result of N application three times (¼ at sowing, ½ at mid-tillering and ¼ at anthesis) at lower N rate. The application of N three times in split (T5) produced higher RE for all the N rates tested in the current experiment. This implies if N is applied in several small doses during the period of rapid crop growth, rather than as a single large dose at the beginning of rapid crop growth, then losses are minimized and crop recovery is maximized. Furthermore, the highest RE in the second growing season might be due to lower rainfall which improved the availability of nitrogen than the first growing season; thus, the crop has used the applied nitrogen more efficiently. The highest RE was recorded at a rate of 120 kg N ha−1 in both growing seasons. In line with the current experiment, increase in apparent nitrogen recovery efficiency was reported at the rate of 150 kg N ha−1 for wheat and barley [38]. In contrast, lower NUE (27.10%) with the highest nitrogen rate of 120 kg N ha−1 and the highest value (39.27%) at the lowest N rate of 30 kg N ha−1 were reported on bread wheat [36]. However, nitrogen recovery may vary with the location, soil type, crop variety and the environmental conditions prevailing during the crop growth [39]. In conformity with this result, studies from Ethiopia reported highest apparent nitrogen recovery efficiency of 65.8% Selamyihun [40] and 39.27% [25] on wheat in Ethiopia. However, the common apparent recovery N-efficiency values ranging between 30 and 50%, and 50 and 80% indicate well-managed system [27].

In the current experiment, application of nitrogen beyond 120 kg N ha−1 did not significantly affect NHI in the second growing season while the application of 360 kg N ha−1 significantly produced lower NHI as compared to the application of 120 kg N ha−1. The first growing season produced the highest NHI than the second growing season under all the levels of N. This showed a strong influence of rainfall, in the variable response of NHI to time of application. The higher NHI during the first growing season might be due to the production of a lower aboveground biomass yield due to water logging, which resulted from higher rainfall. The lower NHI in the second growing as compared to the first growing season might be attributed to the increase in aboveground biomass yield. In general, the highest NHI value was recorded when nitrogen was applied only at tillering during both growing seasons. This might be due to the lowest aboveground biomass and grain yield produced by this treatment. Similarly, a higher nitrogen harvest index for wheat was obtained with treatments which produced the least aboveground biomass and grain yield [41].

Conclusion

The results of this study have demonstrated that application of a large quantity of nitrogen (a minimum of 240 kg N ha−1) in three split doses (T5) was required to obtain optimum wheat yield, which is about 2.5-fold higher than the national average yield of the crop in Ethiopia. The soil requires application of as much as 240 kg N ha−1 to produce about 6 tons of wheat per hectare which implies that the soil is productive unless the nitrogen uptake efficiency of the crop possibly is reduced as a result of its characteristic waterlogging condition. The importance of splitting nitrogen in three split doses (1/4th at sowing, ½ at tillering and the other 1/4th at booting) was also evidenced in the optimum yields and improving nitrogen recovery. Nitrogen fertilizer led to a general decrease of nitrogen use efficiency traits in both growing years. Higher N level increased N content in the grain and nitrogen uptake by wheat crop. In view of the current result, the significant interaction with year indicates that the efficiency of broad bed and furrows to drain excess soil moisture is lower in years which receive a higher amount of rainfall. Therefore, it should be supported by developing wheat varieties tolerant or resistant to such shocks.

Abbreviations

AE: 

agronomic efficiency

APE: 

agro-physiological efficiency

CV: 

coefficient of variation

GNUP: 

grain nitrogen uptake

GY: 

grain yield

MoARD: 

Ministry of Agriculture and Rural Development

N

nitrogen

NHI: 

nitrogen harvest index

NUE: 

nitrogen use efficiency

RE: 

recovery efficiency

TNUP: 

total nitrogen uptake

Declarations

Authors’ contributions

FB conceived the study and design, collected the data, performed the analysis on all samples, interpreted the data, wrote the manuscript and acted as corresponding author. ND, AM and TT assisted in analysis and interpretation of data and drafting of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors would like to thank Debre Berhan Agricultural Research Center for providing the land and allowing us to use their facilities and Debre Berhan University for allowing the corresponding author a leave of absence to conduct PHD dissertation research from which this article was emanated.

Competing interests

The authors declare that they have no competing interests.

Availability of data and materials

The authors want to declare that they can submit the data at whatever time based on your request. The data used for the current study are available from the corresponding author on reasonable request.

Consent for publication

Not applicable.

Ethics approval and consent to participate

Not applicable.

Funding

All authors dedicated their additional working hours to develop this paper with no specific grant from any funding agency.

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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
School of Plant Sciences, College of Agriculture and Environmental Sciences, Haramaya University, P.O. Box 138, Dire Dawa, Ethiopia
(2)
Department of Plant Sciences, College of Agriculture and Natural Resource Sciences, Debre Berhan University, P.O. Box 445, Debre Berhan, Ethiopia
(3)
Chickpea and Malt Barley-Faba Bean Projects ICARDA, Addis Ababa, Ethiopia

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