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

: 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 N 2 O 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 CO 2 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 N 2 O and CO 2 emissions in both PM and NM. The mean CH 4 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 CH 4 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.


Introduction
Global warming, caused by greenhouse gas (GHG) emissions consisting of nitrous oxide (N 2 O), carbon dioxide (CO 2 ), methane (CH 4 ), hydrofluorocarbons (HFCs), sulphur hexafluoride (SF 6 ), and nitrogen trifluoride (NF 3 ), 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 CO 2 (8%), N 2 O (81%), and CH 4 (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

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). effect of organic and inorganic fertilizers on global warming potential and greenhouse gas intensity in apple orchards.

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.  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).

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

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:

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.

. 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 Agriculture 2023, 13, 1211 6 of 23 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 N 2 O concentrations were measured by an electron capture detector (ECD), while the CO 2 and CH 4 concentrations were measured by an ion detector. GHG concentrations (N 2 O, CO 2 , and CH 4 ) were measured in ppm then transformed to mg m −2 h −1 by using the following equation by Mehmood et al. [42]: where F refers to the average flux of N 2 O, CO 2 , and CH 4 (mg m −2 h −1 ) in the soil, µ refers to the molar mass of N 2 O, CO 2 , and CH 4 (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 × 10 5 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 N 2 O, CO 2 , and CH 4 in the closed stainless-steel chamber (mL m −3 h −1 ). The total annual cumulative emissions of N 2 O, CO 2 , and CH 4 in the apple orchard were measured by using the following equation by Liu et al. [43]: where M refers to the average cumulative flux of N 2 O, CO 2 , and CH 4 (kg ha −1 ) in the soil, F refers to the average fluxes of N 2 O, CO 2 , and CH 4 (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 (N 2 O, CO 2 , and CH 4 ), the carbon dioxide equivalent (CO 2 ) based on the integrated global warming potential (GWP) of N 2 O, CO 2 , and CH 4 emissions was calculated using the following equation by Liu et al. [43]. GWP refers to the measurement of how much heat is absorbed by N 2 O, CO 2 , and CH 4 (kg CO 2-eq ha −1 ). The GWP of N 2 O and CH 4 emissions are 298 and 25 times that of CO 2 , with an average residence time of 100 years.
The GHGI was determined according to the method of Xu et al. [44]: The direct N 2 O emission factor (EF d ) was determined by a standard method of Toma et al. [45]: where F N refers to the annual emissions of N 2 O in nitrogen (N) fertilizer treatment (kg ha −1 ) and F CK refers to the annual emissions of N 2 O in the treatment without fertilizer application (kg ha −1 ).

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: where w i refers to the water-filled pore space (%) in the experimental area, W i refers to the soil water holding capacity (mm), ∆S 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 (NO 3 − -N and NH 4 + -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. NO 3 − -N and NH 4 + -N were analyzed using the salicylate-nitroprusside-hypochlorite method, with the absorbance at 600 nm. NO 3 − -N reduced to NO 2 − was analyzed using the VCl3 salicylatenitroprusside-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 (H 2 SO 4 ) 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 NO 3 − -N terminated when H 2 SO 4 was around 0.02. The solution changed its color from purple to red, and the standard of H 2 SO 4 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 (NH 4 OAc) 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 (K 2 Cr 2 O 7 ) 1.00 N solution, and 20 mL of H 2 SO 4 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.

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 (NH 4 + -N, NO 3 − -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.

Apple Yield
As shown in Table 2, 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).  (Table S1).

Ammonium Nitrogen and Nitrate Nitrogen Dynamics and Greenhouse Gases in
The highest NH 4 + -N content was observed mostly in June in MNPK and NPK treat-  (Table S1).     Table 2).

Greenhouse Gas
The average daily CO 2 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 CO 2 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 CO 2 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 CO 2 emissions ranged from 2.10 to 6.68 t ha −1 in PM and from 1.   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).

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 CO2-eq ha −1 , while the GWP for NM was in the range of 184.64-763.78 kg CO2-eq ha −1 . Treatments with higher GWPs include the MNPK treatment in PM (1030.62 kg CO2-eq ha −1 ) and the NPK treatment in NM (570.58 kg CO2-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 The average annual cumulative CH 4 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 CH 4 uptake was ranked as follows: M > MNPK > NPK > CK. The annual CH 4 uptake in M under PM was 3.19 kg ha −1 and 5.71 kg ha −1 in M treatment under NM (Table 2).

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 N 2 O emission factor was higher in MNPK in both PM and NM, with 0.40 and 0.19%, respectively, compared to the N 2 O emission factor of NPK treatments of PM and NM (Table 3).

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 (N 2 O, CO 2 , and CH 4 ) is shown in Figure 8. All selected properties of the soil (SOM, soil pH, TN, NO 3 − -N, WFPS, and soil temperature) except ammonium content in soil were significantly correlated to N 2 O in both PM and NM ( Figure 8A,B). NO 3 − -N, soil pH, and soil temperature were more highly correlated with N 2 O than other soil properties with R 2 of 0.69, while R 2 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 NO 3 − -N, soil pH, and soil temperature were highly correlated with N 2 O followed by SOM, WFPS, and TN, but NH 4 + -N was not statistically correlated to N 2 O in both PM and NM.
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). WFPS: water-filled pore space. *, **, and *** indicate significant effects (p < 0.05), highly significant effects (p < 0.01), and extremely significant effects (p < 0.001), respectively.
The characteristics of CO 2 emissions and increase and decrease periods were similar to those of N 2 O emissions, but there was a significant difference between treatments. The CO 2 emissions were positively associated with soil temperature, WFPS, and SOM (p < 0.05), while CH 4 uptake was positively associated with soil temperature but negatively associated with WFPS, NH 4 + -N, NO 3 − -N, and other environmental factors (p < 0.05; Figure 8A,B).

The Effects of PM and NM Combined with Different Fertilizer Applications on Nitrous Oxide Emissions
Human activities and agricultural production directly affected N 2 O emissions [48]. Fertilizer application, temperature, and soil moisture were also the main factors causing gas emissions [49]. N 2 O emissions in soil were mostly caused by the microbial processes of nitrification and denitrification [50]. In PM, average annual N 2 O 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 N 2 O emissions by 23.90% in comparison with NM. Inversely, PM in paddy fields reduced the N 2 O 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 N 2 O emissions in wheat fields. Some research showed that PM decreased N 2 O 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, N 2 O 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 N 2 O fluxes (Figures 2 and 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 N 2 O 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 N 2 O emissions because it promoted an increase in NO 3 − -N and NH 4 + -N [52]. The integrated use of organic and inorganic fertilizer significantly enhanced N 2 O in soil [56]. Das and Adhya [57] found that the combination of poultry manure and N fertilizer in rice fields increased N 2 O 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 N 2 O 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 N 2 O 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 N 2 O emissions, in addition, PM and NM in MNPK treatment increased soil porosity, permeability, and evaporation of NH 4 + -N and NO 3 − -N, which increased the N 2 O emissions in soil [59][60][61][62]. The highest rate of N 2 O emissions was found 2 to 7 days after fertilization [63].
During fertilization from May to July, NH 4 + -N and NO 3 -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 NH 4 + -N and NO 3 -N promoted the processes of nitrification and denitrification in soil and affected the increase in N 2 O emissions ( Figure 5A,B), which was in line with the previous research. As shown in Figure 8, N 2 O emissions are positively associated with environmental factors, including NO 3 − -N and NH 4 + -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 N 2 O and CH 4 emissions. The study reported further that the integrated use of organic and inorganic fertilizer decreased N 2 O 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 N 2 O 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 N 2 O 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 N 2 O 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 N 2 O emissions were approximately 34.1 to 60.3 kg N ha −1 , which were significantly higher than the results found in this study.

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 CO 2 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 CO 2 emissions. According to the observation, CO 2 emissions increased at the temperature of 21.80 • C and WFPS content of 68.90% [70]. The CO 2 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 CO 2 emissions [72].
According to the results of this study, the average annual CO 2 emissions in PM ranged from 5.97 to 56.45%, which was significantly greater than in NM (p < 0.05; Table 2). The CO 2 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 CO 2 emissions were similar to N 2 O 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 CO 2 emissions. The higher temperature enhanced microbial growth and accelerated SOM decomposition and soil respiration, which affected CO 2 emissions. Figure 8 illustrates the positive association of CO 2 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 CO 2 emissions [60,77].
Moreover, some previous research found that PM impeded CO 2 emissions from the soil to the atmosphere, since the PM decreased turbulent wind blowing from the soil surface [75,78]. Compared to NM, the CO 2 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 CO 2 emissions. Compared to NPK treatment, CO 2 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 CO 2 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 CO 2 emissions [79]. Yang et al. [80] suggested that the integrated use of organic fertilizer (composted rapeseed cake) and N fertilizer increased CO 2 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 CO 2 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 CO 2 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 N 2 O and CO 2 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, NH 4 + -N, and NO 3 − -N significantly affected the increase in N 2 O and CO 2 emissions in the apple orchard.

The Effects of PM and NM Combined with Different Fertilizer Applications on Methane Uptake
In this study, CH 4 fluxes were similar to most dryland farming in China; CH 4 fluxes in the soil were low, and the CH 4 flux value was negative. The negative value indicates the ability of the soil to sink the CH 4 [38]. It could be the sink for atmospheric CH 4 due to methanotrophs in agricultural soils oxidizing CH 4 under relatively dry conditions [82].
The CH 4 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 CH 4 fluxes [84]. In PM, the average annual CH 4 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 CH 4 uptake by 16% because PM impeded the soil-atmosphere exchange of CH 4 uptake [85]. Meanwhile, the findings of the study by Li et al. [22] conversely showed that PM increased CH 4 uptake by 151.40% in comparison with NM.
Compared with NPK treatment, we found that in M treatment, the cumulative CH 4 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, NO 3 − -N, and NH 4 + -N in the soil in NPK were significantly lower, causing an increase in CH 4 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 CH 4 uptake. Figure 8 illustrates that CH 4 uptake was positively but not significantly associated with soil temperature. CH 4 uptake increased at a temperature of between 18 and 30 • C [49], but it was negatively associated with WFPS, SOM, NO 3 − -N, and NH 4 + -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 CH 4 emissions in dryland. In addition, some research reported that the increase in NO 3 − -N and NH 4 + -N in the soil can suppress CH 4 uptake [89]. Therefore, CH 4 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 CH 4 emissions.

The Effects of Plastic Mulching Combined with the Different Fertilizer Applications on Apple Yield, N 2 O 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.  Table 3).
The GWP of farmlands was generally measured by GWP times for CH 4 and N 2 O 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 NH 4 + -N and NO 3 − -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 NH 4 + -N, NO 3 − -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 N 2 O emissions, while GHGI emissions are mostly caused by N 2 O 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].

Conclusions
Plastic mulching (PM) increased soil temperature, WFPS, SOM, TN, NO 3 − -N, and NH 4 + -N in soil so that it significantly increased N 2 O and CO 2 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 CH 4 uptake. In PM, CH 4 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 N 2 O and CO 2 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 CH 4 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, NO 3 − -N, and NH 4 + -N, and these factors were positively associated with N 2 O and CO 2 emissions. While there were only two factors, air and soil temperature, positively associated with CH 4 uptake, other factors were found to have negative associations (p < 0.05).
The increase in GHG emissions (N 2 O and CO 2 ) 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.

Supplementary Materials:
The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/agriculture13061211/s1, Table S1: Effects of different management practices on soil properties in the apple orchard. Data Availability Statement: All data generated or analyzed during this study are included in this manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.