Nitrogen Fertilization and Straw Management Economically Improve Wheat Yield and Energy Use Efficiency, Reduce Carbon Footprint

Abstract

Fertilization is an effective agronomic management technique for increasing crop production. However, the overuse of chemical fertilizer stimulates energy consumption and greenhouse gas (GHG) emissions, which are antagonistic to sustainable wheat production. In this study, we estimated the energy and GHG performances of different fertilization regimes based on a 32−year fertilization experiment. In this long−term experiment, there are five treatments: CK (no fertilizer with wheat residue removal), NPK (chemical fertilizer with wheat residue removal), NPKPM (chemical fertilizer and pig manure with wheat residue removal), NPKCM (chemical fertilizer and cattle manure with wheat residue removal), and NPKWS (chemical fertilizer with wheat residue retention). The results indicated that NPKCM and NPKPM consumed higher total energy than NPK and NPKWS, which was attributed to the extra energy usage of farmyard manure. Although NPKCM and NPKPM increased energy output by 4.7 and 2.8%, NPKWS stood out by delivering the highest energy use efficiency (EUE) of 6.66, energy productivity of 0.26 kg MJ⁻¹, energy profitability of 5.66, net return of 1799.82 US$ ha⁻¹ and lower specific energy of 3.84 MJ kg⁻¹. Moreover, the yield scale carbon footprint of NPKWS decreased by 66.7 and 52.3% compared with NPKCM and NPKPM, respectively. This study shows that the application of chemical fertilizer in combination with wheat residue retention is a good strategy to increase EUE and economic benefits while decreasing the carbon footprint of wheat production.

1. Introduction

Energy consumption, including to meet the high food demand and food security of its growing population, has made China one of the biggest energy producers and GHG emitters in the world since 2009 [1]. Emissions from agricultural activities account for approximately 17% of the total greenhouse gas (GHG) emissions [2], and therefore present a significant opportunity to reduce GHG emissions. The higher energy inputs in China’s wheat production compared with those in developed world result in lower energy use efficiency (EUE) [3]. It is worth noting that energy input, environmental impact, crop productivity and economic benefits are closely related in agriculture. As the production and use of essential farm inputs (e.g., diesel fuel, fertilizers, crop protection products, etc.) results in GHG emissions, improving its resource and EUE is crucial for sustainable agricultural development. Thus, the improvement of energy efficiency not only helps to improve farm profitability by reducing input costs, but also helps minimize GHG emissions and environmental pollution [4,5]. Furthermore, reducing energy consumption and carbon emissions in agricultural production will help the world achieve carbon neutrality in the future [6].

Major farmland decisions, such as choice and application rate of farming chemicals, determine the EUE of production. At the same time, the process of manufacturing, transport and application of these materials produces GHG emissions [7]. In wheat production, optimizing agronomic input (e.g., fertilizer, diesel fuel) could reduce the total energy input by 15% [8]. Hence, it is imperative to explore innovative agronomic practices such as suitable tillage and fertilization practices that can reduce energy use and mitigate GHG emissions [9]. Previous studies show that reduced tillage techniques, improved irrigation practices, and changing planting patterns could increase EUE in crop production. A comparison of five different tillage treatments showed that zero− or minimum−tillage, which is an essential element of conservation agriculture, improved EUE and productivity [10]. Kaur et al. [11] noted that both crop rotation and appropriate irrigation regime considerations are important in optimizing the energy efficiency of a farming system.

Fertilizer is a key input in agricultural production and determines the production efficiency, as well as the environmental footprint, of a farming system depending on the type, rate and method of application [12,13]. Increasing evidence demonstrates that farmyard manure application could improve soil quality and enhance sustainable agriculture [14]; however, its potential negative impact on the environment needs to be considered. For example, the application of chemical fertilizer and farmyard manure significantly increased seasonal total GHG emission compared with the chemical fertilizer in a wheat−maize rotation system [15]. In most areas of China, there has been a shift in recent years from the practice of crop straw incineration to straw retention and/or incorporation into soil to improve soil health, which also reduces the environmental effects of straw burning [16]. In a maize field experiment using different tillage methods, Lu and Lu [17] observed better performance in EUE and carbon footprint when straw was returned in chisel plow tillage or no tillage, compared with when straw was removed in conventional moldboard plowing tillage. However, there is a gap in the understanding of the long−term influences of different fertilization strategies and straw management on energy, carbon footprint and economic benefits of wheat production systems.

Clearly, a sustainable wheat fertilization regime with high productivity and reduced energy input and GHG emissions is needed, and this regime should be assessed based on knowledge of its energy, carbon footprint and economics. This study utilizes a long−term field experiment established in 1983 where different fertilization treatments have been applied in a consistent manner in each cropping season. Such consistency in agronomic management results in a stable performance in the output of the agroecosystem. We analyzed the effects of different fertilization regimes on EUE, carbon footprint and economic return to help develop strategies for optimizing the energy budget while mitigating climate change.

2. Material and Methods

2.1. Experimental Site Description

The long−term fertilization experiment was established in 1983 in the south of the North China Plain in Mengcheng County, Anhui Province, China (32°13′ N, 116°37′ E, Figure 1), which has an average altitude of 21.0~29.5 m with a warm temperate semi−humid monsoon climate. The area has a mean annual temperature of 16.5 °C, annual precipitation of 822 mm, annual sunshine hours of 2341 h, and a frost−free period of 211 d. The soil is classified as vertisol (IUSS Working Group WRB, 2015). The basic characteristics of topsoil (0–20 cm) at the beginning of the long−term experiment were described by Hua et al. [18].

2.2. Experimental Design

The experiment was set up in a randomized block design with plot sizes of 14.9 × 4.7 m² in four replicates. The five treatments included:

• No fertilizer with wheat residue removal (CK)
• Chemical fertilizer with wheat residue removal (NPK)
• Chemical fertilizer and pig manure with wheat residue removal (NPKPM)
• Chemical fertilizer and cattle manure with wheat residue removal (NPKCM)
• Chemical fertilizer with wheat residue retention (NPKWS).

Chemical fertilizer doses of N, P₂O₅, K₂O were applied at the rate of 180, 90, 135 kg ha⁻¹, using urea (46% N) as a N fertilizer, superphosphate (12% P₂O₅) as a P fertilizer, and potassium chloride (60% K₂O) as a K fertilizer. Pig and cattle manure were applied in a wet base at the rate of 15,000 kg ha⁻¹ and 30,000 kg ha⁻¹, respectively, each year. The amount of wheat straw retention was 7500 kg ha⁻¹. Before wheat sowing, all N, P, K fertilizers and organic fertilizers were applied once as base fertilizers to the topsoil (0–20 cm) using rotary tillage. The contents of N, P and K in pig manure, cattle manure and wheat straw were 17.0, 18.5 and 20.7 g kg⁻¹; 7.9, 3.5 and 15.2 g kg⁻¹; 5.5, 0.7 and 8.9 g kg⁻¹, respectively, in a dry base. The moisture of wheat straw, pig manure and cattle manure were about 33%, 70% and 60%, respectively. The detailed fertilization regimes are described in

Table 1. Pure N, P and K nutrient application amount (kg ha⁻¹) under various fertilization regimes.

Treatment	Chemical Fertilizer				Wheat Straw				Pig Manure				Cattle Manure				Total
	N	P	K		N	P	K		N	P	K		N	P	K		N	P	K
CK	−	−	−		−	−	−		−	−	−		−	−	−		−	−	−
NPK	180	39	112		−	−	−		−	−	−		−	−	−		180	39	112
NPKPM	180	39	112		−	−	−		77	84	93		−	−	−		257	123	205
NPKCM	180	39	112		−	−	−		−	−	−		95	42	182		275	81	294
NPKWS	180	39	112		28	4	45		−	−	−		−	−	−		208	43	157

CK: no fertilization; NPK: chemical fertilizers; NPKPM: NPK plus pig manure; NPKCM: NPK plus cattle manure; NPKWS: NPK with wheat residue retention. The same below.

.

To evaluate the energy input and output of each fertilizer treatment, a full production process record of all inputs (seeds, agro−chemicals, fuel, human labor, machinery power, etc.) was considered, and outputs (grain and straw) were systematically calculated from 2014 to 2015. The grain yield of CK, NPK, NPKPM, NPKCM and NPKWS were 444.38, 5895.00, 6327.50, 6624.38 and 6403.75 kg ha⁻¹, respectively. All treatments were sown at the end of October and harvested in early July. Wheat variety was Yannong 19 and applied at the rate of 187.5 kg ha⁻¹. Each plot was irrigated by 66.5 mm at sowing, wintering, and jointing stage, respectively. Agricultural machines were only used in land preparation and harvest. Imidacloprid and carfentrazone−ethyl were uniformly sprayed at 0.98 and 1.58 kg ha⁻¹, respectively, in all plots to control insects and weeds.

2.3. Energy Evaluation

Energy values for the wheat production system were determined using energy equivalents reported in the literature for various agronomic inputs and outputs (

Table 2. Energy equivalent values of different inputs and outputs in wheat production.

Particulars	Unit	Energy Equivalent(MJ unit−1)	Reference
Input
1. Human labor	h	1.96	[22]
2. Machinery	h	13.06	[22]
3. Diesel	L	56.31	[23]
4. Chemical fertilizers
(a) Nitrogen	kg	75.46	[24]
(b) Phosphorus(P2O5)	kg	13.07	[24]
(c) Potassium(K2O)	kg	11.15	[25]
5. Farmyard manure	kg	0.47	[26]
6. Chemical pesticides
(a) Insecticides	kg	101.2	[26]
(b) Herbicide	kg	238	[27]
7. Seed	kg	20.1	[28]
8. Irrigation water	m3	1.02	[29]
Output
1. Wheat grain	kg	14.48	[28]
2. Wheat straw	kg	9.25	[28]

). The retained or incorporated crop residues are an integral part of the soil system, therefore wheat residue retention has not been taken as a component for the estimation of energy budgeting [19,20,21].

Machinery energy was calculated according to Equation (1) [22].

ME = EG/T

(1)

where ME is the machinery energy (MJ h⁻¹), E is the energy equivalent for a tractor (62.7 MJ kg⁻¹), G is the weight of a tractor (kg), and T is the economic life of a tractor (h). The energy output in this study refers to grain energy and straw energy. To better evaluate the energy use under varied fertilizer treatments, indexes such as net energy gain, energy use efficiency, energy productivity, specific energy and energy profitability are used according to the following Equations (2)−(6) [28,30,31].

Net energy gain (MJ ha⁻¹) = Energy output (MJ ha⁻¹) − Energy input (MJ ha⁻¹)

(2)

Energy use efficiency = Energy output (MJ ha⁻¹)/Energy input (MJ ha⁻¹)

(3)

Energy productivity (kg MJ⁻¹) = Grain yield (kg ha⁻¹)/Energy input (MJ ha⁻¹)

(4)

Specific energy (MJ kg⁻¹) = Energy input (MJ ha⁻¹)/Grain yield (kg ha⁻¹)

(5)

Energy profitability = Net energy gain (MJ ha⁻¹)/Energy input (MJ ha⁻¹)

(6)

Total energy input is a vital indicator to show an integral picture of energy use and could be classified into direct (DE) and indirect energy (IDE). Direct energy refers to irrigation water, human labor, diesel fuel used to complete field operations such as tillage, sowing, pesticide application, irrigating, harvesting, etc. Indirect energy refers to energy consumed during extraction, manufacture and/or transport of seed, pesticides, farm machinery, farmyard manure and chemical fertilizer [10,17,32]. From the economic and ecological perspective, total energy consumption is divided into renewable energy (RE) such as seeds, human labor, farmyard manure, and water, and nonrenewable energy (NRE), such as chemical fertilizers, diesel fuel, pesticides, herbicides, and machinery [33].

2.4. Carbon Footprint

The environmental effect of different fertilizer management was revealed by determining the spatial and yield−scale carbon footprint (CF). According to Lal [34], the GHG emissions derived from inputs in wheat production per hectare were calculated by using CO₂ emissions coefficient of these inputs, which considered the global warming potential of different GHGs. The amount of produced CO₂ was calculated by multiplying the input application rate (i.e., seed, chemical fertilizers, farmyard manure, diesel fuel, machinery, pesticides, herbicides) by its corresponding emissions coefficient given in Table 3. It is assumed that CH₄ emissions are negligible when wheat is planted under well−drained upland condition, and only CO₂ and N₂O gases were considered in the study [31,35]. The N₂O emissions from applied nitrogen fertilizer, farmyard manure and crop residue were calculated by following Equation (7) [36]:

N₂O emissions (kg ha⁻¹) = N amount (kg ha⁻¹) × EF × 44/28

(7)

where N is the input of N from fertilizers, farmyard manure and wheat straw, kg N ha⁻¹; EF = emission factor 0.01 for N₂O emissions from N amount (EF is used to evaluate the direct N₂O emission from soil, which is equal to 1% of N amount [36], expressed in kg N₂O−N−kg N⁻¹ input). Global warming potential (GWP) was calculated with data from CO₂ and N₂O emissions, as in Equation (8) [31,35]:

GWP = (emitted N₂O × 265) + emitted CO₂

(8)

The CF was computed by using following Equations (9) and (10) [37,38]:

$$\mathrm{CFs} (\mathrm{kg} {\mathrm{CO}}_2-\mathrm{eq} {\mathrm{ha}}^{-1})={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}\mathrm{GWP}$$

(9)

CFy (kg CO₂−eq kg⁻¹) = CFs (kg CO₂−eq ha⁻¹)/wheat grain yield (kg ha⁻¹)

(10)

where CFs is the carbon footprint at spatial scale; CFy is the carbon footprint at yield scale.

Table 3. Greenhouse gas emissions coefficients of agricultural inputs in wheat production.

Particulars	Unit	CO2−eq (kg unit−1)	References
1.Human labor	Day	0.86	[34]
2. Machinery	MJ	0.071	[39]
3. Diesel	L	2.76	[40]
4. Chemical fertilizer
(a) Nitrogen (N)	kg	1.3	[34]
(b) Phosphate (P2O5)	kg	0.2	[34]
(c) Potassium (K2O)	kg	0.15	[34]
5. Farmyard manure	kg	0.126	[32]
6. Chemical pesticides
(a) Insecticide	kg	5.1	[34]
(b) Herbicide	kg	6.3	[34]
7. Seed	kg	1.22	[34]

Note: Labor worked 8 h a day.

Table 3. Greenhouse gas emissions coefficients of agricultural inputs in wheat production.

Particulars	Unit	CO2−eq (kg unit−1)	References
1.Human labor	Day	0.86	[34]
2. Machinery	MJ	0.071	[39]
3. Diesel	L	2.76	[40]
4. Chemical fertilizer
(a) Nitrogen (N)	kg	1.3	[34]
(b) Phosphate (P2O5)	kg	0.2	[34]
(c) Potassium (K2O)	kg	0.15	[34]
5. Farmyard manure	kg	0.126	[32]
6. Chemical pesticides
(a) Insecticide	kg	5.1	[34]
(b) Herbicide	kg	6.3	[34]
7. Seed	kg	1.22	[34]

Note: Labor worked 8 h a day.

In terms of carbon indexes calculation, total C input was calculated by multiplying the carbon footprint at a spatial scale by 12/44. Wheat biomass, i.e., straw and grain yield, were multiplied by 0.44 to estimate the total carbon output, as it is supposed that wheat biomass contains 44% C content [34]. The carbon output to carbon input ratio provided C efficiency [34].

In this study, the system boundary consisted of off−farm and on−farm stages. It includes the energy consumed in material extraction, production, and transportation of farm inputs in the off−farm stage, and different field operations in on−farm stage. The calculation of the energy consumption and GHG emissions under various fertilizer treatments was based on the processes contained in the system.

2.5. Economic Analysis

We computed total cost of cultivation and gross return from wheat production taking into consideration the market prices of inputs and outputs, respectively. The prices of all inputs and outputs are determined by the market prices in US $ in 2015 from the website of the Ministry of Agriculture and Rural Affairs of the People’s Republic of China (http://zdscxx.moa.gov.cn:8080/nyb/pc/index.jsp (accessed on 18 March 2022)). The input cost included the cost of hiring human labor, diesel, seed, water, fertilizers, farmyard manure and pesticides, whereas the gross return was estimated from the economic value of wheat grain (0.46 US$ kg⁻¹). The net return was estimated by subtracting total production cost (US$ ha⁻¹) from gross return (US$ ha⁻¹).

2.6. Statistical Analysis

The input–output energy relationships, carbon footprint, and economics of cultivation were calculated using the SPSS 26.0 analytical software package and Excel 2016. Before the analysis of variance (ANOVA), we tested the normality of the data. The ANOVA and the least significant difference (LSD) test (p < 0.05) were used for comparing significant differences among the investigated treatments.

3. Results and Discussion

3.1. Energy Input and Output of Wheat Production

3.1.1. Energy Input

The energy inputs in wheat production for each treatment decreased in the order NPKCM > NPKPM > NPKWS > NPK > CK (

Table 4. Energy input and output (MJ ha⁻¹) under different treatments of wheat production.

Particulars	CK	NPK	NPKPM	NPKCM	NPKWS
Input
Human labor	323.54	382.64	464.34	464.34	660.25
Diesel	2111.63	2111.63	2533.95	2533.95	2533.95
Chemical fertilizer	−	14,530.05	14,530.05	14,530.05	14,530.05
Farmyard manure	−	−	7050	14100	−
Chemical pesticides	473.25	473.25	473.25	473.25	473.25
Seed	3768.75	3768.75	3768.75	3768.75	3768.75
Water	2040	2040	2040	2040	2040
Machinery	435.55	435.55	435.55	435.55	435.55
Total energy input	9152.71	23,741.86	31,295.89	38,345.89	24,441.80
Output
Grain output	6434.55 c	85,359.60 b	91,622.20 ab	95,920.95 a	92,726.30 a
Straw output	8615.44 d	55,408.25 c	75,712.31 a	74,590.32 a	70,098.69 b
Total energy output	15,049.99 d	140,767.85 c	167,334.51 ab	170,511.27 a	162,824.99 b

Different lowercase letters represented significant difference in column at 5% level.

). Higher total energy input in NPKCM and NPKPM were primarily due to the addition of farmyard manure. In all treatments except CK, the highest energy input was associated with chemical fertilizer use, with values of 38–61% (Figure 2). The share of energy consumed for farmyard manure became the second largest after chemical fertilizer in NPKPM (23%) and NPKCM (37%), which was reported to be large energy−consuming farm operations [41]. A previous study also reported that fertilizer consumed the highest energy, with a share of 56.9%, due to the application of fertilizer that significantly exceeds the maximum demand for crop growth in the region [42]. However, Šarauskis et al. [32] found that farmyard manure accounted for the largest proportion of 45.0–49.3% of the total energy input in an organic fertilization system, which owes to their manure consumption being about twice as high as ours. Thus, there is room to decrease total energy input by optimizing fertilization management practices. In addition, seed was the second in the order of consumption in NPK and NPKWS, and accounted for 16 and 15% of total energy input, respectively. What is more, in accordance with previous studies [31,43], NPKWS consumed the highest amount of human labor energy, as more workforce was required for cutting, transferring, and incorporating straws.

The indirect energy consumption was higher than direct energy in all treatments (Figure 3). The contribution of indirect energy input to total energy inputs was highest in NPKCM (87%) and lowest in CK (51%). In accordance with previous studies [42,44], the contribution of renewable energy to total energy input was lower than non−renewable energy in NPK, NPKPM and NPKWS. The share of renewable energy was much higher than non−renewable energy in CK, which was due to the lack of chemical fertilizer application. The present results further indicated that wheat production was mainly based on non−renewable energy. However, it was found that the share of renewable energy in NPKCM and NPKPM were higher than that in NPK and NPKWS, which suggested that the application of farmyard manure can effectively reduce the contribution of non−renewable energy in wheat production [31].

3.1.2. Energy Output

Fertilizer applications resulted in significant differences in yield and straw, which together constitute the energy output. Fertilizer treatments significantly improved total energy output compared with CK which did not receive fertilizer (Table 4). Within the treatments that received chemical fertilizer, the addition of farmyard manure and wheat straw increased the energy output by 18.87% (NPKPM), 21.13% (NPKCM), and 15.66% (NPKWS) compared with NPK. The energy output in NPKWS was significantly lower than NPKCM but not significantly different from NPKPM. In the study, the higher energy outputs of NPKPM, NPKCM and NPKWS were due to higher wheat grain yield and straw. Notably, previous studies have reported that farmyard manure application and residue retention play a critical role in enhancing soil fertility and crop production [45,46], thus increasing the energy output.

3.2. Indicators of Energy Use

As shown in

Table 5. Energy indicators under different fertilizer management.

Energy Indicators	Unit	CK	NPK	NPKPM	NPKCM	NPKWS
Net energy gain	MJ ha−1	5897.28 c	117,025.99 b	136,038.62 a	132,165.37 a	138,383.19 a
Energy use efficiency	−	1.64 e	5.93 b	5.35 c	4.45 d	6.66 a
Energy productivity	kg MJ−1	0.05 d	0.25 a	0.20 b	0.17 c	0.26 a
Specific energy	MJ kg−1	20.67 a	4.05 c	4.95 bc	5.80 b	3.84 c
Energy profitability	−	0.64 e	4.93 b	4.35 c	3.45 d	5.66 a

Different lowercase letters represented significant difference in column at 5% level.

, there were significant differences in terms of the energy indicators. In this study, net energy gain varied significantly from 5897.28 MJ ha⁻¹ (CK) to 138,383.19 MJ ha⁻¹ (NPKWS). Net energy gain in CK and NPK was significantly different from each other and from the other three treatments; however, differences in NPKPM, NPKCM and NPKWS were not significant. These results show the benefit of proper organic fertilizer management and residue retention on net energy gain, as increased yield offsets the energy gap caused by fertilizer addition.

Energy use efficiency ranged from 1.64 to 6.66 with significantly higher EUE in fertilizer treatments compared with CK (Table 5). Within fertilized treatments, the EUE decreased in the order NPKWS > NPK > NPKPM > NPKCM with all treatments significantly different from each other. Thus, the retention of wheat residue increased EUE by 12%, while adding farmyard manure decreased EUE by 21% on average compared with NPK. These results indicate that wheat residue retention could improve yield performance at a lower energy input compared with NPKPM and NPKCM. Higher EUE for NPKWS was primarily due to the lower cost of extra nutrition input to achieve higher output when compared with farmyard manure. The advantage of straw mulch for higher output could also reflect as creating a suitable soil environment for crop growth [47]. Furthermore, the significantly 20% higher EUE of NPKPM than NPKCM shows that a similar output could be achieved with a lower application rate of pig manure compared with cattle manure, which highlights the importance of farmyard manure quality (i.e., nutrient content). Our results show that the merit of farmyard manure in addition to chemical fertilizers needs to be weighed carefully, especially when more energy efficient options (i.e., crop residue retention) are available, to reduce pressure on the system from the perspective of EUE. Although not measured in this study, a reasonable reduction in the amount of chemical fertilizer when applying organic fertilizer could be more efficient in energy consumption than other integrated nutrient management, which could improve grain and straw yield [48,49].

Energy productivity reveals the efficiency of conversion of invested energy into yield. CK recorded the lowest energy productivity, whereas NPKWS recorded the highest energy productivity. Energy productivity in NPKPM and NPKCM were significantly lower than NPK. It was also reported that farmyard manure combined with NPK significantly decreased energy productivity compared with NPK only [48]. However, higher energy productivity can be obtained by reasonably replacing chemical fertilizer with farmyard manure [50]. Furthermore, there was no significant difference between NPKWS and NPK, as gains in energy output (yield and straw) were offset by the input energy required to incorporate wheat residue in the field. However, energy productivity in NPKWS was 30% and 53% higher than NPKPM and NPKCM, respectively, because of the output benefits of wheat residue retention at a lower energy investment compared with farmyard manure. As expected, this trend conforms to the results of energy productivity. The specific energy in NPK and NPKWS was significantly lower than in NPKCM. The energy profitability varied from 0.64 to 5.66 (Table 5). Fertilizer treatments significantly improved energy profitability compared with CK. Furthermore, the present study found that NPKWS produced the highest energy profitability, thus consistently showing the benefits of residue retention in achieving higher energy gains with lower energy input.

These results indicate that an excessive amount of farmyard manure may have an adverse effect on the energy efficiency of the crop production system. Therefore, the choice of residue retention might be a better option than farmyard manure application when combined with chemical fertilizer.

3.3. Carbon Footprint and Efficiency

The carbon footprint of the five different fertilizer treatments is represented in

Table 6. Carbon input and output efficiency for different fertilizer treatments in wheat production.

Items	Unit	CK	NPK	NPKPM	NPKCM	NPKWS
CFs	kg CO2−eq ha−1	793.76	1818.82	4122.02	6146.98	1978.72
CFy	kg CO2−eq kg−1	1.79 a	0.33 d	0.65 c	0.93 b	0.31 d
Carbon input	kg ha−1	216.48	496.04	1124.19	1676.45	539.65
Carbon output	kg ha−1	444.38 c	5895.00 b	6327.5 ab	6624.37 a	6403.75 ab
Carbon efficiency	−	2.05 d	11.88 a	5.63 b	3.95 c	11.87 a

CFs: Carbon footprint in spatial scale; CFy: Carbon footprint in yield scale. Different lowercase letters represented significant difference in column at 5% level.

. The highest CFs for NPKCM was 6146.98 kg CO₂−eq ha⁻¹, whereas the lowest CFs for CK was 793.76 kg CO₂−eq ha⁻¹. NPKWS significantly decreased the carbon footprint relative to NPKCM and NPKPM treatments. Meanwhile, NPKWS had the lowest CFy as compared to NPKPM and NPKCM. These results showed that straw returning significantly reduced CFs and CFy compared with the farmyard manure treatment. In addition, studies have found that straw retention can help achieve high yield and low carbon production compared with straw removal [51]. Our study also confirmed that straw returning can significantly improve the output, but straw returning has no significant effect on the carbon footprint (Table 6). The possible reason is that the straw was removed manually with less emission in our experiment, rather than using agricultural machinery with fuel.

The emissions from human labor, machinery, diesel, fertilizer/manure, chemical pesticides and N₂O from the field made up the total CO₂−eq emission (Figure 4). Results showed that the CO₂−eq emissions from fertilizer/manure contributed the highest CFs in NPKCM (66%) and NPKPM (54%), followed by N₂O from the field (26% in NPKPM and 19% in NPKCM). Generally, such a high contribution of farmyard manure to the carbon footprint happens in organically fertilized farms, and is attributable to the substantial input of farmyard manure [32]. Nevertheless, other results reported that diesel fuel use was the main contributor to the carbon footprint, as used for crop management, such as more diesel fuel for irrigation in dryland (76%) wheat agroecosystems compared with irrigated (47%) wheat agroecosystems [52]. Notably, under CK in this study, diesel (63%) was also the biggest CO₂−eq emissions contributor. Furthermore, the composition of CO₂−eq emissions for NPKWS, in which N₂O from the field contributed 44%, followed by diesel with 27% of the total CO₂−eq emission on average. It was also confirmed by Lal et al. [43] that N₂O from field emission accounts for the largest proportion.

Carbon efficiency was also significantly influenced by different fertilizer treatments (Table 6). Because of the large amount of carbon input from farmyard manure, NPKCM maintained the highest carbon outputs and the least carbon efficiency. Compared with NPKPM and NPKCM, NPKWS achieved a comparable carbon output by using much less carbon input, therefore producing a significantly increased carbon efficiency of 111% and 201%, respectively. Residue retention offers a lower carbon footprint compared with conventional straw burning or straw removal [53]. Thus, our results and previous studies suggest that crop residue retention, reduction in chemical fertilizer and farmyard manure application rates, and the adoption of minimum or no−tillage systems should be considered as potential strategies to reduce the carbon footprint of wheat production systems.

3.4. Economic Analyses

The total cost of production ranged from 513.81 US$ ha⁻¹ to 1850.22 US$ ha⁻¹ (

Table 7. Economics (US$ ha⁻¹) analyses of different fertilizer treatments in wheat production.

Items	CK	NPK	NPKPM	NPKCM	NPKWS
Costs
Human labor	264.09	312.32	379.02	379.02	538.92
Diesel	3.44	3.44	3.64	3.64	3.64
Fertilizer	−	353.77	353.77	353.77	353.77
Farmyard manure	−	−	94.34	188.68	0.00
Pesticide	56.82	56.82	56.82	56.82	56.82
Seed	186.00	186.00	186.00	186.00	186.00
Water	3.47	3.47	3.47	3.47	3.47
Total	513.81	918.60	1417.93	1850.22	1145.90
Gross return	204.41c	2711.70 b	2910.65 ab	3047.21 a	2945.73 ab
Net return	−309.4c	1793.10 a	1492.72 ab	1196.99 b	1799.82 a

Different lowercase letters represented significant difference in column at 5% level.

). The production costs estimated in our study are similar to those reported in other studies [27,54]. The economic analysis indicated that NPKCM was the most expensive production mode, followed by NPKPM, NPKWS and NPK. The most expensive input was human labor, which account for 35.93% of the total production costs on average. The result that labor accounts for a large proportion of total economic investment was also reported in a plot experiment by Wang et al. [47]. Another important component of the total costs is fertilizer, which constituted 22.69% of the total production costs on average.

The gross and net return varied under different treatments. The highest gross return was in NPKCM, and the lowest return was recorded in CK due to yield performance, which are also reported in similar studies [55,56]. However, in this study, the net return of NPKCM was significantly lower than NPK, indicating that the inappropriate input of farmyard manure may reduce income. In general, NPKWS gained about 20.57% and 50.36% higher net return than NPKPM and NPKCM, respectively, which is mainly due to higher grain yield, from the better growth conditions for wheat demand with relatively little investment [47].

4. Conclusions

This study compared the energy efficiency and carbon footprint under various fertilizer treatments in wheat production to help develop strategies for improving EUE and reducing environmental footprint. Treatments that received chemical fertilizers outperformed non−fertilized treatment in energy output by providing the essential nutrients required for crop growth. The residue retention treatment (NPKWS) achieved the highest EUE, energy productivity, energy profitability and net return. Although the combined application of chemical fertilizers and organic amendments (manures) increases energy output and net energy gain, it significantly decreased EUE compared with NPKWS. Importantly, the application of farmyard manure increased carbon footprint by approximately 1 to 2 times compared with residue retention. As such, we recommend residue retention in combination with chemical fertilizer as an optimum strategy to reduce energy input, enhance EUE and net return, and decrease carbon footprint. In this study, we estimated the energy and GHG performances of different fertilization regimes in a plot experiment, in which human labor still accounts for a large proportion of energy input to complete farm operations. The latest farmland management techniques, especially developed tools, could help improve production performance. Mechanizing farm operations using modern agricultural equipment will likely further change the composition of energy consumption and carbon footprint. Therefore, the percentages of energy components and carbon footprint for different fertilizer treatments need to be further verified, improving energy efficiency and net return, and reducing carbon footprint at a large scale.