Characteristics of Greenhouse Gas Emissions from Yellow Paddy Soils under Long-Term Organic Fertilizer Application

Abstract

Reducing greenhouse gas emissions from rice fields is essential to respond to the national “dual-carbon” strategy, achieve green agricultural development, and ensure food security. The substitution of organic fertilizers for chemical fertilizers is an important means to achieve zero growth and has a positive impact on crop yield and soil nutrients; however, the impact on the greenhouse effect is inconsistent. The effects of organic fertilizers on soil greenhouse gas emissions vary depending on factors such as soil, geography, ecological environment, and human management. However, previous research has shown that the combined application of organic fertilizer can increase soil carbon storage and increase crop yield, and may be an effective fertilization measure to reduce greenhouse gas emissions from yellow paddy fields. To clarify the effects of different ratios of organic fertilizer on the greenhouse gas emission characteristics of Guizhou yellow paddy soil, CH₄, CO₂, and N₂O emissions from rice fields were monitored by static opaque chamber-gas chromatography, and the effects of different fertilization treatments on the cumulative greenhouse gas emissions and global warming potential (GWP) were investigated. Results showed that organic fertilizer application increased CH₄ emissions from rice fields, and the effect increased with increasing organic fertilizer application. The peak period was from the heading stage to the filling and ripening stage, and there was almost no emission during the fallow period. Compared with the balanced application of chemical fertilizer (NPK), the treatment with organic fertilizer alone (M) significantly increased CO₂ emissions, but the replacement of 1/2 chemical fertilizer nitrogen with 1/2 organic fertilizer (1/2 M + 1/2 N-PK) and the replacement of 1/4 chemical fertilizer nitrogen with 1/4 organic fertilizer (1/4 M + 3/4 N-PK) did not significantly increase CO₂ emissions; emissions were 5% lower in the 1/2 M + 1/2 N-PK treatment than in the NPK treatment. Compared with the NPK treatment, the application of organic fertilizer alone significantly reduced N₂O emissions by 32.16%, while the 1/2 M + 1/2 N-PK and 1/4 M + 3/4 N-PK treatments increased N₂O emissions by 6.31% and 16.02%, respectively. However, there were no significant differences between the organic–inorganic combined treatments and NPK. During the flooding period, N₂O emissions were relatively low, but the emissions increased rapidly after field drying. The application of organic fertilizer increased the GWP of rice fields. Compared with the NPK treatment, the M treatment increased GWP by 47.07%, 1/2 M + 1/2 N-PK increased GWP by 10.16%, and the 1/4 M + 3/4 N-PK treatment increased GWP by 2.93%. Except for the M treatment, the differences between treatments were not significant. Our results concluded that replacement of chemical fertilizers with organic fertilizers at a ratio of 1/4 to 1/2 did not significantly increase greenhouse gas emissions in rice fields, besides, it mitigate the greenhouse effect and increase soil carbon sequestration and yield in rice fields.

1. Introduction

With the rapid development of the economy and the increase in the human population, food security has been seriously challenged. As a decisive factor in increasing grain production, chemical fertilizers have played a pivotal role in production, but the continuous increase in chemical fertilizers over the past 20 years has led to decreased yield, a continuous decline in soil quality and excessive waste of nutrient resources [1]. Nitrogen loss from farmland in China accounts for approximately 30–70% of the total amount of chemical nitrogen fertilizer; the loss from paddy fields is higher than that in drylands, and the uptake and utilization rate of nitrogen in one rice season is only 35% [2]. Fertilizer nitrogen is released into the atmosphere through nitrification–denitrification to produce N₂O, which impacts both CO₂ and CH₄ emissions [3]. Thus, the improvement in farmland quality through rational fertilization and other farmland management measures are of great importance to alleviating China’s food insecurity, resource consumption and improving environmental protection. Organic fertilizer has become important in China’s agricultural production because of its rich organic and inorganic nutrients. The substitution of organic fertilizers for chemical fertilizers is an important way to achieve zero growth, which has a positive impact on crop yield and soil nutrients; however, its impact on the greenhouse effect is uncertain because both the nature of the organic fertilizer and the ecological environment of the farmland have important impacts on greenhouse gas emissions. To this end, it is necessary to systematically conduct regional greenhouse gas emission studies on paddy fields and propose fertilization measures tailored to local conditions; it is the key goal of our research to maintain or increase rice yield through appropriate fertilization patterns while reducing the effect of global warming.

Climate change is a global challenge. To avoid a continuous rise in temperature, the Paris Agreement stated that the temperature increase in this century should be limited to less than 1.5 °C [4]. CO₂, CH₄, and N₂O are major greenhouse gases, and reducing their emissions is critical to suppressing global temperature rise [5]. Farmland greenhouse gas emissions are one of the main sources of greenhouse gases worldwide: 5–20% of CO₂, 15–30% of CH₄ and 80–90% of N₂O in the atmosphere come from farmland soil emissions each year [3]. Paddy soil is one of the most important sources of CH₄ emissions, and the total annual CH₄ emissions from paddy fields account for approximately 20% of the total annual worldwide emissions of anthropogenic CH₄ [6]. Therefore, without reasonable emission reduction measures, it is predicted that farmland soils will increase CH₄ emissions by 50–60% and N₂O emissions by 35–60% by 2030 compared with 2005 [7,8].

Rice is the main food of nearly 50% of the world population; production is mainly distributed within Asia, and China is the world’s largest rice producer. In 2021, the total cultivated area in China reached 29.9 million ha [9]. The anaerobic environment caused by the long-term flooding of rice fields results in CH₄ production [10]. The global CH₄ emissions from rice fields are approximately 2.5 × 10⁴ Gg, accounting for 24% of global agricultural CH₄ emissions. The CH₄ emissions from rice fields in China are approximately 5.4 × 10³ Gg, accounting for approximately 21.5% of the worldwide total CH₄ emissions from rice fields [11]. On the basis of ensuring food security, reducing greenhouse gas emissions from rice fields is essential to supporting the national “dual-carbon” strategy and achieving green agricultural development.

The generation and emission of greenhouse gases are affected by factors such as soil temperature, moisture, fertility, and farmland management measures [12,13,14,15]. The characteristics of greenhouse gas emissions under various fertilization treatments are inconsistent. Compared with the application of chemical fertilizers alone, the application of organic fertilizers alone or the combined application of organic and inorganic fertilizers could significantly promote soil CH₄ emissions while inhibiting N₂O emissions [3,16]. The results of studies on the characteristics of CO₂ emissions are rather inconsistent. Some scholars argue that the combined application of organic and inorganic fertilizers has no significant effect on CO₂ emissions [17]. Some results also showed that CO₂ emissions under the application of organic fertilizer alone were lower [18], while the emissions under the combined application of organic and inorganic fertilizers were higher than those under chemical fertilizer use alone. However, the emissions were higher than those with single application of chemical fertilizer. The application of organic fertilizers can increase global warming potential (GWP) [19,20,21], but the effects of the combined application of organic and inorganic fertilizers on GWP have been variable. Yang et al. [22] found that the combined application of organic and inorganic fertilizers could increase the GWP of soil, while Gao et al. [23] drew the opposite conclusion. Another study concluded that there was no significant effect on GWP [24]. In the early stage, many scholars found that the greenhouse gas emission intensity under organic fertilizer application alone, or in combination with chemical fertilizer treatment, was lower than that of chemical fertilizer application alone [16,20,25]; however, some studies have reported the opposite results [21]. The effects of organic fertilizers on soil greenhouse gas emissions vary depending on factors such as soil, geography, ecological environment, and human management methods. In this study, we conducted a long-term investigation of the characteristics of greenhouse gas emissions from yellow paddy soils in central Guizhou under different substitution ratios of organic fertilizer in a long-term experiment. The aim was to identify suitable organic fertilizer substitution ratios for achieving sustainable production, reducing chemical fertilizer use, and promoting environmentally friendly rice cultivation; the results further provide a technical reference and data support for regional sustainable agricultural development, carbon sequestration, and emissions reductions in farmland. Increasing rice production and reducing environmental costs are important to ensure China’s food security and build a healthy ecological environment. Our previous research found that the application of organic fertilizer can significantly increase the effective panicle number of rice and thus significantly increase the rice yield, and there was no significant difference between the combined application of organic fertilizer and inorganic fertilizer and the application of pure organic fertilizer [26]. We also found that the application of organic fertilizer can significantly improve the soil carbon storage capacity [27]. Thus, the combined application of organic fertilizer can increase soil carbon storage and increase crop yield. and may be an effective fertilization strategy to reduce greenhouse gas emissions from yellow paddy fields.

2. Materials and Methods

2.1. Overview of the Experimental Area

This study is based on the long-term site experiment by the Scientific Observation and Experimental Station of Arable Land Conservation and Agricultural Environment (Guizhou), the Ministry of Agriculture, which is located at the Guizhou Academy of Agricultural Sciences, Huaxi District, Guiyang, Guizhou Province (106°39′52″ E, 26°29′49″ N). The station is situated in the hilly region of yellow soil in central Guizhou, with a subtropical monsoon climate. The average altitude is 1071 m, the average annual temperature is 15.3 °C, the average annual sunshine duration is approximately 1354 h, the relative humidity is 75.5%, the annual frost-free period is approximately 270 days, and the annual precipitation ranges from 1100 to 1200 mm. The soil type at the experimental site is yellow paddy soil. Continuous monitoring of the long-term site experiment began in 1995. The basic physicochemical properties of the initial soil samples (0–20 cm) were as follows: organic matter, 44.9 g·kg⁻¹; total nitrogen, 1.96 g·kg⁻¹; total phosphorus, 0.95 g·kg⁻¹; total potassium, 16.4 g·kg⁻¹; alkaline hydrolyzable nitrogen, 159 mg·kg⁻¹; available phosphorus, 13.4 mg·kg⁻¹; available potassium, 294 mg·kg⁻¹; and pH, 6.75.

2.2. Experimental Design

The long-term site experiment was designed as a large-scale comparative experiment, with a total rice planting area of 2814 m²; each plot was 201 m² (35.7 m length × 5.6 m width). In this study, four plots were selected, including the balanced application of chemical fertilizer (NPK), 1/4 organic fertilizer to replace 1/4 chemical fertilizer nitrogen (1/4 M + 3/4 N-PK), 1/2 organic fertilizer to replace 1/2 chemical fertilizer nitrogen (1/2 M + 1/2 N-PK), and organic fertilizer alone (M). The chemical fertilizers used in the experiment were urea (containing 46.0% N), calcium superphosphate (containing 12.0% P₂O₅), and potassium chloride (containing 60% K₂O). The commercial organic fertilizer was fermented from cow manure (dry matter ratio: 25.5%; organic matter content: 44.5%; N: 3.04%; P₂O₅: 0.44%; K₂O: 2.75%) (Guizhou Wuguhui Ecological Agriculture Technology Co.). Except for the different application rates of P and K in the M treatment, the pure N, P, and K inputs of the other treatments were identical. The chemical nitrogen fertilizer was applied in two topdressings with a 40%:60% ratio of striking root fertilizer to tillering fertilizer (

Table 1. Total N, P, and K nutrient application contents of different fertilization treatments.

Treatments	Organic Fertilizer t·hm−2	N	P2O5	K2O
		kg·hm−2
NPK	0.0	165.0	82.5	82.5
1/4 M + 3/4 N-PK	5.3	165.0	82.5	82.5
1/2 M + 1/2 N-PK	10.6	165.0	82.5	82.5
M	21.3	165.0	23.9	149.4

). The tested rice variety was Yixiangyou 800, which was transplanted on 9 June 2021. The first topdressing (striking root fertilizer) was conducted on 20 June, the second topdressing (tillering/heading fertilizer) was conducted on 6 July, and field drying began on approximately 14 September. The rice fields were not irrigated until 8 October, and the fields were tilled and dried in winter. Except for the difference in fertilization, all the treatments were consistent in other aspects of agricultural management.

The sampling device was positioned in the field relative to the three equal parts of the test site, and three static boxes (one on the top, one in the middle and one on the bottom) were placed at each test site. By eliminating the side row, the base was selected in the test area for more convenient later sample collection with minimal human disturbance; the base remained unchanged throughout the test cycle, and the rice planting density in the box was unchanged. Each time a sample was taken, the field was entered and the water surface was allowed to stabilize for five minutes without fluctuation; the gas sample was then collected according to the collection specifications. Gas samples were collected after rice transplanting on 11 June 2021 and collected once every 7 days as well as after fertilization; collection was postponed in the case of heavy rain. The sampling frequency was reduced to twice a month after rice harvest, and the last date of testing was December 31, 2021; the total collection period was up to 7 months.

2.3. Sample Collection and Analysis

A closed static opaque chamber method was used to collect the gas. The sampling device was made of stainless steel and mainly consisted of a top box (50 cm × 50 cm × 50 cm), a middle box (50 cm × 50 cm × 50 cm), and a base (50 cm × 50 cm × 15 cm). The exterior of the entire box was wrapped with a reflective, thermally insulated film; and a small fan, a temperature meter, a pressure balance tube, and a gas sampling port were installed inside the top box. In this study, soil CH₄, CO₂, and N₂O emissions were monitored after rice transplanting. Samples were collected once every 7 days. After topdressing, the sampling time was fixed at 8:00 to 11:00 am. Before each measurement, the tank was filled with water and inserted into the box to form an airtight space. At 0, 15, and 30 min after the fan was turned on, 200 mL of air was pumped into the aluminum foil air bag using a syringe (connected with a three-way valve), and the temperature in the box was recorded at the same time.

Gas sample analysis was performed using an Agilent GC-7890A gas chromatograph. The CH₄ and CO₂ detectors were flame ionization detectors (FIDs), the carrier gas was nitrogen, the detector temperature was 250 °C, the separation column temperature was 55 °C, and the N₂O detector was an electron capture detector (ECD). The carrier gas was nitrogen, the detection temperature was 350 °C, and the column temperature was 55 °C. The reference gas was purchased from the National Sharing Platform for Reference Materials of the National Institute of Metrology.

2.4. Calculation and Data Processing

The formula for calculating gas emission flux [28] is:

$$\mathrm{F}=\mathsf{\rho }\times \mathrm{H}\times \frac{\Delta \mathrm{c}}{\Delta \mathrm{t}}\times \frac{273}{273+\mathrm{T}}$$

(1)

where F is the greenhouse gas emission flux (the unit of CO₂ and CH₄ emission fluxes is mg·m⁻²·h⁻¹;the unit of N₂O emission flux is μg·m⁻²·h⁻¹), ρ is the density of the greenhouse gas in the standard state in kg·m⁻³, H is the net height of the sampling box in m, Δc/Δt is the rate of change in greenhouse gas concentration in the sampling box in μL·l⁻¹·h⁻¹, and T is the temperature in the sampling box in °C.

The formula for calculating cumulative emissions is:

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

(2)

where f is the cumulative greenhouse gas emissions in kg·hm⁻², F is the greenhouse gas emission flux, n is the total number of samples, i is the number index of a sample, and ${\mathrm{t}}_{\mathrm{i}+1}-{\mathrm{t}}_{\mathrm{i}}$ is the number of days between two sampling events.

GWP on a 100-yr time scale is generally expressed in terms of CO₂ equivalents:

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

(3)

where GWP is the global warming potential in kg·hm⁻², and f (CH₄) and f (N₂O) are the total seasonal CH₄ and N₂O emissions, respectively, in kg·hm⁻².

SPSS 22.0 software was used for the one-way ANOVA, and the significance of differences was analyzed using LSD (Least Significant Difference) test; and univariate analysis of General Linear Model (GLM) was performed to determine interactions between variables; the significance level was set to α = 0.05. Excel 2010 was used for data processing, analysis, and plotting.

3. Results

3.1. Methane Emission Characteristics

There were significant differences in CH₄ emission fluxes from rice fields with different fertilization treatments. As shown in Figure 1, the CH₄ emission flux continuously increased after rice transplanting, and the emission peak lasted from the end of the heading and flowering stages to the beginning of the grain filling stage. The treatments, in order of emission flux amount (mg·m⁻²·h⁻¹), were M (15.90) > 1/2 M + 1/2 N-PK (8.93) > 1/4 M + 3/4 N-PK (5.18) > NPK (4.27), with a significant decrease in the later stage until the CH₄ emission flux was almost zero during the fallow period. The mean CH₄ emission flux (mg·m⁻²·h⁻¹) during the monitoring period of each treatment in descending order was M (5.17) > 1/2 M + 1/2 N-PK (2.90) > 1/4 M + 3/4 N-PK (1.60) > NPK (1.14). The results of this study showed that the complete replacement of chemical fertilizers by organic fertilizers could significantly increase CH₄ emissions from yellow paddy soil, but the appropriate ratio of organic replacement could significantly reduce peak and average emissions.

As shown in

Table 2. Cumulative emissions of CH₄ in different periods of fertilizer use (kg·hm⁻²).

Treatment	Seedling	Tillering	Jointing and Booting	Heading and Flowering	Grain Filling	Fallow Period
NPK	0.83 ± 0.27 b	5.85 ± 1.49 b	11.63 ± 3.57 c	14.68 ± 5.91 c	14.66 ± 4.03 c	−0.32 ± 0.39 ab
M	2.49 ± 1.02 a	25.54 ± 10.1 a	57.16 ± 4.75 a	71.78 ± 9.96 a	60.46 ± 10.48 a	−0.71 ± 0.17 b
1/2 M + 1/2 N-PK	2.34 ± 1.16 ab	14.65 ± 1.25 b	28.25 ± 0.65 b	43.74 ± 7.19 b	32.26 ± 2.57 b	0.01 ± 0.09 a
1/4 M + 3/4 N-PK	1.18 ± 0.67 ab	8.15 ± 3.64 b	12.92 ± 4.81 c	23.41 ± 5.58 c	22.04 ± 7.97 bc	−0.10 ± 0.10 a

Note: Different lowercase letters indicate significant differences between different treatments (p < 0.05).

below, the order of the contribution of CH₄ emissions to the total emissions during the entire monitoring period (d) at different stages of each treatment was as follows: heading and flowering stage (21) > grain filling stage (33) > jointing and booting stage (22) > tillering stage (21) > seedling stage (15) > fallow period (92). During the fallow period, except for the 1/2 M + 1/2 N-PK treatment, which generated a small amount of CH₄ emissions, the treatments all showed net CH₄ absorption. The characteristics of CH₄ emissions from different treatments during the entire monitoring period indicate that CH₄ emissions were concentrated in the middle and late stages of rice growth, and there was almost no emission during the fallow period. Overall, there was no significant difference in emissions between the NPK treatment and 1/4 organic fertilizer substitution, which is the ratio of organic fertilizer replacement recommended based on comprehensive consideration of factors such as the economic benefits of production.

3.2. Carbon Dioxide Emission Characteristics

After rice transplantation, the CO₂ emission flux increased continuously, with a total of three emission peaks. The first peak occurred on August 7 (the end of the heading and flowering period), the second peak occurred on September 3 (the beginning of the grain filling period), the third peak occurred approximately two weeks after field drying, and the CO₂ emission flux was significantly reduced during the fallow period. The treatments ordered by the highest peak CO₂ emissions (mg·m⁻²·h⁻¹) were as follows: M (1866.18) > 1/4 M + 3/4 N-PK (1401.52) > NPK (1361.71) > 1/2 M + 1/2 N-PK (1231.95). Treatments were ranked by average emission flux (mg·m⁻²·h⁻¹) as follows: M (621.51) > 1/4 M + 3/4 N-PK (521.34) > NPK (486.76) > 1/2 M + 1/2 N-PK (448.34). The application of organic fertilizer alone significantly increased the CO₂ emissions from the rice field, but the difference between the organic fertilizer substitution treatments and the NPK treatment was not significant, and the 1/2 organic fertilizer substitution slightly reduced the emission value (Figure 2).

The growing stages (d) were ordered by CO₂ emissions as follows: grain filling stage (33) > heading and flowering stage (21) > fallow period (92) > jointing and booting stage (22) > tillering stage (21) > seedling stage (15) (

Table 3. Characteristics of cumulative CO₂ emissions in different periods of fertilizer use (kg·hm⁻²).

Treatment	Seedling	Tillering	Jointing and Booting	Heading and Flowering	Grain Filling	Fallow Period
NPK	155.37 ± 11.51 a	1674.19 ± 356.62 ab	4099.53 ± 780.71 ab	4157.01 ± 568.48 b	8102.6 ± 1689.03 b	4370.40 ± 71.60 b
M	175.67 ± 58.10 a	1552.47 ± 121.54 ab	4666.34 ± 232.12 a	5959.72 ± 504.92 a	12,153.74 ± 834.65 a	5180.45 ± 362.07 a
1/2 M + 1/2 N-PK	175.58 ± 42.80 a	1409.30 ± 248.44 b	3488.98 ± 329.49 b	4427.75 ± 856.44 b	7317.56 ± 501.88 b	4580.19 ± 587.21 ab
1/4 M + 3/4 N-PK	221.26 ± 38.23 a	1869.02 ± 128.07 a	4490.75 ± 458.28 a	5936.88 ± 1039.4 a	7972.58 ± 1109.28 b	3961.52 ± 355.43 b

Note: Different lowercase letters indicate significant differences between different treatments (p < 0.05).

). There were no significant differences in CO₂ emissions among the treatments during the initial stage. There were no significant differences between the 1/2 organic fertilizer substitution and NPK treatments in the jointing and booting stage or the heading and flowering stage, and emissions were higher in the remaining treatments. Emissions were higher in the M treatment than in the other treatments during the grain filling stage and fallow period, and there were no significant differences among the other treatments. If only the effect of fertilizer use on CO₂ emissions is considered, 1/2 organic substitution is a suitable organic substitution ratio.

3.3. Characteristics of Nitrous Oxide Emissions

The characteristics of N₂O emissions from rice fields are greatly affected by soil moisture. During the flooding period, N₂O emissions are relatively small and are often accompanied by N₂O absorption. There were two emission peaks in this study. The first peak occurred on September 30 (two weeks after field drying), and the second emission peak occurred on October 21 (fallow period). Soil N₂O emissions were high throughout the fallow period. Treatments were ordered by highest peak N₂O emissions (µg·m⁻²·h⁻¹) as follows: 1/4 M + 3/4 N-PK (61.73) > 1/2 M + 1/2 N-PK (61.65) > NPK (55.70) > M (36.06). Treatments were ordered by average N₂O emission flux (µg·m⁻²·h⁻¹) as follows: NPK (11.37) > 1/4 M + 3/4 N-PK (11.17) > 1/2 M + 1/2 N-PK (9.97) > M (6.19). Compared with the NPK treatment, the application of organic fertilizer reduced N₂O emissions from the rice field, and the M treatment exhibited the most significant effect (Figure 3).

As shown in

Table 4. Characteristics of cumulative N₂O emissions in different periods of fertilizer use (kg·hm⁻²).

Treatment	Seedling	Tillering	Jointing and Booting	Heading and Flowering	Grain Filling	Fallow Period
NPK	0.015 ± 0.003 a	0.014 ± 0.002 a	0.018 ± 0.006 a	−0.025 ± 0.009 ab	0.106 ± 0.009 a	0.664 ± 0.086 bc
M	0.012 ± 0.003 a	0.003 ± 0.002 c	−0.020 ± 0.005 b	−0.026 ± 0.016 ab	0.003 ± 0.008 c	0.566 ± 0.047 c
1/2 M + 1/2 N-PK	0.007 ± 0.001 b	0.008 ± 0.002 b	−0.027 ± 0.015 b	−0.012 ± 0.007 a	0.059 ± 0.008 b	0.809 ± 0.066 ab
1/4 M + 3/4 N-PK	0.008 ± 0.001 b	0.006 ± 0.005 bc	−0.012 ± 0.004 b	−0.035 ± 0.006 b	0.100 ± 0.010 a	0.853 ± 0.132 a

Note: Different lowercase letters indicate significant differences between different treatments (p < 0.05).

, N₂O emissions during the rice growth period accounted for a very small proportion of the emissions during the entire monitoring period, and N₂O emissions during the fallow period were relatively high. The rice growth period accounted for 16.27% of the total N₂O emissions in the NPK treatment, and the fallow period accounted for 16.27%. The M treatment showed a net N₂O absorption throughout the reproductive period and 105.20% emissions during the fallow period. The contributions of the growth and fallow periods to N₂O emissions under 1/2 M + 1/2 N-PK were 4.03% and 95.97%, respectively. The contributions under 1/4 M + 3/4 N-PK were 7.28% and 92.72% during the growth period and fallow period, respectively.

GHG fluxes varied greatly between treatments during the same growth period and during different growth periods for the same fertilizer treatment, which supports the conclusion that GHG emissions are affected by the interaction of the multiple factors described above. The data in

Table 5. Effects of different fertilization treatments and growth periods on greenhouse gas emission flux.

Factors	Greenhouse Gas Emission Flux
	CH4	CO2	N2O
Treatment	50.847 **	3.661 *	15.944 **
Growth period	95.749 **	90.420 **	42.834 **
Treatment × Growth period	11.641 **	2.764 **	4.368 **

Note: * significant correlation at the 5% level (bilateral), ** significant correlation at the 1% level.

indicate that the effects of treatment and growth period and their interactions on CH₄ and N₂O emission fluxes were highly significant, CO₂ emission fluxes were significantly affected by treatment, and the effects of growth period and the interactions between the growth period and treatment on CO₂ emission fluxes were also highly significant.

3.4. Cumulative Emissions and Warming Potential

Treatments were ordered by total CH₄ emissions as follows: M > 1/2 M + 1/2 N-PK > 1/4 M + 3/4 N-PK > NPK. Compared with NPK, emissions were 357.75% higher in M, 156.13% higher in 1/2 M + 1/2 N-PK, and 42.80% higher in 1/4 M + 3/4 N-PK, and the values were not significantly different between the NPK and 1/4 M + 3/4 N-PK treatments. Treatments were ordered by total CO₂ emissions as follows: M > 1/4 M + 3/4 N-PK > NPK > 1/2 M + 1/2 N-PK (

Table 6. Changes in total greenhouse gas emissions and GWP under different treatments.

Treatment	Total Greenhouse Gas Emission (kg hm−2)			GWP (kg CO2-e hm−2)
	CH4	CO2	N2O
NPK	47.34 ± 14.73 c	22,559 ± 3044 b	0.793 ± 0.092 a	23,979 ± 2918 b
M	216.71 ± 31.28 a	29,688 ± 707 a	0.538 ± 0.056 b	35,266 ± 554 a
1/2 M + 1/2 N-PK	121.25 ± 8.77 b	21,399 ± 1914 b	0.843 ± 0.061 a	24,682 ± 2137 b
1/4 M + 3/4 N-PK	67.60 ± 19.05 c	24,452 ± 2818 b	0.920 ± 0.124 a	26,416 ± 2829 b

Note: Different lowercase letters indicate significant differences between different treatments (p < 0.05).

). Compared with NPK, CO₂ emissions were 31.60% higher in M and 8.39% higher in 1/4 M + 3/4 N-PK, while they were 5.14% lower in 1/2 M + 1/2 N-PK. Except for the M treatment, the differences between treatments were not significant. Treatments were ordered by total N₂O emissions as follows: 1/4 M + 3/4 N-PK > 1/2 M + 1/2 N-PK > NPK > M. Compared with NPK, emissions declined in the M treatment by 32.16%, and the 1/2 M + 1/2 N-PK and 1/4 M + 3/4 N-PK treatments increased emissions by 6.31% and 16.02%, respectively, indicating that there were no significant differences between treatments except for the M treatment. Treatments were ranked in descending order of GWP as M > 1/4 M + 3/4 N-PK > 1/2 M + 1/2 N-PK > NPK. Compared with the NPK treatment, emissions were higher in M by 47.07%, in 1/2 M + 1/2 N-PK by 10.16%, and in 1/4 M + 3/4 N-PK by 2.93%, indicating that there were no significant differences between treatments except for the M treatment.

In summary, the application of organic fertilizer significantly increased the amount of CH₄ emissions from the paddy soil, and the level of CH₄ emissions increased proportionally with increasing organic fertilizer application. There was no significant difference in CH₄ emissions between the 1/4 M + 3/4 N-PK treatment and the NPK treatment. Organic fertilizer alone significantly reduced soil N₂O emissions but significantly promoted soil CO₂ emissions and increased the GWP of rice fields. However, there were no significant differences in the indicators between partial replacement with organic fertilizer and the application of chemical fertilizer alone.

4. Discussion

4.1. Methane Emissions

Organic fertilizer plays important roles in improving soil structure, regulating water and fertilizer, and balancing soil nutrients [29,30]. The application of organic fertilizer to the soil provides a large amount of carbon for methanogens, thereby significantly promoting the production of CH₄ [31]. The decomposition of organic matter reduces the soil oxidation–reduction potential, thereby increasing the activity of methanogens [32]. In addition, the available nutrients provided by organic fertilizers create a better ecological environment for microorganisms, which improves soil microbial activity and diversity and promotes the growth of methanogens to increase CH₄ emissions [25]. The application of organic fertilizers improves the physical and chemical properties of soil, promotes the growth and development of rice roots, and releases more root exudates. The hydrocarbons, organic acids, and amino acids secreted by roots are fermented into CH₃COOH, CO₂, and H₂ under anaerobic conditions, thereby forming CH₄ under the action of methanogens [33]. Soil CH₄ emissions were lower in the treatments with chemical fertilizer application, perhaps because the application of chemical fertilizer significantly increased the contents of ammonium nitrogen and nitrate nitrogen in the soil, ammonium nitrogen increased the activity of methane-oxidizing bacteria, and the generated CH₄ was oxidized. Under flooded anaerobic conditions, nitrate nitrogen competes with methanogens for substrate denitrification, thereby reducing the amount of CH₄ produced [25]. It has been found that 98% of CH₄ emissions are discharged to the atmosphere through plant aeration tissues [34]. The results of this study showed that CH₄ emissions changed correspondingly with the growth of rice, and the trend was consistent with the results of Wei et al. [33]. From the seedling stage to the tillering stage, the root system of rice is well developed and can produce many root exudates, thereby promoting the production and emission of CH₄. At the booting stage (approximately July 23), the transfer of organic matter to the panicle leads to a decrease in root exudates, and the oxidative capacity of root cells is stronger at this time, so CH₄ production and emission are reduced. At the heading stage, the aging and death of some roots are accompanied by an increase in black roots, which provide abundant substrates for methanogens and promote the production of CH₄. At this stage, the oxidative capacity of the root system is low, so emissions increase. Consistent with the results of Yang et al. [22], the aeration of the soil was improved after field drying, the level of CH₄ production decreased, and the amount of oxidation increased, so there was almost no emission during the fallow period.

4.2. Carbon Dioxide Emissions

CO₂ emissions are determined by both plant and soil respiration and are also affected by conditions such as rice plant growth and environmental temperature [22]. One month after rice transplantation, plant respiration is relatively weak. In this study, after the tillering stage, the plants grew rapidly, respiration was enhanced, and CO₂ emissions gradually increased. The peak period of CO₂ emissions was from the heading stage to the maturity stage. As the rice plants grew and the ambient temperature increased, the CO₂ emission levels were high until the rice matured and was harvested. After rice harvest, plant respiration was significantly reduced, and the CO₂ emissions in the different treatments ranged from 55.29 to 298.90 mg·m⁻²·h⁻¹. The peak CO₂ emissions and the average emission flux were higher under organic fertilizer alone than under chemical fertilizer alone. However, there was no significant difference between the organic fertilizer replacement treatments and chemical fertilizer alone. The discrepancies between the results of previous studies on whether organic fertilizer application promotes CO₂ emissions are due to the combined effects of environmental factors and other variables. However, in general, organic fertilizers can increase CO₂ emissions by improving soil microbial activity, promoting rice root growth, increasing plant biomass, and increasing soil mineralization [20,35]; the appropriate ratio of organic fertilizer to chemical fertilizer can help reduce CO₂ emissions.

4.3. Nitrous Oxide Emissions

N₂O is an intermediate product of nitrification and denitrification, and changes in soil moisture and fertilization conditions cause dynamic changes [22]. The long-term anaerobic environment of flooded rice fields inhibits the progress of nitrification, and the N₂O produced by denitrification is further reduced to N₂. Therefore, in this study, the N₂O emission fluxes were all at low levels during the flooding period and were often accompanied by absorption. Before field drying, the soil N₂O emission flux varied between −21.24 and 18.04 µg·m⁻²·h⁻¹. After field drying, the N₂O emission flux increased rapidly, and the maximum emission during the fallow period was as high as 61.73 µg·m⁻²·h⁻¹. The N₂O emissions over the entire growth period accounted for only −5.20 to 16.27% of the total emissions in the different treatments, while the fallow period accounted for 83.73–105.20% of the total emissions. The treatments were ranked by total N₂O emissions (kg·hm⁻²) during the growth period as follows: NPK (0.129) > 1/4 M + 3/4 N-PK (0.067) > 1/2 M + 1/2 N-PK (0.034) > M (−0.027), indicating that the application of chemical fertilizer alone promoted N₂O emissions during the flooding period, while the application of organic fertilizer reduced the total N₂O emissions during the growth period [36]. In addition, N₂O emissions decreased with increasing proportion of organic fertilizer application because the application of chemical fertilizer can rapidly increase the content of available N in the soil, which provides an abundant substrate for nitrification and denitrification, resulting in an increase in N₂O emissions; in contrast, the effect of organic fertilizer on increasing soil available N lags behind that of chemical fertilizer. The application of organic fertilizers can inhibit the activity of denitrifying enzymes and reduce the nitrification potential, thereby reducing N₂O emissions [25]. The N₂O emission characteristics in the fallow period were different. The N₂O emissions under organic fertilizer combined with chemical fertilizer were 21.84–28.46% higher than those under chemical fertilizer alone. This finding may be related to the fact that combined organic–inorganic fertilizer application provides both a substrate for nitrification and denitrification and a carbon source available to microorganisms and promotes the growth of denitrifying bacteria [35].

4.4. Cumulative Emissions and Global Warming Potential

Organic fertilizer alone significantly increased the GWP of rice fields, which is consistent with the results of Qian et al. [21]. The GWP value under the combined application of organic fertilizer and chemical fertilizer was slightly higher than that in the treatment with chemical fertilizer alone, but the differences between treatments were not significant exception for the M treatment, which is also consistent with the results of a previous study [24,37]. The partial replacement of chemical fertilizers by organic fertilizers reduces chemical fertilizer input, improving soil fertility and increasing rice yield [38,39,40]. For this reason, to avoid significantly increasing greenhouse gas emissions from rice fields, replacing chemical fertilizer N with organic fertilizer N at a ratio of 1/4 to 1/2 is recommended on the basis of this experiment as a fertilizer application pattern that strengthens farmland and is environmentally friendly.

5. Conclusions

CH₄ emissions from soil increased with increasing organic fertilizer application. Organic fertilizer alone significantly increased soil CO₂ emissions, but there was no significant difference between the partial replacement of chemical fertilizer by organic fertilizer and chemical fertilizer alone. Compared with the application of chemical fertilizer alone, the application of organic fertilizer reduced N₂O emissions from the rice fields, and the M treatment exhibited the most significant effect. The application of organic fertilizer increased the GWP of the rice fields. Compared with chemical fertilizer alone, the application of organic fertilizer alone increased emissions by 47.07%, whereas the increases under partial organic fertilizer replacement were less than 10%; there were no significant differences between these treatments and the treatment with chemical fertilizer alone. Based on the results of this experiment and from previous experimental studies, replacing chemical fertilizer nitrogen with organic fertilizer nitrogen at a ratio of 1/4 to 1/2 is an excellent fertilization mode to reduce the greenhouse effect, increase soil carbon sequestration, and increase the yield of paddy fields.