Resource Use Efficiencies of C₃ and C₄ Cereals under Split Nitrogen Regimes

Abstract

Resources are limited, thus improving resource use efficiency is a key objective for cereal-based cropping systems. This field study was carried out to quantify resource use efficiencies in selected C₃ and C₄ cereals under split nitrogen (N) application regimes. The study included the following treatments: six cereals (three C₃: wheat, oat, and barley; and three C₄: maize, millet, and sorghum) and four split N application regimes (NS₁ = full amount of N at sowing; NS₂ = half N at sowing + half N at first irrigation; NS₃ = ⅓ N at sowing + ⅓ N at first irrigation + ⅓ N at second irrigation; NS₄ = ¼ N at sowing + ¼ N at first irrigation + ¼ N at second irrigation + ¼ N at third irrigation). Results revealed that C₄ cereals out-yielded C₃ cereals in terms of biomass production, grain yield, and resource use efficiencies (i.e., radiation use efficiency (RUE) and nitrogen use efficiency (NUE)), while splitting N into three applications proved to be a better strategy for all of the selected winter and summer cereals. The results suggest that C₄ cereals should be added into existing cereal-based cropping systems and N application done in three installments to boost productivity and higher resource use efficiency to ensure food security for the burgeoning population.

1. Introduction

Cereals constitute a large proportion of food supplements. These are used in diets as end-products and provide greater than 70% of the worldwide caloric intake. Wheat (Triticum aestivum L.), oat (Avena sativa L.), barley (Hordeum vulgare L.), maize (Zea mays L.), millet (Pennisetum americanum L.), and sorghum (Sorghum bicolor L.) are the main C₃ winter and C₄ summer cereals being grown in Asia [1,2,3,4]. The classification and physiological characteristics of cereals are presented in

Table 1. Classification of commonly grown C₃ and C₄ cereal crops in Asia.

Crop/Botanical Name	Family	Life cycle	Season	Photoperiod	Growth habit	Pollination	Propagation	Photosynthesis	Nutrient uptake	Root system
Wheat (Triticum aestivum L.)	Poaceae	Annual	Winter	Long-day	Determinate	Self-pollinated	Seed	C3	Exhaustive	Fibrous
Oat (Avena sativa L.)
Barley (Hordeum vulgare L.)
Maize (Zea mays L.)			Summer	Day-neutral		Cross-pollinated		C4
Millet (Pennisetum americanum L.)				Short-day
Sorghum (Sorghum bicolor L.)						Self-pollinated
Rice (Oryza sativa L.)								C3

Source: Nazir et al. [5].

[5]. The historical area and production of first-, second-, and third-order cereals in Pakistan is presented in Figure 1 [6]. Currently, productivity and resource use efficiency of C₃ winter and C₄ summer cereals are low in Pakistan compared to other countries in the region [7,8,9,10,11,12,13,14,15,16,17], indicating a substantial potential to increase resource use efficiencies [6,18,19,20,21,22,23,24,25,26].

The world nitrogen-use efficiency (NUE) for cereal production is around 33% [27,28]. Reported N losses (Figure 2) are presented in

Table 2. Losses of N fertilizer in cereal crops.

N losses	Loss (%)	Crop/Condition	Reference
Nitrate	52–73	Corn	[30]
	>21	Winter wheat	[31]
Denitrification	9.5	Winter wheat	[32]
	10	Rice	[33]
	>10	Corn	[34]
Runoff	1–13		[35,36]
Volatilization	40	Without incorporated	[37,38]
Drainage	23	Tile	[39]

Source: Raun et al. [28].

[29,30,31,32,33,34,35,36,37,38]. Lower production is due to the meager use of all available resources along with climate variability and change [3,39,40,41,42]. Paradoxically, growers growing C₃ and C₄ cereals adopt conventional practices instead of approved practices that make more efficient use of resources [43,44,45,46,47,48,49,50]. Nitrogen is the key constituent of agricultural inputs to maintain production of these cereals throughout their lifecycle [51,52,53,54] (Figure 3). Surplus N and/or N applied without splitting can be lost through pathways such as nitrification and volatilization [8,40,41] (Figure 1). Principally, it is essential to boost NUE through better approaches to increase RUE. So, split N application regimes result in considerable boost in NUE, production, and resultantly RUE by reducing losses and improving uptake [41]. It is a rising concern that most of the available or applied N is lost, thereby reducing NUE, which is just 29% for cereals in developing agrarian economies [55,56]. This research study was carried out to validate the influence of split N application on biomass accumulation, grain yields, and resource use efficiencies such as NUE and RUE for C₃ winter and C₄ summer cereals.

2. Materials and Methods

2.1. Site and Experiment Description

Research experiments were carried out at the Agronomic Research Area at Bahauddin Zakariya University, Pakistan (30°15′ N latitude, 71°30′ E longitude, and 126.6 m a.s.l.). The research site was situated under irrigated conditions in an arid environment of a silt clay loam textural class. The detailed description of the physical and chemical features of the soil has been published previously [1,10]. The meteorological conditions for C₃ winter and C₄ summer cereal seasons are presented in Figure 4, and the treatments and experimental details are outlined in

Table 3. Agronomic practices for C₃ winter (wheat, oat, and barley) and C₄ summer (maize, millet, and sorghum) cereals.

Crops/Cultural Practices	Wheat	Oat	Barley	Maize	Millet	Sorghum
	C3 Cereals			C4 Cereals
Sowing date	November 14			August 18
N (kg ha−1)	125	115	50	227	170	100
Irrigations	December 7, January 14, February 1, March 11			September 12, September 28, October 17, October 31
Fertilizer dates	November 14, December 10, January 17, February 3			August 18, September 12, September 28, October 31
Split N treatments	NS1 = whole N at sowing; NS2 = ½ N at sowing + ½ N at first irrigation; NS3 = ⅓ N at sowing + ⅓ N at first irrigation + ⅓ N at second irrigation; NS4 = ¼ N at sowing + ¼ N at first irrigation + ¼ N at second irrigation + ¼ N at third irrigation
Net plot size	2 m × 5 m	2 m × 5 m	2 m × 5 m	8 m × 10 m	6 m × 10 m	3.5 m × 10 m
Soil properties	Sand 28%, silt 52%, and clay 20%, pH 9.6, EC 3.42 ds m−1, OM 0.74%, total N 0.033%, P 4.92 ppm, and K 255 ppm			pH 8.02, EC 2.3 ds m−1, Corg 0.76%, Ntot 0.039%, POlsen 5.1 mg kg−1, and Kext 110 mg kg−1
Harvest date	24 April	26 April	22 April	11 December	6 December	14 December

EC = Electrical conductivity; OM = Organic matter; N_tot = Nitrogen total; P_Olsen = Phosphorus Olsen; K_ext = Potassium extractable.

. There were three replications and net plot size has been mentioned in Table 3.

2.2. Data Collection

Common techniques were employed to record growth data. The leaf-area index (LAI) of a sample (10 g) of fully expanded fresh leaves for C₃ winter and C₄ summer cereal crops were taken, and leaf-area was recorded by means of a leaf-area meter. LAI was recorded using the methodology of Watson [57].

LAI _{(C3, C4 Cereals)} = Leaf area _{(C3, C4 Cereals)}/Land area _{(C3, C4 Cereals)}

(1)

The RUE was computed for C₃ winter and C₄ summer cereal crops as follows:

RUE _{TDM (C3, C4 Cereals)} = TDM (_{C3, C4 Cereals})/∑Sa (_{C3, C4 Cereals})

(2)

where ∑Sa is the cumulative photosynthetically active radiation PAR for C₃ winter and C₄ summer cereal crops that was anticipated to be half of the total daily instance radiation, and TDM is total aboveground biomass [58] and calculated using the following equation:

Sa (_{C3, C4 Cereals}) = F_i (_{C3, C4 Cereals}) × S_i (_{C3, C4 Cereals})

(3)

where S_i is the incident PAR for C₃ winter and C₄ summer cereal crops, and F_i was appraised from corresponding cereal crop LAIs by means of the Monteith and Elston [59] equation.

F_i (_{C3, C4 Cereals}) = 1 − exp (−k _{(C3, C4 Cereals)} × LAI _{(C3, C4 Cereals)})

(4)

where F_i is the fraction of intercepted radiation, k is an extinction coefficient for the total solar radiation, and the LAI is for C₃ winter and C₄ summer cereal crops [60]. The standards of k for wheat, oat, barley, maize, sorghum, and millet were 0.70, 0.63, 0.74, 0.65, 0.63, and 0.52, respectively [61,62,63,64]. Multiplying the totals by proper estimates of Fi plus Si produced the quantity of intercepted radiation (Sa) for C₃ winter plus C₄ summer cereals.

The NUE was calculated as the ratio of grain yield (GY) to quantity of N application [1,10,65]:

NUE _{(C3, C4 Cereals)} = Grain yield (N_{X (C3, C4 Cereals)})/N application rate _{(C3, C4 Cereals)}

(5)

2.3. Statistical Analysis

Data thus collected after field experiments were analyzed by Statistix 8.1 (Tallahassee, FL, USA) for ANOVA. Treatment differences were addressed through the methodology of Steel et al. [66].

3. Results

3.1. Biomass

Seasonal differential accumulation of biomass occurred throughout the life cycle (including vegetative and reproductive stages till maturity) in all C₃ winter and C₄ summer cereals (Figure 5). Almost half of the biomass was accumulated till anthesis by all the C₃ and C₄ cereal crops. Overall, C₄ cereals performed better than C₃ cereals. Across winter and summer cereals and split N regimes, average biomass varied from 425 to 703 g/m² and 1083 to 1660 g/m² among C₃ and C₄ cereals, respectively (Figure 5). Among these cereals, wheat and maize produced higher biomass compared to other C₃ and C₄ crops, respectively. The biomass productivity of these cereal crops was higher when N was applied in three equal doses compared to other regimes. The lowest biomass was recorded when N was applied in four splits. These selected C₃ winter and C₄ summer cereals reached peak LAI just before the anthesis stage, which varied substantially among crops and split N application regimes (statistics not given).

3.2. Grain Yield (GY)

The data for GY for these winter and summer cereals significantly differed among crops with split N application regimes (Figure 6). Overall, GY ranged from 198 to 883 g/m² in the case of cereals and split N application regimes. The C₄ cereals also out-yielded C₃ in terms of GY, and it varied from 198 to 338 g/m² and 205 to 883 g/m² for the C₃ winter and C₄ summer cereals, respectively. Among winter and summer cereal wheat (338 g/m²) and maize (883 g/m²), crops produced higher GY, respectively, compared to other cereal crops. However, in all C₃ winter and C₄ summer cereals, higher GY was recorded when N was applied in three splits compared to other regimes. The lowest GY was observed for oat (198 to 253 g/m²) and millet (205 to 266 g/m²) crops. Among N application regimes, the lowest GY was recorded when N was applied in four splits.

3.3. Radiation Use Efficiency (RUE)

The RUE for wheat, oat, barley, maize, sorghum, and millet crops and split N application regimes significantly differed (Figure 7). Overall, among all these cereal crops, C₄ summer cereals also out-yielded C₃ winter cereals in terms of capturing photosynthetically active radiation. The RUE varied from 0.90 to 1.42 g MJ⁻¹ and 1.95 to 2.31 g MJ⁻¹ in the case of C₃ and C₄ cereals, respectively. Among split N application regimes, the maximum RUE (2.31 g MJ⁻¹) was found in the treatment where N was applied in three splits, and the lowest (0.90 g MJ⁻¹) was recorded for the four splits condition.

3.4. Nitrogen Use Efficiency (NUE)

The NUE for C₃ (wheat, oat, barley) and C₄ (maize, millet, sorghum) cereal crops and split N application regimes differed significantly (Figure 8). Overall, among all these cereal crops, C₄ summer cereals also out-yielded C₃ winter cereals in terms of NUE, varying from 17.84 to 38.88 kg kg⁻¹ and 17.18 to 51.86 kg kg⁻¹ in the case of C₃ plus C₄ cereals. Among split N application regimes, the highest NUE (51.86 kg kg⁻¹) was found in the treatment where N was applied in three splits, while the lowest (17.84 kg kg⁻¹) was recorded for four splits. The 1:1 lines between the RUE and NUE of C₃ and C₄ cereals are presented in Figure 9.

4. Discussion

C₄ summer cereals (maize, millet, and sorghum) having C₄ carbon metabolism were found to be superior in accumulating biomass to C₃ winter cereals at different split N application regimes. The C₄ cereals produce higher biomass and grain yields as compared to C₃ cereal crops. This variation for biomass and grain yield was possibly due to the supremacy of C₄ cereals as compared to C₃ cereal crops towards harnessing higher resource use efficiencies for N [67,68].

The N application in three splits proved to be an effective strategy for all six C₄ and C₃ cereal crops as compared to other split application regimes. N application at sowing without splitting likely increases the losses through volatilization, nitrification, denitrification, and leaching (Figure 1). However, N application in four splits creates hidden hunger and did not fulfil the optimum nutrient requirements of all C₄ and C₃ summer and winter cereals during the crop lifetime. This deficiency is reflected in the form of low biomass, grain yields, NUE, and RUE in this study as well as elsewhere [69,70].

Growth dilution effect with variations in N in C₄ and C₃ cereal crops necessitate the splitting of N. The N is directly linked with leaf photosynthesis as well as higher NUE [40,71]. Variation in N dynamics as well as NUE has substantial effects on photosynthetic efficiency and growth [72].

In this study, C₄ summer cereals out-yielded C₃ winter cereal in terms of RUE and NUE. The RUE and NUE varied from 0.90 to 2.31 g MJ⁻¹ and 17.84 to 51.86 kg kg⁻¹ for the C₃ and C₄ cereals. Among split N application regimes, the highest RUE and NUE were found in the treatment in which N was applied in three splits, possibly due to the continuous and optimum availability of resources. It is a well-established fact that at optimum availability of N, the RUE of C₃ and C₄ cereals is enhanced, producing more height, LAI, light interception, and canopy development [1,10,73,74,75]. Similar trends of RUE against applied N in C₄ cereals indicated that RUE might be even somewhat better on a total biomass basis. The C₄ cereals displayed additional LAI compared to C₃. Conversely, it seems inadequate for C₃ cereals to accrue leaf N to obtain the level of C₄ cereals. Splitting N approach for C₃ and C₄ crops will increase productivity in the form of grain yield, then likewise increase NUE as well as biological harvest. The strategies in which the N losses of C₃ and C₄ crops are reduced will boost the C₃ and C₄ crop productivity in future.

5. Conclusions

Reduced resource use efficiencies, such as NUE and RUE, in selected C₃ winter (wheat, oat, barley) and C₄ summer (maize, millet, sorghum) cereal crops could be augmented through splitting N fertilizer in irrigated arid conditions. The poor resource use efficiencies are due to lesser NUE in cereal crops and its possible losses by nitrification and runoff as well as leaching. Therefore, N application in three splits (at sowing time and first irrigation as well as second irrigation) to C₃ winter (wheat, oat, barley) and C₄ summer (maize, millet, sorghum) cereals may be considered as a substitute strategy to enhance resource use efficiencies by decreasing N losses in irrigated arid conditions.