The Effects of Plastic Mulching Combined with Different Fertilizer Applications on Greenhouse Gas Emissions and Intensity, and Apple Yield in Northwestern China

Abstract

Plastic mulching reduces weeds, conserves soil water, and boosts crop yield. However, most studies are insufficient when determining how plastic mulching affects greenhouse gas (GHG) emissions, particularly when used in conjunction with fertilizers. The purpose of this study was to determine the combined effect of plastic mulching and fertilizers on GHG emissions in apple orchards. A 3-year field experiment was conducted with two factors: mulching and fertilizers; (1) mulching treatments: plastic film (PM) and no mulching (NM); and (2) four fertilizer treatments: control (CK), organic fertilizer (M), inorganic fertilizer (NPK), and organic combined with inorganic fertilizer (MNPK), arranged in a two factorial randomized complete block design. The results showed that the mean annual N₂O emissions ranged from 0.87 to 5.07 kg ha⁻¹ in PM and from 0.75 to 2.90 kg ha⁻¹ in NM. The mean CO₂ emissions ranged from 2.10 to 6.68 t ha⁻¹ in PM and from 1.98 to 4.27 t ha⁻¹ in NM. MNPK contributed more to N₂O and CO₂ emissions in both PM and NM. The mean CH₄ uptake rate ranged from 1.19 to 4.25 kg ha⁻¹ in PM and from 1.14 to 6.75 kg ha⁻¹ in NM. M treatment contributed more to CH₄ uptake in both PM and NM. NKP treatments had higher greenhouse gas intensity (GHGI) in PM and NM, while MNPK and NPK treatments had higher greenhouse gas potential (GWP) in PM and NM, respectively. These results suggest that plastic film mulching significantly raises the potential for soil GHG emissions and increases apple yield.

1. Introduction

Global warming, caused by greenhouse gas (GHG) emissions consisting of nitrous oxide (N₂O), carbon dioxide (CO₂), methane (CH₄), hydrofluorocarbons (HFCs), sulphur hexafluoride (SF₆), and nitrogen trifluoride (NF₃), is one of the biggest threats to humanity and the ecosystem today. Agricultural activity is one of the leading causes of GHG emissions [1]. Farm management and control systems such as fertilizer application, irrigation, land management, and tillage are major contributors to GHG emissions [2]. GHG emissions caused by agricultural production account for 9 to 15% of total GHG emissions, which include CO₂ (8%), N₂O (81%), and CH₄ (44%), and are expected to increase by 30% by 2050 [3,4].

Nowadays, plastic mulching using conventional, low-density polyethylene mulch films (PM) is widely used in crop production throughout the world [5]. It covers an area of 20 million ha⁻¹ which accounts for 60% of the total global use, especially in upland areas [6,7,8]. China is the world’s largest user of PM, covering around 19, 320 ha⁻¹ of farmland [9].

However, using PM in agriculture combined with fertilizer application is the major factor in the increase in yield and GHG emissions [10]. Thus, a good yield management system can assure food security by increasing yield and reducing GHG, GWP, and GHGI emissions in the production of sustainable agriculture and a friendly environment [11].

The Plateau region in China is the major apple cultivating region [12], covering 138.86 ha⁻¹, which accounts for 55.20% and 45.00% of the Chinese and the world’s total farmland [13]. As a result of the lack of water in the production system and the limitation of agricultural development in plateau regions [14], using PM can help to increase soil temperature in order to suppress weed growth [15], promote decomposition and transformation of soil organic carbon (SOC), and improve soil nutrient content [16]. Moreover, it can help to promote plant growth which can solve the decrease in agricultural products in dryland [15,17]. Fang et al. [18] reported that biodegradable film mulch can increase water-filled pore space (WFPS) content. In addition, the results of Lee et al. [19] confirmed that the use of horticultural fabric mulching in apple orchards has significantly increased apple production.

However, the impact of PM use on GHG emissions is still unclear, and the issue has been raised for discussion. Some study results show that PM increases soil temperature and WFPS content in the soil, which promotes the nitrification and denitrification system in soil, speeds up soil breathing which affects the diffusion of N₂O [20], and increases CO₂ emissions [10]. Moreover, using PM impedes the exchange between soil air and atmospheric air, increases WPFS in soil, increases reparation in the root, and decreases the concentration of O₂ in soil and methanotroph; therefore, it affects CH₄ uptake [21]. Nevertheless, according to the report by Li et al. [22], the cumulative amount of CH₄ uptake increased under soil PM. The study conducted by Guo et al. [23] found that PM can decrease CO₂ emissions, which was consistent with other studies showing that transparent degradable mulching film reduces GHG emissions [24].

The number of inorganic fertilizer applications in apple orchards has been excessive over the past two decades. It has affected the rise of production costs due to rising fertilizer use in apple orchards. The decline in soil minerals affected soil acidity and increased GHG emissions [25,26]. Using organic combined with inorganic fertilizers, including animal manure and agricultural waste, instead of using inorganic fertilizers can improve soil health and increase yield [27]. Zhang et al. [28] explained that the integrated use of organic and inorganic fertilizers can increase the apple yield by 47.40% in comparison with CK. However, the effect on GHG emissions is still unknown. Some studies have shown that using organic combined with inorganic fertilizer increases nitrification and N₂O emissions [29,30]. The N content in organic fertilizer is lower than that in inorganic fertilizer, which affects the reduction of N₂O emissions [31,32]. According to the meta-analysis found in the study of mineral fertilizer with organic fertilizer in maize, organic fertilizer application increased CO₂ emissions by 26.80% in comparison with using mineral fertilizer [33] because it could significantly increase SOC in soil [34]. On the contrary, the study of Zhang et al. [30] and Xie et al. [35] found that there is not any effect on CO₂ emissions. The studies of rice conducted by Hou et al. [36] and Pandey et al. [37] found that organic fertilizer use can increase CH₄ emissions, while the studies by Fan et al. [38] and Brenzinger et al. [39] found that organic fertilizer use in bean, wheat, and rice production can promote methane oxidant in soil and affect the increase of CH₄ uptake. Thus, this indicates that different kinds of fertilizer affect soil CH₄ flux differently.

There are no studies on using PM combined with the integrated use of organic and inorganic fertilizer in apple orchards. Therefore, this study is very important to help farmers understand how to manage their orchards properly and apply fertilizer appropriately during the production process to obtain the highest production capacity and decrease GHG emissions at the same time.

The objectives of this study were: (1) to determine the effect of organic and inorganic fertilizers on apple yield under plastic mulching; (2) to evaluate the effect of organic and inorganic fertilizers on greenhouse gas emissions of apple orchards; and (3) to assess the effect of organic and inorganic fertilizers on global warming potential and greenhouse gas intensity in apple orchards.

2. Materials and Methods

2.1. Experimental Area

The experimental area is located at the Apple Test Station of Northwest Agriculture and Forestry University, Baishui County, Weinan, China (35°21′ N, 109°56′ E), at 850 m above sea level (Figure 1). The climate is humid subtropical, and dry in winter with a mean annual precipitation of 109.65 mm and a mean annual temperature of 10.04 °C. The average annual rainfall from 2018 to 1019 was around 390, 627, and 719.10 mm, and it increased in 2020, which was 9.00 and 26.20% higher than those in 2018 and 2019 (Figure 2). In January 2018, the lowest air temperature was −10 °C and the highest temperature was 42 °C in July 2020. The annual average temperature was 19.44, 19.23, and 18.02 °C from 2018 to 2020, respectively. The average soil temperatures in PM and NM in 0 to 20 cm soil depth were 24.60 to 22.61 °C, and 22.87 to 21.29 °C, respectively (Figure 2).

There was no irrigation system in the experimental area. The amount of WFPS mostly increased after rain. The WFPS content in PM in July was lower than in NM because PM sites lacked rain. The WFPS content in both PM and NM decreased after the rainy season. However, the evaporation of water in soil and water storage capacity in PM was higher than in NM. WFPS average content in PM and NM was around 17.38 to 31.49% and 16.44 to 26.23%, which increased from 3.74 to 27.81% in PM, compared to that in NM (Table S1).

According to the United States Department of Agriculture’s (USDA) soil classification, the experimental soil type is silty loam. The characteristic of soil at a depth of 0 to 20 cm was loam consisting of silt (67%), clay (25%), and sand (8%). The soil organic matter (SOM), total nitrogen (TN), superphosphate (P₂O₅), potassium sulfate (K₂O), soil bulk density, and pH were 13.00, 1.00, 15.90, 151.30 g kg⁻¹, 1.42 g cm⁻³, and 8.20, respectively.

2.2. Experimental Design

The Fuji apple tree (Malus domestica Borkh) was planted in this study, and the orchard was planted in 2008 and began bearing fruit in 2010. The plant density was 1200 apple trees ha⁻¹, 22 were planted in a row and 2 × 4 m apart from one another. The experiment was laid out in a two factorial randomized complete block design with three replicates. The experiment was divided into two factors (mulching and fertilizers): (1) mulching treatments: plastic film (PM) and no mulching (NM); and (2) four fertilizer treatments: control (CK), organic fertilizer (M), inorganic fertilizer (NPK), and organic combined with inorganic fertilizer (MNPK) (Figure 3).

The soil was manually covered by black polyethylene plastic film with 0.02 mm thick on both sides of the tree, 1 m on each side. No mulching treatments covered the areas between apple tree rows, which had natural grass. The fertilization of the plant occurred by digging a ditch at 40 cm × 20 cm (depth × width) near the apple trees (Figure 3). The fertilizer application programs for the apple orchard during the growing season included: 1. Inorganic fertilizer, including urea containing 46% N (Shaanxi Coal and Chemical Industry Group Co., Ltd., Xi’an, China) was applied three times a year (65% in November for maintaining apple trees after harvesting, 15% in May for flowering, and 20% in August for crop growth). 2. Superphosphate containing 16% P₂O₅ (Shifang KonLon Chemical Co., Ltd., Liaocheng, Shifang, China) was applied three times a year (70% in November for maintaining apple trees after harvesting, 20% in May for flowering, and 10% in August for crop growth). 3. Potassium sulfate containing 50% K₂O (Jiangsu Kolod Food Ingredients Co., Ltd., Lianyungang, China) was applied three times a year (40% in November for maintaining apple trees after harvesting, 20% in May for flowering, and 40% in August for crop growth). 4. Organic fertilizer from sheep manure (Hebei Fengnong Organic Fertilizer Manufacturing Co., Ltd., Baoding, China) was applied once a year in November to maintain apple trees after harvesting. The organic fertilizer included OM, TN, P₂O₅, and K₂O, accounting for 35.05 g kg⁻¹, 6.01 g kg⁻¹, 39.05 mg kg⁻¹, and 41.10 mg kg⁻¹, respectively (

Table 1. The amount of fertilizer applied and different types of fertilizers used in different treatments (kg ha⁻¹).

Treatment 1	Inorganic Fertilizer 2			Organic Fertilizer 3	The Total Amount of N Fertilizer Applied 4
	N	P2O5	K2O
CK	0	0	0	0	0
M	0	0	0	45,000	68
NPK	675	405	405	0	675
MNPK	640	405	405	22,500	675

¹ Treatment: control (CK), organic fertilizer (M), inorganic fertilizer (NPK), organic combined with inorganic fertilizer (MNPK), ² inorganic fertilizers: urea (N), superphosphate (P₂O₅), potassium sulfate (K₂O), ³ organic fertilizers, and ⁴ the total amount of fertilizer applied.

).

2.3. Collection and Calculation of Samples

2.3.1. Collection of Gas Samples

The experiment ran between March 2018 and December 2020. The collection of gas samples was conducted [40] by using the closed-chamber method (Beijing Rongxingxiai Design Studio, Beijing, China). The closed chamber made of stainless-steel was divided into two parts. The first part was the chamber base with a size of 41 × 21 × 17 cm (height × length × width). The lid was opened, and the top edge of the chamber was specially designed with a groove size of 2 × 3 cm (width × depth). The chamber was able to contain water to prevent gas leakage during the collection of gas samples.

The second part of the closed chamber was designed with a size of 40 × 20 × 15 cm (height × length × width). The lid was closed to cover the base of the upper part. The chamber was wrapped by insulation (Linguan Co., Ltd., Renqiu, China), to prevent temperature change caused by sunlight reflecting directly from outside to inside the closed chamber. Fans (Shenzhen Fluence Tech Co., Ltd., Shenzhen, China) were installed in the chamber. The airflow direction was formed by the fans for mixing gas within a closed chamber. A 5 mm hole was drilled on top of the chamber to install a thermometer (People’s Electrical Appliances Group., Ltd., Wenzhou, China), and a 10 mm hole was drilled on one side of the chamber to install a silicone tube (Dongguan Mingjing Silicone Products Co., Ltd., Dongguan, China) from inside to outside the chamber in order to suck out gas samples from the chamber.

The PM was removed a bit when the sample was collected in the PM, and the PM was put back after finishing collecting the samples.

The gas samples were normally collected once a week and were collected four times in one chamber within 0, 10, 20, and 30 min. However, it depended on suitability, such as after rain in summer and one to two days after fertilization. Gas could collected from 08.00 a.m. to 11.30 a.m. [41].

The stainless steel chamber was made up of two parts: the base and the cover. To prevent air leaks from the stainless steel base during gas sampling, a water trap was installed and gas samples were taken from the chamber using a needle and 50 mL syringe. Gas samples were kept in a 0.50 L air bag with aluminum foil to regulate the constant temperature and maintain the quality of the samples before being taken to the laboratory within 24 h for GHG analysis (Changde Beekman Biotechnology Co., Ltd., Changde, China).

A gas chromatograph-7890B (Agilent, Santa Clara, CA, USA) was used to analyze the concentrations and GHG fluxes by using the syringe to remove a gas sample from Dalian Pulaite and inject it into the gas chromatograph-7890B every 4 min. The N₂O concentrations were measured by an electron capture detector (ECD), while the CO₂ and CH₄ concentrations were measured by an ion detector. GHG concentrations (N₂O, CO₂, and CH₄) were measured in ppm then transformed to mg m⁻² h⁻¹ by using the following equation by Mehmood et al. [42]:

$$\mathrm{F}=\mathrm{H}\times \frac{\mathsf{\mu }\times \mathrm{P}}{\mathrm{T}+273}\times \frac{1}{\mathrm{R}}\times \frac{\mathrm{dc}}{\mathrm{dt}}$$

(1)

where F refers to the average flux of N₂O, CO₂, and CH₄ (mg m⁻² h⁻¹) in the soil, μ refers to the molar mass of N₂O, CO₂, and CH₄ (g mol⁻¹) in the soil, H refers to the height of closed stainless-steel chamber (cm), P refers to the standard atmospheric pressure in the experimental area (1.01 × 10⁵ Pa), R refers to the value of gas constant (8.31 J mol⁻¹ kg⁻¹), T refers to the temperature inside the closed chamber measured during the sampling period (°C), and dc/dt refers to the exchange rate of concentrations of N₂O, CO₂, and CH₄ in the closed stainless-steel chamber (mL m⁻³ h⁻¹).

The total annual cumulative emissions of N₂O, CO₂, and CH₄ in the apple orchard were measured by using the following equation by Liu et al. [43]:

$$\mathrm{M}={\displaystyle \sum }_{\mathrm{n}=1}^{\mathrm{n}}\left(\frac{{\mathrm{F}}_{\mathrm{i}+1}+{\mathrm{F}}_{\mathrm{i}}}{2}\right)\times \left({\mathrm{t}}_{\mathrm{i}+1}-{\mathrm{t}}_{\mathrm{i}}\right)\times 24$$

(2)

where M refers to the average cumulative flux of N₂O, CO₂, and CH₄ (kg ha⁻¹) in the soil, F refers to the average fluxes of N₂O, CO₂, and CH₄ (mg m⁻² h⁻¹) in the soil, i refers to the gas sampling, (t_i+1 − t_i) refers to the sampling period during which samples were continually collected for two to three days, n refers to the total number of sample collection, and 24 is used for unit conversion.

According to annual cumulative GHG emissions (N₂O, CO₂, and CH₄), the carbon dioxide equivalent (CO₂) based on the integrated global warming potential (GWP) of N₂O, CO₂, and CH₄ emissions was calculated using the following equation by Liu et al. [43].

GWP refers to the measurement of how much heat is absorbed by N₂O, CO₂, and CH₄ (kg CO_2-eq ha⁻¹). The GWP of N₂O and CH₄ emissions are 298 and 25 times that of CO₂, with an average residence time of 100 years.

$$\mathrm{GWP}=\left({\mathrm{CH}}_4\times 25\right)+\left({\mathrm{N}}_2\mathrm{O}\times 298\right)$$

(3)

The GHGI was determined according to the method of Xu et al. [44]:

$$\mathrm{GHGI}=\frac{\mathrm{GWP}}{\mathrm{Apple} \mathrm{yield}}$$

(4)

The direct N₂O emission factor (EF_d) was determined by a standard method of Toma et al. [45]:

$${ \mathrm{EF}}_{\mathrm{d}}=\frac{{\mathrm{F}}_{\mathrm{N}}-{\mathrm{F}}_{\mathrm{CK}}}{\mathrm{N}}\times 100\%$$

(5)

where F_N refers to the annual emissions of N₂O in nitrogen (N) fertilizer treatment (kg ha⁻¹) and F_CK refers to the annual emissions of N₂O in the treatment without fertilizer application (kg ha⁻¹).

2.3.2. Soil Sample Collection Process

Soil samples were collected once a week. The sample collection processes were: digging soil at 0 to 40 cm soil depth with a soil auger (8 cm diameter), separating the refuse from the soil, packing the consequent soil samples in a 100 g plastic bag, and sending them to the laboratory for further analysis.

The soil samples were divided into two parts: fresh soil and air-dried soil. Air-dried soil was soil dried in the oven for 12 h at a temperature of 105 °C. After being dried, the air-dried soil was ground by a soil grinder mill and filtered by a 2 mm test sieve. The consequent soil was packed in a plastic bag for further analysis.

For soil moisture and soil bulk density analysis, the soil samples were put in aluminum moisture content tins (6 cm diameter and 5 cm depth) to weigh before being dried and then dried in the oven for one day at 100 to 120 °C to analyze soil bulk density and water-filled pore space. The formula of Weijie and Li [46] was used to calculate WFPS:

$$\mathrm{w}=\left(\frac{{\mathrm{M}}_{\mathrm{i}}-{\mathrm{m}}_1}{{\mathrm{m}}_{\mathrm{i}}}\right){\mathsf{\gamma }}_{\mathrm{i}}\times 100\%$$

(6)

$${\mathrm{W}}_{\mathrm{i}}=10\times {\mathrm{w}}_{\mathrm{i}}\times {\mathrm{h}}_{\mathrm{i}}$$

(7)

$$\Delta \mathrm{S}={\mathrm{W}}_{\mathrm{o},\mathrm{i}}-{\mathrm{W}}_{\mathrm{o}}$$

(8)

where w_i refers to the water-filled pore space (%) in the experimental area, W_i refers to the soil water holding capacity (mm), ΔS refers to the moisture content in the soil (mm), M_i refers to the weight of the wet soil (g), M_i refers to the weight of the oven-dried soil (g), γ_i refers to the soil bulk density (g cm⁻³) in the experimental area, h refers to the depth of the sampling soil (cm), and W_o refers to the soil water.

To analyze the intensity of ammonium and nitrate (NO₃⁻-N and NH₄⁺-N), 5 g of fresh soil and potassium chloride (KCL) 2M solution (1:5 soil:solution) were put in a centrifuge (50 mL) and shaken at 250 rounds per minute. Then, the soil sample was put in the centrifuge refrigerator with a centrifugal speed of 5000 rounds per minute.

The consequent samples were filtered with a filter paper and stored at −10 °C. NO₃⁻-N and NH₄⁺-N were analyzed using the salicylate-nitroprusside-hypochlorite method, with the absorbance at 600 nm. NO₃⁻-N reduced to NO₂⁻ was analyzed using the VCl3 salicylate-nitroprusside-hypochlorite method, with the absorbance at 540 nm [47].

To analyze the total nitrogen (TN) in the soil, the soil samples were divided using sulfuric acid (H₂SO₄) with the Kjeldahl method at a temperature of 360 °C, and then they were refined by N distillation system, Kjeltec™ 9. The distilled solution was titrated with a standard solution. The observation showed that the NO₃⁻-N terminated when H₂SO₄ was around 0.02. The solution changed its color from purple to red, and the standard of H₂SO₄ was recorded to calculate the TN in the soil.

To analyze the intensity of phosphorus (P), the Bray II method was used; 1.00 g of air-dried soil and 10 mL of solution were put in an Erlenmeyer flask (50 mL) and shaken for 1 min. The consequent samples were filtered with 11 cm NO.5 filter paper. The extracted and working solutions (1:16) were sucked by pipette and put in a centrifuge and left for half an hour. The light absorption of the samples was measured using a spectrophotometer at the absorbance of 882 nm.

To analyze the potassium (K), 2.50 g of air-dried soil and 25 mL of 1 M ammonium acetate (NH₄OAc) solution were put in an Erlenmeyer flask (50 mL), shaken for 30 min, and then filtered with 11 cm NO.5 filter paper. The consequent samples were analyzed using a Flame spectrophotometer.

To analyze the organic matter in the soil (SOM), the Walkley black modified method was used by mixing 1 g of air-dried soil, 10 mL of potassium dichromate (K₂Cr₂O₇) 1.00 N solution, and 20 mL of H₂SO₄ solution in an Erlenmeyer flask (250 mL), and slightly shaking for 1 to 4 min. The solution was left until it reached room temperature, then, 50 mL of distilled water and five drops of O-phenanthroline were added. The samples were titrated using FAS 0.50 N solution. When the solution changed color from green to red-brown, the titration ended. The FAS content was recorded to calculate SOM in the soil.

To analyze the soil pH 1:1 (w/w), 20 g of air-dried soil and 20 mL of distilled water were put in a beaker and mixed with a glass rod for 30 min and left for 30 min.

Rainfall and temperature data are based on the Weinan meteorological bureau of Baishui County, which is about 2 km from the experimental area. The measurement of soil temperature was conducted using the meter (Dongguan Wanchuang Electronic Products Co., Ltd., Dongguan, China), at a depth of 0 to 20 cm.

2.4. Data Analysis and Evaluation

In this study, the primary data were organized and calculated by Microsoft Excel 2020 software (Microsoft Corporation, Washington, DC, USA). The statistical data were analyzed, and the normality of the data was tested by IBM SPSS 29.00 (SPSS Inc., Chicago, IL, USA). The differences in annual cumulative GHG emissions, GWP, GHGI, and apple yield in each treatment were evaluated using a one-way analysis of variance (ANOVA). The significant differences occurring between treatments at a significance level of 0.05 were determined by using Tukey’s multiple-range tests. The effects of each treatment and their interactions on GHG emissions, GWP, GHGI, and apple yield were analyzed by a two-way ANOVA. R version 4.3.0 software (R Core Team, Indianapolis, IN, USA) was used to determine the correlation between GHG emissions and soil properties (NH₄⁺-N, NO₃⁻-N, TN, SOM, WFPS content, and soil pH) and climate variables (rainfall and temperature). OriginPro software 2023.10 (OriginLab Corporation, Northampton, MA, USA) and Adobe Photoshop 2020.21.1.0 (Adobe Inc., San Jose, CA, USA) were used to create the graphs.

3. Results

3.1. Apple Yield

As shown in

Table 2. Effects of different management practices on GHG emissions in the apple orchard.

Year	T 1	Apple Yield (t ha−1) 2	CH4 Emission (kg ha−1) 3		CO2 Emission (t ha−1) 4		N2O Emission (kg ha−1) 5
			PM	NM	PM	NM	PM	NM
2018	CK	28.54 ± 4.05 c	−1.32 ± 0.02 a	−1.51 ± 0.03 a	2.10 ± 0.12 d	1.98 ± 0.13 d	0.87 ± 0.03 d	0.75 ± 0.06 d
	M	40.04 ± 4.08 b	−4.06 ± 0.09 d	−5.45 ± 0.19 d	4.07 ± 0.04 b	2.94 ± 0.15 b	1.63 ± 0.50 c	1.34 ± 0.03 c
	NPK	23.97 ± 2.51 c	−2.16 ± 0.12 b	−2.97 ± 0.05 b	3.68 ± 0.02 c	2.54 ± 0.06 c	2.13 ± 0.11 a	1.71 ± 0.05 a
	MNPK	51.72 ± 5.00 a	−3.20 ± 0.17 c	−4.23 ± 0.21 c	4.83 ± 0.08 a	3.28 ± 0.16 a	1.89 ± 0.15 b	1.43 ± 0.03 b
2019	CK	28.53 ± 5.92 d	−1.34 ± 0.25 a	−1.51 ± 0.19 a	2.36 ± 0.26 d	2.08 ± 0.22 d	1.15 ± 0.02 d	1.05 ± 0.04 d
	M	65.18 ± 8.50 b	−4.25 ± 0.05 d	−6.75 ± 0.37 d	4.35 ± 0.15 b	3.32 ± 0.07 b	3.47 ± 0.11 c	1.75 ± 0.02 c
	NPK	50.35 ± 10.60 c	−2.58 ± 0.24 b	−3.90 ± 0.20 b	3.77 ± 0.11 c	2.78 ± 0.22 c	3.81 ± 0.12 b	2.18 ± 0.03 b
	MNPK	93.43 ± 11.96 a	−3.85 ± 0.18 c	−5.36 ± 0.34 c	5.65 ± 0.18 a	4.06 ± 0.29 a	4.22 ± 0.24 a	2.38 ± 0.14 a
2020	CK	30.98 ± 5.73 c	−1.19 ± 0.05 a	−1.14 ± 0.05 a	3.13 ± 0.18 d	2.29 ± 0.14 d	1.52 ± 0.02 d	1.28 ± 0.24 d
	M	61.11 ± 12.96 a	−2.27 ± 0.06 c	−4.94 ± 0.14 d	5.34 ± 0.26 b	3.86 ± 0.13 b	4.02 ± 0.03 c	2.23 ± 0.02 c
	NPK	45.44 ± 5.86 b	−1.67 ± 0.09 b	−2.94 ± 0.16 b	4.60 ± 0.28 c	2.98 ± 0.13 c	4.60 ± 0.12 b	2.67 ± 0.06 b
	MNPK	58.60 ± 9.48 a	−2.52 ± 0.06 d	−4.07 ± 0.19 c	6.68 ± 0.19 a	4.27 ± 0.10 a	5.07 ± 0.03 a	2.90 ± 0.06 a
AV	CK	29.35 ± 2.05 d	−1.28 ± 0.06 a	−1.38 ± 0.05 a	2.53 ± 0.19 c	2.12 ± 0.16 d	1.18 ± 0.07 d	1.03 ± 0.01 d
	M	55.45 ± 2.14 b	−3.53 ± 0.02 d	−5.71 ± 0.15 d	4.59 ± 0.14 b	3.37 ± 0.10 b	3.04 ± 0.06 c	1.77 ± 0.01 c
	NPK	39.92 ± 1.75 c	−2.14 ± 0.02 b	−3.27 ± 0.14 b	4.02 ± 0.06 b	2.77 ± 0.12 c	3.51 ± 0.04 b	2.19 ± 0.02 b
	MNPK	67.91 ± 2.90 a	−3.19 ± 0.14 c	−4.55 ± 0.14 c	5.72 ± 0.15 a	3.87 ± 0.18 a	3.73 ± 0.12 a	2.24 ± 0.04 a

¹ T: treatment, PM: plastic mulching, NM: no mulching, CK: control, M: organic fertilizer, NPK: inorganic fertilizer, MNPK: organic combined with inorganic fertilizer, AV: average, ² apple production, ³ CH₄: methane, ⁴ CO₂: carbon dioxide, ⁵ N₂O: nitrous oxide. The data in the table are the mean ± standard deviation, n = 3; different letters in the same column indicate significant differences among treatments (p < 0.05).

, throughout the study, the average apple yield ranged from 23.97 to 93.49 t ha⁻¹. MNPK increased apple yield by approximately 51.72 to 93.43 t ha⁻¹ in 2018 and 2019, with an overall average increase of 115.76% and 85.57% when compared to NPK. However, compared to M in 2020, apple yield in the MNPK treatment decreased by 4.12%.

This indicated that the integrated use of organic and inorganic fertilizer in MNPK treatment and the single use of organic fertilizer in M treatment were more effective for improving apple yield than NPK (p < 0.05).

3.2. Ammonium Nitrogen and Nitrate Nitrogen Dynamics and Greenhouse Gases in Apple Orchards

3.2.1. Ammonium Nitrogen and Nitrate Nitrogen Dynamics in Apple Orchards

The highest NO₃⁻-N content was observed mostly in July in MNPK and NPK treatments of both PM (49.78 mg kg⁻¹) and NM (45.17 mg kg⁻¹) from 2018 to 2020 (Figure 4A,B). The average content of NO₃⁻-N in PM and NM ranged from 9.05 to 27.73 mg kg⁻¹ in PM and from 8.65 to 23.13 mg kg⁻¹ in NM. The highest NO₃⁻-N content in different sub treatments was observed in MNPK in both PM and NM with 25.54 and 20.18 mg NO₃⁻-N kg⁻¹, respectively (Table S1).

The highest NH₄⁺-N content was observed mostly in June in MNPK and NPK treatments of both PM (36.86 mg kg⁻¹) and NM (30.56 mg kg⁻¹) from 2018 to 2020 (Figure 4C,D). The cumulative amount of NH₄⁺-N ranged from 7.64 to 22.42 mg kg⁻¹ in PM and from 8.15 to 15.33 mg kg⁻¹ in NM. The highest NH₄⁺-N content in different sub treatments was observed in MNPK in both PM and NM with 18.61 and 12.83 mg NH₄⁺-N kg⁻¹, respectively (Table S1).

3.2.2. Greenhouse Gas Emissions in the Apple Orchard (N₂O, CO₂, and CH₄)

The highest N₂O flux was observed in July of every season (2018 to 2020) in NPK and MNPK treatments under PM and NM. The highest N₂O fluxes in PM were 0.11, 0.19, and 0.17 mg m⁻² h⁻¹ on 28 July 2018, 24 July 2019, and 7 July 2020, respectively, while the highest N₂O fluxes in MN were 0.15, 0.11, and 0.13 mg m⁻² h⁻¹ on 8 July 2018, 29 June 2019, and 27 July 2020, respectively (Figure 5A,B).

The average annual N₂O emissions ranged from 0.87 to 5.07 kg ha⁻¹ in PM and from 0.75 to 2.90 kg ha⁻¹ in NM. The N₂O emissions in PM under MNPK (3.73 kg ha⁻¹) were significantly different from 2.24 kg ha⁻¹ in NM under MNPK, and the N₂O emissions of different treatments were ranked as follows: MNPK > NPK > M > CK (Table 2).

The average daily CO₂ fluxes ranged from 12.20 to 356.09 mg m⁻² h⁻¹ in PM and from 11.24 to 185.00 mg m⁻² h⁻¹ in NM. The highest rise of CO₂ fluxes appeared in MNPK treatment under PM after yearly fertilization and 2 to 3 days after rain, increasing by 329.25, 331.05, and 356.09 mg m⁻² h⁻¹ recorded on 5 August 2018, 24 July 2019, and 13 July 2020, respectively, while in NM, the highest CO₂ fluxes were 147.12, 173.52, and 185.00 mg m⁻² h⁻¹ recorded on 5 August 2018, 28 August 2019, and 13 July 2020, respectively (Figure 6A,B).

The average annual cumulative CO₂ emissions ranged from 2.10 to 6.68 t ha⁻¹ in PM and from 1.98 to 4.27 t ha⁻¹ in NM. The MNPK treatment in both PM and NM differed considerably from other treatments and was ranked as follows: CK > MNPK > MNPK > NPK > CK. The average annual cumulative CO₂ emissions in MNPK were 5.72 t ha⁻¹ in MNPK under PM and 3.87 t ha⁻¹ in MNPK under NM (Table 2).

The average daily CH₄ fluxes in the PM ranged from 0.01 to 0.41 mg m⁻² h⁻¹ and from 0.01 to 0.61 mg m⁻² h⁻¹ in the NM. CH₄ fluxes increased between the end of June to the beginning of September throughout the experimental years. The CH₄ fluxes in M treatment under NM were recorded as 0.52, 0.60, and 0.46 mg m⁻² h⁻¹ on 26 May 2018, 24 May 2019, and 6 June 2020, respectively, while 0.37, 0.40, and 0.21 mg m⁻² h⁻¹ were recorded on 16 June 2018, 24 May 2019, and 6 June 2020 in PM (Figure 7A,B).

The average annual cumulative CH₄ uptake rate ranged from 1.19 to 4.25 kg ha⁻¹ in PM and from 1.14 to 6.75 kg ha⁻¹ in NM. In the PM and NM, the CH₄ uptake was ranked as follows: M > MNPK > NPK > CK. The annual CH₄ uptake in M under PM was 3.19 kg ha⁻¹ and 5.71 kg ha⁻¹ in M treatment under NM (Table 2).

3.3. Global Warming Potential (GWP) and Green House Gas Intensity (GHGI)

The GWP for PM in the years 2018–2020 was in the range of 227.68–1446.27 kg CO_2-eq ha⁻¹, while the GWP for NM was in the range of 184.64–763.78 kg CO_2-eq ha⁻¹. Treatments with higher GWPs include the MNPK treatment in PM (1030.62 kg CO_2-eq ha⁻¹) and the NPK treatment in NM (570.58 kg CO_2-eq ha⁻¹).

GHGI in PM ranged from 8.06 to 29.31 g kg⁻¹ from 2018 to 2020, while GHGI in NM ranged from 5.42 to 18.31 g kg⁻¹. NKP treatments had higher GHGI than other treatments in both PM and NM, with 24.99 and 15.08 g kg⁻¹, respectively. The N₂O emission factor was higher in MNPK in both PM and NM, with 0.40 and 0.19%, respectively, compared to the N₂O emission factor of NPK treatments of PM and NM (

Table 3. Effects of different management practices on apple yield and GHG emissions in the apple orchard.

Year	T 1	GWP (Emission CO2-eq ha−1) 2		GHGI (g kg−1) 3		N2O Emission EFd (%) 4
		PM	NM	PM	NM	PM	NM
2018	CK	227.68 ± 17.69 d	184.64 ± 19.12 d	8.06 ± 0.88 c	6.50 ± 1.40 b	-	-
	M	385.46 ± 13.31 c	264.08 ± 18.26 c	9.68 ± 0.77 b	6.65 ± 2.77 b	-	-
	NPK	580.60 ± 7.30 a	435.91 ± 13.52 a	24.40 ± 2.58 a	18.30 ± 1.78 a	0.19 ± 0.10 a	0.14 ± 0.05 b
	MNPK	483.44 ± 39.92 b	319.70 ± 14.83 b	9.36 ± 0.32 b	6.22 ± 1.53 b	0.16 ± 0.21 a	0.11 ± 0.02 a
2019	CK	309.57 ± 17.28 d	276.68 ± 11.50 c	10.86 ± 0.50 d	9.70 ± 2.10 a	-	-
	M	927.65 ± 36.31 c	352.76 ± 16.26 b	14.25 ± 0.76 b	5.42 ± 0.29 b	-	-
	NPK	1069.93 ± 31.49 b	553.05 ± 13.47 a	21.25 ± 0.43 a	10.99 ± 0.37 a	0.39 ± 0.20 b	0.17 ± 0.03 b
	MNPK	1162.15 ± 28.19 a	576.07 ± 40.10 a	12.44 ± 0.61 c	6.17 ± 0.54 b	0.48 ± 0.42 a	0.21 ± 0.02 a
2020	CK	422.75 ± 17.59 d	352.35 ± 10.50 d	13.68 ± 1.89 d	11.40 ± 1.65 b	-	-
	M	1141.13 ± 11.79 c	539.75 ± 26.60 c	18.68 ± 3.38 c	8.83 ± 2.23 c	-	-
	NPK	1330.07 ± 15.46 b	722.77 ± 19.75 b	29.31 ± 1.21 a	15.94 ± 2.09 a	0.46 ± 0.20 b	0.21 ± 0.01 b
	MNPK	1446.27 ± 10.54 a	763.78 ± 23.80 a	24.74 ± 1.48 b	13.06 ± 2.75 b	0.55 ± 0.60 a	0.25 ± 0.06 a
AV	CK	320.00 ± 7.40 d	271.22 ± 17.26 d	10.87 ± 0.62 d	9.20 ± 2.34 b	-	-
	M	818.08 ± 19.13 c	385.53 ± 22.19 c	14.20 ± 0.49 c	6.97 ± 2.34 d	-	-
	NPK	993.54 ± 10.55 b	570.58 ± 22.61 a	24.99 ± 1.13 a	15.08 ± 2.86 a	0.35 ± 0.05 b	0.17 ± 0.01 b
	MNPK	1030.62 ± 33.15 a	553.18 ± 19.84 b	15.51 ± 0.37 b	8.48 ± 4.60 c	0.40 ± 0.02 a	0.19 ± 0.01 a

¹ T: treatment, PM: plastic mulching, NM: no mulching, CK: control, M: organic fertilizer, NPK: inorganic fertilizer, MNPK: organic combined with inorganic fertilizer, AV: average, ² GWP: global warming potential, ³ GHGI: greenhouse gas intensity, ⁴ EF_d: N₂O emission factor. The data in the table are the mean ± standard deviation, n = 3; different letters in the same column indicate significant differences among treatments (p < 0.05).

).

3.4. The Influence of Environmental Factors on GHG Emissions in Soil

The analysis of the relationship between selected soil properties and climate variables with GHG emissions (N₂O, CO₂, and CH₄) is shown in Figure 8. All selected properties of the soil (SOM, soil pH, TN, NO₃⁻-N, WFPS, and soil temperature) except ammonium content in soil were significantly correlated to N₂O in both PM and NM (Figure 8A,B). NO₃⁻-N, soil pH, and soil temperature were more highly correlated with N₂O than other soil properties with R² of 0.69, while R² of SOM, WFPS, and TN were 0.68, 0.63, and 0.61, respectively, in PM. This observation was also the same for NM, where NO₃⁻-N, soil pH, and soil temperature were highly correlated with N₂O followed by SOM, WFPS, and TN, but NH₄⁺-N was not statistically correlated to N₂O in both PM and NM.

The characteristics of CO₂ emissions and increase and decrease periods were similar to those of N₂O emissions, but there was a significant difference between treatments. The CO₂ emissions were positively associated with soil temperature, WFPS, and SOM (p < 0.05), while CH₄ uptake was positively associated with soil temperature but negatively associated with WFPS, NH₄⁺-N, NO₃⁻-N, and other environmental factors (p < 0.05; Figure 8A,B).

4. Discussion

4.1. The Effects of PM and NM Combined with Different Fertilizer Applications on Nitrous Oxide Emissions

Human activities and agricultural production directly affected N₂O emissions [48]. Fertilizer application, temperature, and soil moisture were also the main factors causing gas emissions [49]. N₂O emissions in soil were mostly caused by the microbial processes of nitrification and denitrification [50]. In PM, average annual N₂O emissions increased from 9.16% to 98.31%, which was significantly higher than NM (p < 0.05; Figure 2), which was in line with the previous research [20].

Yu et al. [51] showed that PM increased N₂O emissions by 23.90% in comparison with NM. Inversely, PM in paddy fields reduced the N₂O emissions due to the low content of WFPS [41]. According to the study conducted by Abubakar et al. [52], WFPS in soil did not result in any significant N₂O emissions in wheat fields. Some research showed that PM decreased N₂O emissions because PM can improve root systems in plants for absorbing N in soil, which lowers N content in the soil. The N in soil was largely important for nitrification and denitrification [53]. The difference in temperature, WFPS content in the soil, and agricultural land management affected the difference in gas emissions [20]. In this study, N₂O emissions reached the highest rate between June to August every year due to the higher temperature (30 to 35 °C) and rainfall and the increase in WFPS in soil (30 to 60%) which affected the increase in N₂O fluxes (Figure 2 and Figure 5A,B). PM increased soil temperature, moisture, activities, and soil microbes. It also decreased oxygen levels in the soil and encouraged denitrification. Such factors had a positive association and affected the increase in N₂O emissions in the soil (Figure 8) [54].

There have not been any obvious results for what kind of fertilizer and amount of fertilizer are appropriate for GHG emissions in current studies [55]. The overapplication of fertilizer directly affected N₂O emissions because it promoted an increase in NO₃⁻-N and NH₄⁺-N [52]. The integrated use of organic and inorganic fertilizer significantly enhanced N₂O in soil [56]. Das and Adhya [57] found that the combination of poultry manure and N fertilizer in rice fields increased N₂O emissions more than either fertilizer alone because it promoted nitrification and denitrification and provided a carbon source for microorganisms to grow denitrifying bacteria. The report was in line with the study by Yang et al. [58] suggesting that the use of urea plus cow manure and urea plus pig manure increased N₂O emissions by a greater amount than the single use of urea or manure. The present study indicated that compared to NPK, M, and CK treatments, PM and NM in MNPK treatment significantly increased N₂O emissions by 6.06, 22.52, and 215.40%, respectively (p < 0.05, Table 2).

Because the amount of N content in fertilizer is one of the main factors causing N₂O emissions, in addition, PM and NM in MNPK treatment increased soil porosity, permeability, and evaporation of NH₄⁺-N and NO₃⁻-N, which increased the N₂O emissions in soil [59,60,61,62]. The highest rate of N₂O emissions was found 2 to 7 days after fertilization [63].

During fertilization from May to July, NH₄⁺-N and NO₃-N in MNPK treatment were higher than those in NPK treatment, accounting for 3.51 and 9.44% in PM and 11.27 and 5.61% in NM, respectively. The increase in NH₄⁺-N and NO₃-N promoted the processes of nitrification and denitrification in soil and affected the increase in N₂O emissions (Figure 5A,B), which was in line with the previous research. As shown in Figure 8, N₂O emissions are positively associated with environmental factors, including NO₃⁻-N and NH₄⁺-N [49], soil temperature, and WPFS level in soil [64].

Inversely, the study by Wei et al. [33] on meta-analysis in corn production found that organic fertilizer did not result in any N₂O and CH₄ emissions. The study reported further that the integrated use of organic and inorganic fertilizer decreased N₂O emissions by a greater amount than the single use of organic or inorganic fertilizer, as the use of organic fertilizer lowered N emissions in soil which suppressed the enzyme denitrification process and reduced the potential of nitrification, which means it decreased N₂O emissions [65,66].

Most of the studies on the effects of GHG emissions mostly only focused on the use of N fertilizer application, and there are very few studies on the use of different fertilizers combined with PM. The results of this study showed that the average annual cumulative N₂O emissions in the apple orchard ranged from 1.03 to 3.37 kg N ha⁻¹ (Table 2), which was greater than previous research by Pang et al. [67].

Moreover, the results showed that the average annual N₂O emissions in the apple orchard that applied different amounts of N fertilizer was around 2.40 kg N ha⁻¹. Compared to the research by Xie et al. [68], the average annual N₂O emissions were approximately 34.1 to 60.3 kg N ha⁻¹, which were significantly higher than the results found in this study.

4.2. The Effects of PM and NM Combined with Different Fertilizer Applications on Carbon Dioxide Emissions

The processes of agricultural management and production, temperature change, and WFPS in soil are associated with the increase in CO₂ emissions [42,69]. Those were the main factors causing soil respiration and improving soil microbial activities. When the soil temperature was sufficient, it promoted gas production and increased CO₂ emissions. According to the observation, CO₂ emissions increased at the temperature of 21.80 °C and WFPS content of 68.90% [70]. The CO₂ emissions in the soil were mostly caused by SOM decomposition, microbial respiration in soil, root respiration, and rhizosphere microorganisms [71]. The amount of carbon (C) was the main source of food and energy for the microorganisms in the soil. Soil microbes obtained food and energy from C by using microbial enzymes, microbes decompose, and organic matter, which resulted in CO₂ emissions [72].

According to the results of this study, the average annual CO₂ emissions in PM ranged from 5.97 to 56.45%, which was significantly greater than in NM (p < 0.05; Table 2). The CO₂ flux largely increased between July and September and decreased between November and April, especially 2 to 3 days after rain and yearly fertilization [34]. The characteristics and periods of CO₂ emissions were similar to N₂O emissions [73,74]. The results obtained in this research were in line with previous research [75]. Li et al. [22] and Chen and Wu [76] suggested that PM largely affected CO₂ emissions. The higher temperature enhanced microbial growth and accelerated SOM decomposition and soil respiration, which affected CO₂ emissions.

Figure 8 illustrates the positive association of CO₂ emissions, soil temperature, and WFPS. On the contrary, some research suggested that PM enhanced soil moisture, causing soil porosity and decreasing the diffusion of gas, which caused a decrease in CO₂ emissions [60,77].

Moreover, some previous research found that PM impeded CO₂ emissions from the soil to the atmosphere, since the PM decreased turbulent wind blowing from the soil surface [75,78]. Compared to NM, the CO₂ emissions were lower in PM. Both PM and NM, combined with the integrated use of organic and inorganic fertilizer in MNPK treatment, caused a large increase in CO₂ emissions. Compared to NPK treatment, CO₂ emissions in MNPK and M treatments were significantly higher, accounting for 42.34 and 14.22%, respectively (p < 0.05; Figure 6A,B; Table 2).

According to previous research, it has been shown that the use of organic fertilizer increased CO₂ emissions by 26.80% in comparison with the use of inorganic fertilizer because the use of organic fertilizer effectively improved soil microbial activity and soil respiration [20,33]. Compared to the other treatments, SOM in the MNPK and M treatments was higher, which caused the decomposition of C and increased CO₂ emissions [79]. Yang et al. [80] suggested that the integrated use of organic fertilizer (composted rapeseed cake) and N fertilizer increased CO₂ emissions more greatly than the single use of N fertilizer because organic fertilizer had more potential for water-soluble organic matter emissions that made it easier for soil microbes to decompose and the CO₂ emissions from soil to the atmosphere were greater in comparison with the use of N fertilizer. On the contrary, some researchers found that the integrated use of sheep manure and inorganic fertilizer showed a lower rate of CO₂ emissions in comparison with inorganic fertilizer because inorganic fertilizer can increase the respiration of plants and promote the decomposition of C in soil [81].

According to the results reported in this study, using PM combined with the integrated use of organic and inorganic fertilizer in MNPK treatment affected the increase in N₂O and CO₂ emissions more greatly than both the single use of organic fertilizer in M treatment and the single use of inorganic fertilizer in NPK treatment. Besides the use of fertilizer, environmental factors including air temperature, soil temperature, WPFS, SOM, NH₄⁺-N, and NO₃⁻-N significantly affected the increase in N₂O and CO₂ emissions in the apple orchard.

4.3. The Effects of PM and NM Combined with Different Fertilizer Applications on Methane Uptake

In this study, CH₄ fluxes were similar to most dryland farming in China; CH₄ fluxes in the soil were low, and the CH₄ flux value was negative. The negative value indicates the ability of the soil to sink the CH₄ [38]. It could be the sink for atmospheric CH₄ due to methanotrophs in agricultural soils oxidizing CH₄ under relatively dry conditions [82].

The CH₄ fluxes were the consequence of methanogens [83]. Moreover, soil moisture, temperature, fertilizers, oxygen diffusion in soil, and microbial activity were the major factors affected by the increase in CH₄ fluxes [84]. In PM, the average annual CH₄ uptake significantly decreased by 10.92% in comparison to NM, which accounted for 54.03% (p < 0.05; Table 2). This was in line with Yu et al. [51], who suggested that PM in upland agriculture decreased CH₄ uptake by 16% because PM impeded the soil–atmosphere exchange of CH₄ uptake [85]. Meanwhile, the findings of the study by Li et al. [22] conversely showed that PM increased CH₄ uptake by 151.40% in comparison with NM.

Compared with NPK treatment, we found that in M treatment, the cumulative CH₄ uptake increased by 65.01% in PM and 74.91% in NM, respectively (Table 2; p < 0.05), as a result of the higher SOM in soil. However, compared to the other treatments, N, NO₃⁻-N, and NH₄⁺-N in the soil in NPK were significantly lower, causing an increase in CH₄ uptake ability [86].

The study on using manure fertilizer in potato fields by Meng et al. [85] found that CH uptake was 1.1 to 2.1 times higher than using inorganic fertilizer, while Macharia et al. [87] found that goat manure reduced the potential of oxidation in soil and the activity of methanotrophs, which suppressed CH₄ uptake.

Figure 8 illustrates that CH₄ uptake was positively but not significantly associated with soil temperature. CH₄ uptake increased at a temperature of between 18 and 30 °C [49], but it was negatively associated with WFPS, SOM, NO₃⁻-N, and NH₄⁺-N in soil, which was in line with previous research [21]. Sullivan et al. [88] reported that the higher C level and soil temperature were the major controllers of CH₄ emissions in dryland. In addition, some research reported that the increase in NO₃⁻-N and NH₄⁺-N in the soil can suppress CH₄ uptake [89]. Therefore, CH₄ uptake in NPK treatment decreased in comparison with M and MNPK treatments.

However, the results of this study showed that the integrated use of organic and inorganic fertilizer in M and MNPK treatment was the best way to reduce CH₄ emissions.

4.4. The Effects of Plastic Mulching Combined with the Different Fertilizer Applications on Apple Yield, N₂O EF_d, GWP, and GHGI

In PM treatment, the type and amount of fertilizer were important for the quality of soil and the volume of yield [18]. Overfertilization led to soil deterioration and a decline in soil minerals and SOM [90]. However, PM combined with the integrated use of organic and inorganic fertilizer can improve soil physical properties, soil chemistry, and crop yield [91]. The average apple yield, the integrated use of organic and inorganic fertilizer in MNPK treatment, was more significantly effective for an increase in apple yield than NPK and CK treatment, accounting for 70.13% and 131.41%, respectively (p < 0.05; Table 2). This was in line with previous research conducted by Zhao et al. [92], who suggested that inorganic fertilizer combined with goat manure can improve soil structure, decrease soil water loss, increase water storage capacity, and increase apple yield.

The integrated use of inorganic fertilizer and organic fertilizer (oil residues, straw, manure) increased apple yield by 47.50% in comparison with CK treatment, which was in line with the research conducted by Zhang et al. [28], suggesting that the integrated use of MNPK can increase apple yield.

Using PM and NM combined with the integrated use of organic and inorganic fertilizers in MNPK treatment can improve the physical and chemical properties of soil and greatly increase apple yield. Nevertheless, it affected the increase in GHG emissions.

N₂O EF_d was the indicator measuring the standard value of the Intergovernmental Panel on Climate Change (IPCC) [93] by determining N₂O EF_d as 1%. The results showed that the average N₂O EF_d in PM and NM ranged from 0.35 to 0.40% and 0.17 to 0.19%, respectively, which were lower than IPCC and also lower than the results of N₂O EF_d found in the study by Xie et al. [68] accounting for 1.34%. Compared to NPK treatment, N₂O EF_d in MNPK treatment in both PM and NM increased by 14.91% and 9.77%, respectively (Table 3).

The GWP of farmlands was generally measured by GWP times for CH₄ and N₂O flux emissions on a time horizon of 100 years, which were estimated to have 25 and 298, respectively [94]. Comparing PM and NM during three seasons (2018–2020), GWP ranged from 227.68 to 1446.27 kg CO_2-eq ha⁻¹, which increased by 11.89 to 162.97%, since MNPK increased NH₄⁺-N and NO₃⁻-N in soil in the second and third seasons (Figure 4 and Table S1). In the first season, GWP was higher in NPK treatments in both PM and NM, while in the second and third seasons, GWP was higher in MNPK treatments for both PM and NM (Figure 5 and Table 3). Our results also confirmed the results of other studies which showed that combining organic manure and inorganic fertilizers increased NH₄⁺-N, NO₃⁻-N, and GWP in PM but not in NM [52].

The results were in line with previous research, suggesting that PM increased GWP from 12 to 82%, compared to the NM [95]. However, a meta-analysis by Yu et al. [51] reported that PM had slightly affected the increase in GWP in the uplands.

Yang et al. [96] reported that the integrated use of cow manure and inorganic fertilizer increased GWP more greatly than the single use of inorganic fertilizer, while Yang et al. [97] and Hongjun et al. [98] reported the converse results. In this study, the average GWP in the apple orchard was higher than those in previous research. For example, the average GWP in C oleifera and citrus orchards were 579.11 and 418.13 kg CO_{2 −eq} ha⁻¹ [49].

The increase in GWP is mostly caused by N₂O emissions, while GHGI emissions are mostly caused by N₂O emissions determined by apple yield [99]. The GHGI is associated with GHG emissions and crop yields. GHGI was one of the main indicators determining the appropriate standard of agricultural management in accordance with sustainable agriculture [100].

The average annual GHGI in PM ranged from 8.06 to 29.31 g kg⁻¹, accounting for 12.01 to 162.88%, which was significantly higher than NM (p < 0.05; Table 3). However, it was found to be lower in comparison with the previous research on pineapple orchards [81]. The GHGI in MNPK treatment was 37.92% in PM and 43.73% in NM. It was significantly higher than those in the NPK treatment (p < 0.05; Table 3) because there were higher apple yield despite the increase in GWP [30].

The results found in this study were in line with some previous research. Therefore, using PM combined with the integrated use of organic and inorganic fertilizer in MNPK treatment was the most suitable method with regard to the theory for sustainable agriculture because using MNPK fertilizer can reduce the production cost, decrease GHGI, and increase apple yield [101].

5. Conclusions

Plastic mulching (PM) increased soil temperature, WFPS, SOM, TN, NO₃⁻-N, and NH₄⁺-N in soil so that it significantly increased N₂O and CO₂ emissions, increasing from 9.16 to 98.31% in PM and from 5.97 to 56.45% in NM (p > 0.05). Moreover, PM decreased CH₄ uptake. In PM, CH₄ uptake in the apple orchard decreased by 10.92%, while it decreased by 54.03% in NM (p > 0.05). Using PM and NM combined with the integrated use of organic and inorganic fertilizer in MNPK treatment affected the increase in N₂O and CO₂ emissions. It was significantly higher in comparison with NM treatment (p > 0.05). Compared to the other treatments, the use of organic fertilizer in M treatment decreased CH₄ uptake from air into the soil by a larger amount (p > 0.05).

Besides PM, the major factors causing GHG emissions were soil temperature, WFPS, SOM, TN, NO₃⁻-N, and NH₄⁺-N, and these factors were positively associated with N₂O and CO₂ emissions. While there were only two factors, air and soil temperature, positively associated with CH₄ uptake, other factors were found to have negative associations (p < 0.05).

The increase in GHG emissions (N₂O and CO₂) influenced the increase in GWP. GWP in PM was 11.89 and 162.97% in NM. In PM and NM, GHG emissions in the MNPK treatment increased by a larger amount in comparison with NPK and the other treatments (p > 0.05), while GHGI in MNPK was significantly lower than in NPK, accounting for 37.92% in PM and 43.37% in NM (p > 0.05).

Although using PM combined with the integrated use of organic and inorganic fertilizer in MNPK treatment significantly increased GHG emissions and GWP, it increased soil physical and chemical properties and apple yield. Moreover, it decreased GHGI. Therefore, it is the most appropriate way to be applied to apple orchard management to promote sustainable agriculture, which aims to increase crop yields and decrease GHGI.