The Evolutionary Traits of Carbon Emissions from the Planting Industry in Beijing, China

Abstract

Making clear the exact amount of carbon emissions from the planting industry is of great significance for developing low-carbon agriculture and helping achieve carbon neutrality. The current carbon emissions from the planting industry in Beijing, the capital of China, are still unclear, and there is a lack of quantitative research on the production, economic, and ecological benefits of carbon emissions. This paper used the carbon emissions factor method to study the inter-annual variation characteristics of carbon emissions and carbon benefits in Beijing’s planting industry since 2000. The results show that the carbon emissions from the planting industry in Beijing in 2023 were 256,400 tons, of which the carbon emissions from agricultural inputs, nitrous oxide (N₂O) from farmland, and methane (CH₄) from rice cultivation were 149,300, 105,200, and 2000 tons, respectively. From 2000 to 2023, the total carbon emissions from the planting industry in Beijing have shown a downward trend. Compared with 2000, the carbon emissions from agricultural inputs and N₂O in 2023 decreased by 59.88% and 74.52%, respectively. The carbon emissions of CH₄ from rice cultivation were only 2.38% of those in 2000, and the total carbon emissions from the planting industry in Beijing decreased by 70.43%. The average carbon emissions from agricultural inputs and N₂O accounted for 50.85% and 47.95% of the total level of the planting industry, respectively, and were the main sources of carbon emissions in Beijing. Chemical fertilizer and agricultural film inputs were important sources of carbon emissions from agricultural inputs. Reducing inputs for agriculture and sources of N₂O from farmland is an important way to reduce carbon emissions from agriculture in Beijing. In the end, some suggestions were proposed for reducing carbon emissions from the planting industry.

1. Introduction

Reducing greenhouse gas emissions and enhancing the carbon sequestration capacity of ecosystems are effective measures to mitigate climate change. In response to climate change, China has made a solemn commitment to “peak carbon emissions before 2030 and achieve carbon neutrality before 2060”, starting a green revolution under the new development concept.

Agriculture is not only one of the main sources of greenhouse gas emissions, but also plays a key role as a carbon sink. Due to reasons such as the low level of agricultural production technology, outdated equipment, and low energy efficiency, agriculture has also become the main source of greenhouse gases. According to the data of the Third Biennial Update Report on Climate Change of the People’s Republic of China, greenhouse gas emissions from agricultural activities in China accounted for 6.1% of the country’s total emissions in 2018, of which CH₄ and N₂O emissions from agricultural sources accounted for 37.22% and 49.16% of the country’s total emissions, respectively [1].

In recent years, the foreign research related to agricultural carbon has mainly focused on aspects such as the sources [2,3], model-based estimations [4], influencing factors [5], reduction measures [6,7], as well as the related management and policy research [8,9,10]. Scholars in China conducted a spatiotemporal analysis of agricultural carbon emissions at different scales in Hebei Province, the Yangtze River Delta, the Yellow River Basin, and China, and studied the driving factors of agricultural carbon emissions [11,12,13,14]. Some researchers further explored the relationship between agricultural carbon emissions and economic growth, agricultural technological progress, and agricultural mechanization in China or Liaoning Province [15,16,17,18]. Other experts have studied the carbon emissions efficiency of agriculture in Hebei Province, or the production efficiency of Chinese soybeans, or the greenhouse gas emissions characteristics of a certain crop [19,20,21]. The research scope continues to expand and the research perspective is broader.

Beijing, the capital of China, is located in the north of the China Plain. As a pioneer area for the high-quality development of agriculture in China, there is still a great potential for carbon emissions reduction. In the previous research, it was found that although the total amount of inputs, such as chemical fertilizer, pesticides, and agricultural film used in planting production in Beijing, showed a downward trend in recent years, the input intensity per unit area remained at a high level and showed an upward trend, and the phenomenon of unreasonable use was still relatively common. The data show that in 2023 (

Table 1. Major agricultural chemical inputs in Beijing in 2023.

Agricultural Inputs	Application Amount (tons)	Planting Area (Thousand Hectares)	Application Amount per Unit Area (kg/hm2)	National Average (kg/hm2)
Chemical fertilizers	67,000	159.0	421.38	292.60
Pesticides	2100	159.0	13.21	6.73
Agricultural films	7000	159.0	44.03	14.08

), the amount of chemical fertilizer, pesticides, and agricultural plastic films applied per unit sown area was 421.38, 13.21, and 44.03 kg/hm², which was higher than the national average of 292.60, 6.73, and 14.08 kg/hm² [22]. At the same time, it is found that there is still a shortage in the resource utilization of agricultural waste in Beijing, especially crop straw, tail vegetables, livestock, and poultry manure. A large amount of agrochemical inputs and the accumulation of organic wastes have imposed a heavy environmental burden, posing a serious threat to soil [23,24], water [25,26,27], and atmospheric environments [28], exacerbating the risk of carbon emissions, and further affecting the quality of agricultural products and even human health [29,30].

Under the guidance of the “double carbon” goal, in recent years, Beijing has advocated the development of low-carbon agriculture and taken the path of green agricultural development. The planting industry is an important component of agriculture in Beijing, including grain crops (wheat, corn, rice, and potatoes), vegetables (tomatoes, cucumbers, cabbage, eggplant, scallions, cabbage, leafy vegetables, etc.), and fruit trees (peaches, apricots, pears, etc.). However, at present, the baseline of carbon emissions from the planting industry in Beijing is still unclear, and there is a lack of quantitative research on the production, economical, and ecological benefits of carbon emissions in Beijing. Therefore, it is urgent to clarify the quantity of carbon emissions from Beijing’s planting industry, quantify the level of carbon efficiency, and further provide support for the carbon emissions reduction goal of Beijing’s planting industry.

This paper studied the inter-annual variation characteristics of carbon emissions and carbon efficiency in Beijing’s planting industry since 2000. Combined with the development orientation of Beijing’s agriculture, it puts forward development suggestions for carbon sequestration and emissions reduction in Beijing’s planting industry. This research is of great significance for accelerating the pace of carbon sequestration and emissions reduction in Beijing’s agriculture, establishing a development model for low-carbon agriculture in Beijing, and following a high-quality, sustainable development path that is green, ecological, and circular.

2. Materials and Methods

2.1. Main Sources of Carbon Emissions from the Planting Industry

According to the “Guidelines for Compiling Provincial Greenhouse Gas Inventories (Trial)” [31], methane (CH₄) and nitrous oxide (N₂O) are the two non-CO₂ greenhouse gases with the highest emissions in the world, and they are the main greenhouse gases emitted by the planting industry. Therefore, the greenhouse gases accounted for in this study are CO₂, CH₄, and N₂O in the planting production process, including carbon emissions from agricultural inputs in planting production, N₂O carbon emissions from agricultural land, and CH₄ carbon emissions from rice production.

2.2. Data Sources and Calculation Methods

The relevant data of the planting industry in Beijing were retrieved from the Beijing Statistical Yearbooks from 2000 to 2023. The emissions factor method was adopted to calculate the carbon emissions, and the specific accounting indicators and methods are as follows.

2.2.1. Carbon Emissions from Agricultural Inputs

Carbon emissions from agricultural inputs in Beijing mainly include chemical fertilizers, pesticides, agricultural films, diesel oil, and irrigation, and the calculation formula is as follows [32]:

$${\mathrm{C}}_{\mathrm{i}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{{\mathrm{N}}_{\mathrm{i}}\times \mathrm{E}}_{\mathrm{i}}$$

Among them, ${\mathrm{C}}_{\mathrm{i}}$ represents the carbon emissions level of agricultural inputs, ${\mathrm{N}}_{\mathrm{i}}$ represents the amount of the ith type of agricultural input used, and ${\mathrm{E}}_{\mathrm{i}}$ represents the carbon emissions coefficient corresponding to the ith type of agricultural input. The carbon emissions coefficients of the five major categories of agricultural inputs, namely chemical fertilizers, pesticides, agricultural films, diesel oil, and irrigation, are shown in

Table 2. Carbon emissions coefficients for agricultural inputs.

Agricultural Inputs	Carbon Emissions Coefficient	Unit (Calculated by C)	Data Sources
Production, transportation, and use of fertilizers	0.8956	kg/kg	ORNL (Oak Ridge National Laboratory, USA)
Production, transportation, and use of pesticides	4.9341	kg/kg	ORNL (Oak Ridge National Laboratory, USA)
Production, transportation, and use of agricultural film	5.18	kg/kg	IREEA (Institute of Agricultural Resources and Ecological Environment, Nanjing Agricultural University)
Agricultural machinery diesel	0.5927	kg/kg	IPCC (United Nations Intergovernmental Committee of Experts on Climate Change)
Irrigation	266.48	kg/hm2	Song et al. [33]

.

2.2.2. Carbon Emissions of N₂O from Agricultural Land

Carbon emissions of N₂O from farmland are mainly caused by nitrogen-containing substances entering farmland soil, resulting in N₂O emissions from farmland. The N₂O emissions from farmland are mainly divided into direct carbon emissions and indirect carbon emissions. The direct emissions refer to the carbon emissions of N₂O caused by returning straw to the field, the use of nitrogen-containing chemical fertilizers, and returning livestock and poultry manure to the field. The indirect carbon emissions mainly refer to the N₂O emissions caused by two aspects: soil nitrogen leaching and runoff, and atmospheric ammonia deposition. The formula is as follows:

$${\mathrm{C}}_{\mathrm{N}2\mathrm{O}}={\mathrm{D}}_{\mathrm{N}2\mathrm{O}}+{\mathrm{I}}_{\mathrm{N}2\mathrm{O}}$$

Among them, ${\mathrm{C}}_{\mathrm{N}2\mathrm{O}}$ represents the carbon emissions level of N₂O from agricultural land, ${\mathrm{D}}_{\mathrm{N}2\mathrm{O}}$ represents the direct carbon emissions of N₂O from agricultural land, and ${\mathrm{I}}_{\mathrm{N}2\mathrm{O}}$ represents the indirect carbon emissions of N₂O from agricultural land. The carbon emission levels of the two were calculated separately.

Direct Carbon Emissions

The direct N₂O carbon emissions from agricultural land are caused by three factors: nitrogen-containing fertilizer input, organic fertilizer being returned to the field, and crop straw being returned to the field. The accounting formula is as follows:

$${\mathrm{D}}_{\mathrm{N}2\mathrm{O}}={\mathrm{F}}_{\mathrm{N}2\mathrm{O}}+{\mathrm{O}}_{\mathrm{N}2\mathrm{O}}+{\mathrm{S}}_{\mathrm{N}2\mathrm{O}}$$

Among them, ${\mathrm{F}}_{\mathrm{N}2\mathrm{O}}$ is the soil N₂O carbon emissions caused by the input of nitrogen-containing fertilizer, including nitrogen fertilizer and compound fertilizer; ${\mathrm{O}}_{\mathrm{N}2\mathrm{O}}$ is the soil N₂O carbon emissions caused by organic fertilizer being returned to fields; and ${\mathrm{S}}_{\mathrm{N}2\mathrm{O}}$ is the soil N₂O carbon emissions caused by crop straw being returned to the field.

The calculation formula for the direct carbon emissions of N₂O caused by nitrogen-containing chemical fertilizers is as follows:

$${\mathrm{F}}_{\mathrm{N}2\mathrm{O}}={(\mathrm{N}}_{\mathrm{w}}+{\mathrm{C}}_{\mathrm{w}}\times {\mathrm{W}}_0)\times {\mathrm{E}}_{\mathrm{N}}\times 298$$

Among them, ${\mathrm{N}}_{\mathrm{w}}$ is the pure equivalent of nitrogen fertilizer application; ${\mathrm{C}}_{\mathrm{w}}$ is the pure equivalent of agricultural compound fertilizer application; ${\mathrm{W}}_0$ is the mass fraction of nitrogen in the chemical fertilizer, which is 28.41% [34]; ${\mathrm{E}}_{\mathrm{N}}$ is the direct emissions coefficient of N₂O from agricultural land, which is 0.0057 kg N₂O-N·per unit of N input [31] (the same below); and 298 is the global warming potential of N₂O [35] (the same below).

The calculation formula for the direct carbon emissions of N₂O caused by the return of organic fertilizers to the field is as follows:

$${\mathrm{O}}_{\mathrm{N}2\mathrm{O}}={\mathrm{N}}_{\mathrm{O}}\times {\mathrm{E}}_{\mathrm{N}}\times 298$$

$${\mathrm{N}}_{\mathrm{O}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{A}}_{\mathrm{i}}\times {\mathrm{T}}_{\mathrm{i}}}{365}}\times {\mathrm{E}}_{\mathrm{i}}{\times \mathrm{W}}_{\mathrm{i}}$$

Among them, ${\mathrm{N}}_{\mathrm{O}}$ is the nitrogen input of organic fertilizer. It is based on the situation of returning the manure generated from livestock and poultry breeding to the field. Therefore, the accounting of the situation of returning the manure generated from livestock and poultry breeding to the field is shown in the second formula. ${\mathrm{A}}_{\mathrm{i}}$ represents the year-end inventory of the ith type of livestock and poultry in Beijing (of which poultry is the annual slaughter volume). ${\mathrm{T}}_{\mathrm{i}}$ represents the growth cycle of the ith type of livestock and poultry. ${\mathrm{E}}_{\mathrm{i}}$ represents the average annual nitrogen excretion coefficient of the ith type of livestock and poultry manure. ${\mathrm{W}}_{\mathrm{i}}$ represents the return rate of the manure of the ith type of livestock and poultry to the field. The various coefficients are shown in

Table 3. Carbon emissions coefficients of livestock and poultry manure returning to the field.

Item	Ti	Ei (kg N Each)	Wi (%)	Data Source
Cattle	365	60 (30~60)	54	Guidelines for Compiling Provincial Greenhouse Gas Inventories [31]
Horse	365	40	36	Guidelines for Compiling Provincial Greenhouse Gas Inventories [31]
Donkey	365	40	36	Guidelines for Compiling Provincial Greenhouse Gas Inventories [31]
Pig	180	12	36	Fu [36]
Sheep	365	9	23	Cai [37]
Poultry	55	0.6	22	Guidelines for Compiling Provincial Greenhouse Gas Inventories [31]

.

The calculation of the direct carbon emissions of N₂O caused by returning crop straws to the field is as follows.

$${\mathrm{S}}_{\mathrm{N}2\mathrm{O}}={\mathrm{N}}_{\mathrm{R}}\times {\mathrm{E}}_{\mathrm{N}}\times 298$$

$${\mathrm{N}}_{\mathrm{R}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{{\mathrm{P}}_{\mathrm{i}}\times \mathrm{W}}_{\mathrm{C}\mathrm{i}}{\times \mathrm{W}}_{\mathrm{N}\mathrm{i}}{\times \mathrm{W}}_{\mathrm{R}\mathrm{i}}$$

Among them, ${\mathrm{S}}_{\mathrm{N}2\mathrm{O}}$ is the direct carbon emissions of N₂O caused by crop straw being returned to the field. ${\mathrm{N}}_{\mathrm{R}}$ is the amount of nitrogen from crop straws returned to the field. ${\mathrm{P}}_{\mathrm{i}}$ is the economic yield of the ith crop. ${\mathrm{W}}_{\mathrm{C}\mathrm{i}}$ is the straw-to-grain ratio of the ith crop. ${\mathrm{W}}_{\mathrm{N}\mathrm{i}}$ is the nitrogen content of the ith crop straw. ${\mathrm{W}}_{\mathrm{R}\mathrm{i}}$ is the rate of the ith crop straw returning to the field. According to the types of crops planted in Beijing, only rice, wheat, corn, soybean, potato, and peanut are counted in this study. The coefficients of various agricultural products are shown in

Table 4. Calculation coefficient for nitrogen from crop straw returned to the field.

Item	WCi 1	WNi (%)	WRi (%) 2
Rice	0.93	0.91	99
Wheat	1.34	0.65	99
Corn	1.73	0.75	99
Soybean	1.57	2.10	99
Tubers	1.00	2.50	99
Peanut	1.22	1.80	99

¹ The emissions coefficients are mainly calculated based on the studies of scholars such as Cai [37] and Han [38]. ² The situation of straw returning to the fields in Beijing comes from the Beijing Agricultural and Rural Publicity Center [39].

.

Indirect Carbon Emissions of N₂O from Agricultural Land

The indirect emissions of N₂O from agricultural land (${\mathrm{I}}_{\mathrm{N}2\mathrm{O}}$) originate from N₂O emissions caused by the atmospheric nitrogen deposition resulting from the volatilization of nitrogen oxide (NOx) and ammonia (NH₃) from fertilized soil and livestock and poultry manure (${\mathrm{I}}_{\mathrm{V}}$), as well as N₂O emissions caused by soil nitrogen leaching or runoff into water bodies (${\mathrm{I}}_{\mathrm{L}}$).

$${\mathrm{I}}_{\mathrm{N}2\mathrm{O}}={\mathrm{I}}_{\mathrm{V}}+{\mathrm{I}}_{\mathrm{L}}$$

The calculation formula for carbon emissions from atmospheric ammonia deposition is as follows.

$${\mathrm{I}}_{\mathrm{V}}=({\mathrm{N}}_{\mathrm{C}}\times {\mathrm{W}}_{\mathrm{C}}+{\mathrm{N}}_{\mathrm{O}}{\times \mathrm{W}}_{\mathrm{O}\mathrm{N}})\times {\mathrm{E}}_{\mathrm{V}}\times 298$$

Among them, ${\mathrm{I}}_{\mathrm{V}}$ is the emissions level of N₂O caused by atmospheric nitrogen deposition. ${\mathrm{N}}_{\mathrm{C}}$ is the nitrogen input from chemical fertilizer, calculated by $({\mathrm{N}}_{\mathrm{w}}+{\mathrm{C}}_{\mathrm{w}}\times {\mathrm{W}}_0)$. ${\mathrm{W}}_{\mathrm{C}}$ is the volatilization level of chemical fertilizers, which is 21.3% [40]. ${\mathrm{N}}_{\mathrm{O}}$ is the nitrogen input from organic fertilizer. ${\mathrm{W}}_{\mathrm{O}\mathrm{N}}$ is the volatilization level of organic fertilizers, which is 23% [40].

The calculation formula for carbon emissions from soil nitrogen leaching and runoff is as follows.

$${\mathrm{I}}_{\mathrm{L}}=({\mathrm{N}}_{\mathrm{C}}+{\mathrm{N}}_{\mathrm{O}}+{\mathrm{N}}_{\mathrm{R}})\times {\mathrm{R}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}}\times {\mathrm{E}}_{\mathrm{L}}\times 298$$

Among them, ${\mathrm{I}}_{\mathrm{L}}$ is the carbon emissions level of N₂O caused by soil nitrogen leaching and runoff. ${\mathrm{R}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}}$ is the level of nitrogen loss due to leaching and runoff, which is 13% [40]. ${\mathrm{N}}_{\mathrm{C}},{\mathrm{N}}_{\mathrm{O}},{\mathrm{N}}_{\mathrm{R}}$ are the input amount of nitrogen from chemical fertilizers, the input amount of nitrogen from organic fertilizers, and the amount of nitrogen from straw returned to the field, respectively.

According to the “Guidelines for Compiling Provincial Greenhouse Gas Inventories (Trial)” [31], the N₂O emissions factor caused by atmospheric nitrogen deposition (${\mathrm{E}}_{\mathrm{V}}$) is recommended to adopt the default value of 0.01 in the “1996 IPCC Guidelines for National Greenhouse Gas Inventories”. The N₂O emissions factor caused by nitrogen leaching and runoff loss (${\mathrm{E}}_{\mathrm{L}}$) recommends the default value of 0.0075 provided by the “2006 IPCC Guidelines for National Greenhouse Gas Inventories”.

2.2.3. Carbon Emissions of CH₄ Caused by Rice Cultivation

The accounting formula for CH₄ emissions from paddy fields is as follows:

$${\mathrm{C}}_{\mathrm{r}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{{\mathrm{r}}_{\mathrm{i}}\times \mathrm{W}}_{\mathrm{r}\mathrm{i}}\times 25$$

Among them, ${\mathrm{C}}_{\mathrm{r}}$ is the carbon emissions of CH₄ from paddy fields. ${\mathrm{r}}_{\mathrm{i}}$ is the area of different types of paddy fields. ${\mathrm{W}}_{\mathrm{r}\mathrm{i}}$ is the CH₄ emissions coefficient of paddy fields, and based on the IPCC and the “Guidelines for the Compiling Provincial Greenhouse Gas Inventories (Trial)”, the value of ${\mathrm{W}}_{\mathrm{r}\mathrm{i}}$ in Beijing is 234 kg/hm². The value of 25 is the global warming potential of CH₄.

3. Results

3.1. Carbon Emissions from the Planting Industry

3.1.1. Carbon Emissions from Agricultural Inputs

The carbon emissions from agricultural inputs in Beijing’s planting industry were 372,100 tons in 2000, which decreased to 298,200 tons in 2004, and then maintained a relatively stable trend until 2011 (Figure 1). The downward trend was relatively obvious from 2012 to 2018, and remained stable after 2018. The annual carbon emissions from agricultural chemicals were about 146,000 tons.

Specifically, in 2000, the carbon emissions from chemical fertilizers in Beijing’s planting industry were 160,300 tons, and from pesticides were 26,600 tons. After that, both of them maintained a relatively stable or slow downward trend. Since 2012, due to the implementation of the national “two reductions (fertilizer reduction and pesticide reduction)” project and “zero growth” of fertilizer and pesticide, the scientific and technological level of fertilization and pesticide application has been improved, and the downward trend of carbon emissions from chemical fertilizers and pesticides has become more obvious. By 2020, the carbon emissions of chemical fertilizers and pesticides reached the lowest value, which were 54,300 tons and 10,400 tons, respectively. There was a slight increase in the past three years.

In 2000, the carbon emissions from the use of agricultural film in Beijing’s planting industry were 50,900 tons. Before 2015, it showed an upward trend first and then a downward trend. The highest value was in 2007, reaching 75,700 tons, and then showed a continuous downward trend, falling to 36,300 tons by 2023.

In 2000, the carbon emissions from diesel consumption in agricultural machinery, such as ploughing, sowing, and harvesting machines in Beijing’s planting industry, were 46,800 tons. It has shown a fluctuating downward trend in the past 20 years and dropped to 11,300 tons by 2023.

In 2000, the carbon emissions from irrigation in Beijing’s planting industry were 87,500 tons, which remained at around 48,100 tons from 2003 to 2007. In 2008, it rose to 64,400 tons, and then showed a continuous downward trend, dropping to 31,400 tons by 2023.

From the perspective of the level of carbon emissions from various agricultural inputs (Figure 2), the carbon emissions of chemical fertilizers, pesticides, agricultural films, diesel oil, and irrigation accounted for 41.60%, 7.25%, 22.90%, 9.38%, and 18.88% of the total carbon emissions, respectively. Chemical fertilizers and agricultural film inputs were the important sources of carbon emissions. The amount of fertilizers and diesel has shown an overall downward trend, falling by 2.59 and 5.04 percentage points, respectively, by 2023. The proportion of agricultural film showed an upward trend, the amount of pesticides was relatively stable, and the amount of irrigation fluctuated and declined in the early stage. After 2013, it showed a slow upward trend. However, compared with 2000, it still decreased by 2.44 percentage points in 2023.

From a phased perspective, the average annual carbon emissions of agricultural inputs in Beijing’s planting industry were 316,100 tons in 2000–2010, of which the carbon emissions of chemical fertilizers, pesticides, agricultural films, diesel oil, and irrigation were 131,900, 22,500, 63,100, 36,900, and 61,600 tons, respectively, accounting for 41.73%, 7.12%, 19.96%, 11.67%, and 19.49% of the total carbon emissions, respectively. That is, the contribution of fertilizer input to carbon emissions in the planting industry was over 40%, while the carbon emissions from agricultural film and irrigation were basically equivalent, each accounting for one-fifth of the total carbon emissions. Diesel ranked fourth, and the carbon emissions contribution of pesticide inputs was less than 10%.

During the period of 2011–2023, the average annual carbon emissions of agricultural inputs in Beijing’s planting industry were 197,200 tons, a decrease of 37.61% compared to the average value during the period of 2000–2010. Among them, the carbon emissions of chemical fertilizers, pesticides, agricultural films, diesel, and irrigation were 82,600, 14,300, 49,100, 15,100, and 36,100 tons, respectively, accounting for 41.87%, 7.28%, 24.89%, 7.65%, and 18.32% of the total carbon emissions respectively. Compared with the period from 2000 to 2010, the contributions of chemical fertilizers, pesticides, and irrigation to the carbon emissions from the planting industry have not changed significantly. The carbon emissions created by agricultural film increased by nearly 5 percentage points, and the carbon emissions from diesel decreased by about 4 percentage points.

3.1.2. Carbon Emissions of N₂O from Agricultural Land

Direct Carbon Emissions of N₂O

Direct carbon emissions of N₂O come from nitrogenous fertilizers, organic fertilizers, and straw returning to fields. From the results (Figure 3), it can be seen that the carbon emissions of the three pathways all show a downward trend. The carbon emissions of N₂O caused by nitrogen-containing fertilizers, organic fertilizers, and straw returning to fields dropped from 206,100, 38,400, and 29,100 tons in 2000 to 52,700, 7500, and 10,200 tons in 2023, respectively, which were only 25.56%, 19.57%, and 35.16% of the initial value. The total direct carbon emissions of N₂O from farmland dropped from 273,600 tons in 2000 to 70,400 tons in 2023, which was only about one-fourth of the initial value. Overall, the carbon emissions of the three pathways accounted for 73.64%, 15.89%, and 10.46% of the direct carbon emissions of N₂O from agricultural land.

Indirect Carbon Emissions of N₂O

The indirect carbon emissions of N₂O from farmland include two parts: the carbon emissions from atmospheric ammonia deposition and the carbon emissions from soil nitrogen leaching and runoff. The results show that the indirect carbon emissions of N₂O are mainly from atmospheric ammonia deposition, accounting for 66.48% of the total indirect carbon emissions (Figure 4). Overall, the carbon emissions from atmospheric ammonia deposition, soil nitrogen leaching, and runoff also showed a downward trend. They decreased from 92,500 and 46,800 tons in 2000 to 22,700 and 12,000 tons in 2023, respectively, dropping by 75.44% and 74.26%, which was only about one-fourth of the initial value.

Total Carbon Emissions of N₂O from Agricultural Land

The results show that the carbon emissions of N₂O from farmland in 2000 are 412,900 tons, of which direct carbon emissions are 273,600 tons and indirect carbon emissions are 139,300 tons (Figure 5). The carbon emissions of N₂O from farmland in 2010 were 296,500 tons, a decrease of 28.19% compared to 2000, with direct and indirect carbon emissions decreasing by 27.92% and 28.72%, respectively. The carbon emissions of N₂O from farmland in 2023 were 105,200 tons, of which direct carbon emissions were 70,423 tons and indirect carbon emissions were 34,769 tons, and a further decrease of 64.53% compared to 2010. The average level of direct carbon emissions of N₂O from farmland caused by chemical fertilizers, organic fertilizers, and straw returning to fields was 66.21%, while the level of indirect carbon emissions of N₂O caused by atmospheric ammonia deposition, soil nitrogen leaching, and runoff was 33.79%. Therefore, direct carbon emissions were the most important part of N₂O carbon emissions from farmland in Beijing, with a ratio of nearly 2:1, and there was no significant change in the amount during the period from 2000 to 2023.

3.1.3. Carbon Emissions of CH4 from Rice Cultivation

Due to the sharp decline in rice planting areas in Beijing, the carbon emissions of CH₄ created by rice cultivation have also decreased rapidly, dropping from 82,300 tons in 2000 to less than 10,000 tons in 2003. Since 2010, the average annual carbon emissions of CH₄ is 1339 tons (Figure 6).

3.1.4. Total Carbon Emissions from the Planting Industry

The carbon emissions from the planting industry in Beijing are mainly divided into three categories: carbon emissions from agricultural inputs, carbon emissions of N₂O from farmland, and carbon emissions of CH₄ generated by rice cultivation. The results show that from 2000 to 2023, the total carbon emissions from the planting industry in Beijing show a downward trend (Figure 7). In 2000, the total carbon emissions from the planting industry in Beijing were 867,300 tons, among which the carbon emissions from agricultural inputs were 372,100 tons, the carbon emissions of N₂O from agricultural land were 412,900 tons, and the carbon emissions of CH₄ from rice cultivation were 82,200 tons. There was a short-term upward trend from 2003 to 2008. After 2008, the total carbon emissions from Beijing’s planting industry continued to decline. By 2023, they were 256,400 tons, including 149,300 tons from agricultural inputs, 105,200 tons of carbon emissions of N₂O from farmland, and 2000 tons of carbon emissions of CH₄ from rice. Compared with 2000, the carbon emissions from agricultural inputs decreased by 59.88%, the carbon emissions of N₂O from farmland decreased by 74.52%, and the carbon emissions of CH₄ from rice cultivation were only 2.38% of those in 2000. The total carbon emissions from the planting industry in Beijing decreased by 70.43%, indicating that the scientific and technological levels of the planting industry in Beijing have been continuously improved, the usage of agricultural inputs is gradually recovering to a reasonable level, and the carbon emissions reduction in the planting industry has achieved good results.

3.1.5. Carbon Emissions Structure in the Planting Industry

The level of carbon emissions from agricultural inputs, carbon emissions of N₂O from agricultural land, and carbon emissions of CH₄ from rice cultivation from 2000 to 2023 is shown in Figure 8. On the whole, the carbon emissions from the planting industry in Beijing showed a downward trend. The carbon emissions of agricultural inputs and of N₂O accounted for 50.85% and 47.95% of the overall level on average, respectively, which were the main sources of carbon emissions from the planting industry in Beijing.

In terms of the degree of change, the carbon emissions of CH₄ generated by rice cultivation showed the largest change from 2000 to 2023 because the sown area changed. In 2000, the sown area of rice in Beijing was 14,060 hm², while in 2023, this value was only 335 hm². Due to the adjustment of the planting structure in Beijing, sown areas of rice have been continuously decreasing, which has reduced the carbon emissions of CH₄ to a certain extent.

From the perspective of the structure changes in the carbon emissions from the planting industry, in 2000, the carbon emissions from agricultural inputs, N₂O, and CH₄ in Beijing accounted for 42.91%, 47.61%, and 9.48%, respectively. In 2008, the above-three types of carbon emissions accounted for 50.04%, 49.54%, and 0.42%. The level of carbon emissions in 2023 were 58.22%, 41.02%, and 0.76%. Over the past 20 years, the level of carbon emissions from agricultural inputs has increased by 15.30 percentage points, carbon emissions of N₂O from farmland have decreased by 6.59 percentage points, and carbon emissions of CH₄ from rice cultivation have accounted for less than 1%. Compared with 2010, the level of carbon emissions from agricultural inputs increased by 8.76 percentage points, the level of carbon emissions of N₂O from agricultural land decreased by 9.23 percentage points, and the level of carbon emissions of CH₄ from rice cultivation increased by 0.47 percentage points. The change in rice sown areas and the improvement of green production technology may be the main reasons for the overall decrease in carbon emissions from the planting industry.

3.2. Characteristics of Carbon Benefits in the Planting Industry of Beijing

3.2.1. Production Benefit of Carbon

The production benefit of carbon in the planting industry refers to the output of agricultural products that can be produced per unit of carbon input. It is obtained from the ratio of agricultural product yield to the amount of carbon input. In this study, the output of agricultural products was obtained from the output of grain and vegetables, and the carbon input was calculated from agricultural inputs. The results show that the agricultural carbon production benefit in Beijing is 17.02 kg/kg in 2000 (Figure 9), and keeps an upward trend until 2004, reaching 18.75 kg/kg. After 2005, it showed a fluctuating downward trend until it dropped to the lowest point in 2019, which was 9.59 kg/kg. This means that the crop yield produced by the carbon input per unit of agricultural inputs has maintained a downward trend. The reason for this may be attributed to the adjustment of Beijing’s agricultural planting structure, where a sharp and significant decline in agricultural product output was caused by the reduction in the crop sown area, while the decrease in inputs of agricultural inputs (fertilizers, pesticides, agricultural films, irrigation water, and diesel) was relatively moderate. In other words, the increase in agricultural inputs per unit area was higher than the output of agricultural products. Only from the change in chemical fertilizer input per unit area it can be seen that the use of chemical fertilizer per unit area in Beijing has maintained an increasing trend before 2018, especially after 2010, and the growth rate is much higher than the change rate of agricultural product output.

After 2020, the production benefit of carbon in Beijing’s planting industry increased rapidly, from the original 9.59 kg/kg to 17.10 kg/kg in 2023, which nearly doubled and was equal to the carbon production benefit in 2000. This change may be related to the improvement of agricultural production technology in Beijing, which further improved crop yields while the quantity of agricultural inputs remained relatively stable. It can also be seen from the changes in chemical fertilizer input per unit area that after 2019, the chemical fertilizer input per unit area of Beijing’s agriculture has dropped rapidly, while the output of agricultural products per unit area has maintained a relatively stable trend.

3.2.2. Economic Benefit of Carbon

The economic benefit of carbon refers to the gross value of the planting industry that can be produced per unit of carbon input. It is obtained from the ratio of the gross output value of the planting industry to the amount of carbon input. In this study, the gross agricultural value is the output from grains and vegetables, and the carbon input is calculated from agricultural inputs. The results show that the economic benefit of agricultural carbon in Beijing has maintained an overall growth trend since 2000, and the growth rate is relatively stable in 2000–2015 (Figure 9). The economic benefit per unit kilogram of carbon increased by an average annual increase of RMB 3100 (RMB 1000 = USD 138.35, the same below), and it did not change much in 2016–2019, remaining between CNY 70,000 and 72,000. After 2020, it showed an obvious growth trend, reaching RMB 90,800 by 2023.

3.2.3. Ecological Benefit of Carbon

The ecological benefit of carbon refers to the amount of carbon sequestration by unit of carbon input delivered to crops. It is calculated by the ratio of the amount of carbon fixed in crops through photosynthesis to the amount of carbon input. The results show that from 2000 to 2003, due to the significant decrease in crop yields, the ecological benefit of carbon shows a downward trend, from 16.00 kg of carbon sequestration per kilogram of carbon input in 2000 to 9.73 kg of carbon sequestration (Figure 9). From 2004 to 2010, the ecological benefit of carbon showed an overall upward trend and recovered to the level of 2000. After that, it showed a significant downward trend until 2019, which was speculated to be related to the decrease in agricultural product output. After 2020, with the increase in the output of agricultural products in the planting industry, it showed an upward trend. Overall, each kilogram of carbon input resulted in an average of 12.85 kg of carbon sequestration in crops.

3.3. Characteristics of Carbon Emissions from the Planting Industry in Beijing

3.3.1. Carbon Emissions per Unit of Area

From the change in carbon emissions per unit of sown area in Figure 10,an upward trend can be seen from 2000 to 2003 in Beijing, which remains relatively stable from 2004 to 2010, and an upward trend from 2011 to 2017 can be seen, reaching 2.88 t/ha. After 2018, it decreases significantly and drops to 1.61 t/ha by 2023. Overall, the average carbon emissions per unit area in 2000–2010 and 2011–2023 are 1.99 and 2.35 t/ha, respectively.

3.3.2. Carbon Emissions per Unit of Output

From the perspective of carbon emissions per unit of output, a downward trend from 2000 to 2003 can be seen, and a long-term steady growth trend from 2004 to 2019 is maintained. The carbon emissions per ton of output increased from 113.75 kg to 177.93 kg, indicating that the carbon emissions per ton of agricultural products continued to rise (Figure 10). After 2020, there was a linear decline, and by 2023, the carbon emissions per ton of output reduced to 100.46 kg. This may be related to the reduction in agricultural chemical input and the increase in agricultural product output brought about by the improvement of the agricultural green production technology level.

3.3.3. Carbon Emissions per Unit of Output Value

In terms of carbon emissions per unit of output value, since 2000, the carbon emissions per unit of agricultural output value have shown a continuous and slow downward trend. That is, the carbon emissions per RMB 10,000 of output value have been slowly decreasing, from 0.95 t in 2000 to 0.19 t in 2023, indicating that Beijing’s agricultural production was gradually transforming to green development (Figure 10).

4. Discussion

4.1. Carbon Emissions and Carbon Source Contributions from the Planting Industry

In this study, the carbon emissions from the planting industry in Beijing in 2023 were 256,400 tons, of which the carbon emissions from agricultural inputs, N₂O, and CH₄ were 149,300 tons, 105,200 tons, and 2000 tons, respectively. From 2000 to 2023, the carbon emissions from agricultural inputs and N₂O accounted for 50.85% and 47.95% of the total planting industry on average, respectively, and were the main sources of carbon emissions from the planting industry in Beijing. Chemical fertilizers and agricultural films were the important sources of carbon emissions. The advantage of this study lies in the detailed calculation of the annual carbon emissions, carbon emissions composition, carbon production, economic and ecological benefits, and contribution to carbon sources of Beijing’s planting industry in the past 24 years. The detailed calculation of carbon emissions from Beijing’s planting industry, which has lasted for more than 20 years, is the first of its kind and also a highlight of this study, providing important basic data for the continuous monitoring of carbon emissions from Beijing’s planting industry. Although the conclusions of different studies regarding agricultural carbon emissions in different regions are not the same, it is a common result that chemical fertilizer application is the main carbon source of crop production. Moreover, the impact on the environment caused by the N₂O emissions resulting from the application of nitrogen fertilizers cannot be ignored [41]. This is also consistent with the results of others’ research. Li et al. [42] constructed a list of carbon sources involving six emissions factors: chemical fertilizer, pesticide, agricultural film, plowing, diesel, and irrigation. They mainly focused on exploring the increase in agricultural carbon emissions caused by human activities, such as production factor inputs, among which chemical fertilizer was considered to be the largest carbon source of agricultural carbon emissions. Qian et al. [43] analyzed the composition characteristics of carbon emissions from black soil in the three northeastern provinces from 2000 to 2021, and found that chemical fertilizer application was the main carbon source. Jia et al. [44] studied the agricultural carbon emissions in China from 2003 to 2021, and also found that the use of chemical fertilizers was the main factor leading to agricultural carbon emissions, with an average annual contribution of 58%, while the use of pesticides, agricultural films, and diesel oil was a secondary factor, accounting for 10%, 14%, and 14%, respectively. Therefore, this study proposed that carbon emissions reduction in the planting industry can start with optimizing the amount of agricultural inputs, such as chemical fertilizers, pesticides, and agricultural films, and reducing agricultural carbon emissions through scientific farmland management measures.

4.2. Factors Affecting Carbon Emissions from the Planting Industry

The results of this study show that the carbon emissions from the planting industry in Beijing have generally shown a downward trend since 2000. By 2023, the carbon emissions from agricultural inputs and N₂O from farmland decreased by 59.88% and 74.52%, respectively. The carbon emissions of CH₄ from rice cultivation were only 2.38% of those in 2000, and the total carbon emissions from the planting industry in Beijing have decreased by 70.43%. This study found that changes in carbon emissions from Beijing’s planting industry were correlated with changes in carbon source composition. The carbon emissions from agricultural inputs and N₂O emissions from farmland were the main sources of carbon emissions from the planting industry in Beijing, while the amount of fertilizer and agricultural film input directly affected the carbon emissions from agricultural inputs. Meanwhile, N₂O carbon emissions from agricultural land were also significantly related to fertilizer inputs. This discovery provides a pathway for carbon reduction in the planting industry in Beijing.

The changes in carbon emissions from Beijing’s agriculture industry suggest that the reasons for these changes may be related to the evolution of Beijing’s agricultural policies and the adjustment of its planting structure. According to the data from the Beijing Municipal Bureau of Statistics, the sown area of crops dropped from 457,300 hm² in 2000 to 143,800 hm² in 2022, and the pure amount of agricultural fertilizers and pesticides dropped from 179,000 tons and 5400 tons in 2000 to 66,100 tons and 2300 tons in 2022, respectively. The decline in the sown area and the quantity of agricultural inputs has led to a decline in carbon emissions from the planting industry. At the same time, before 2012, the increase in grain production mainly depended on a large amount of agricultural materials input, and then with the active implementation of policies, such as the “double reduction of chemical fertilizers and pesticides” and the development of green agriculture, as well as relevant control measures, the carbon emissions from the planting industry have been effectively controlled, and the total agricultural carbon emissions have shown a downward trend.

In addition, the improvement of agricultural production methods and production technologies has promoted the green development of agriculture; optimized the management and use of water, fertilizers, and pesticides in the planting industry; and reduced the carbon emissions from the planting industry to a certain extent. This result is also consistent with others. Previous studies have found that agricultural carbon emissions have strong spatial heterogeneity, which was related to differences in natural conditions, production methods, production layout, planting structure, resource input, and technical level in different regions [13,14,15]. Meanwhile, some studies also believed that the level of agricultural economic development, agricultural structure, the scale of labor force, mechanization, and agricultural production efficiency were the main driving factors of agricultural carbon emissions [16,17]. This study only expounded the possible influencing factors of carbon emissions in Beijing from the composition, while the quantitative relationship between carbon emissions and local planting structure, production layout, agricultural economy, labor force scale, production efficiency, and other factors will also be the focus of further in-depth research in the future.

5. Conclusions

This study provides a detailed calculation of the annual carbon emissions, carbon emissions composition, carbon production, economic and ecological benefits, and changing trends of Beijing’s planting industry in the past 23 years, providing important basic data for the continuous monitoring of carbon emissions from Beijing’s planting industry. In 2023, the carbon emissions from the planting industry in Beijing were 256,400 tons, of which the carbon emissions from agricultural inputs, N₂O, and CH₄ were 149,300, 105,200, and 2000 tons, respectively. From 2000 to 2023, the total carbon emissions from the planting industry in Beijing showed a downward trend. Compared with 2000, carbon emissions from agricultural inputs and N₂O of farmland in 2023 decreased by 59.88% and 74.52%, respectively, Carbon emissions of CH₄ from rice cultivation were only 2.38% of those in 2000, and the total carbon emissions from the planting industry in Beijing decreased by 70.43%.

The identified factors affecting agricultural emissions in this study provide a possible pathway for carbon reduction in Beijing’s agriculture. The changes in carbon emissions from agriculture in Beijing are related to the changes in carbon source composition. The carbon emissions from agricultural inputs and N₂O of farmland accounted for 50.85% and 47.95% of the total level from the planting industry on average, respectively, which were the main sources of carbon emissions from the planting industry in Beijing. The amount of fertilizer and agricultural film input directly affect the carbon emissions from agricultural inputs. Meanwhile, carbon emissions of N₂O from agricultural land are also significantly related to fertilizer inputs. Reducing inputs for agriculture and sources of N₂O from farmland is an important way to reduce carbon emissions from agriculture in Beijing, providing a way to support carbon reduction in agriculture.

Based on the above research results, some suggestions for reducing the carbon emissions from Beijing’s planting industry are as follows.

Firstly, optimize the planting structure and reduce fertilizer input through multiple channels to reduce carbon emissions. This study proposes that fertilizers are one of the important factors affecting carbon emissions from agricultural inputs, and reducing the amount of fertilizer input can be considered to reduce carbon emissions. One way is to reduce the cultivation of high-nitrogen crops. As is well known, the amount of fertilizer applied per unit area for vegetables is much higher than that for grain crops. The 2016 annual report on the long-term monitoring of farmland quality in the suburbs of Beijing showed that the total input of organic and inorganic nutrients in grain fields was 522.0 kg/hm²; in open-field vegetable fields and facility vegetable fields it was 2050.5 kg/hm² and 3427.5 kg/hm², respectively. However, the size of vegetable planting areas in Beijing has increased from 21.3% in 2010 to 32.39% in 2023, showing a clear and rapid upward trend. The increase in the size of vegetable planting area has led to a relatively high level of agricultural fertilizer input in Beijing. Therefore, appropriately reducing the vegetable planting area, increasing the grain field area, and optimizing the planting structure can reduce the total amount of fertilizer input, thereby reducing carbon emissions in the planting industry. In addition, nitrogen-fixing crops can reduce nitrogen fertilizer input by fixing a certain amount of nitrogen in their roots. The cultivation of carbon sequestration crops can offset carbon emissions due to their strong carbon sequestration capacity. Increasing the number of crops with strong nitrogen fixation and carbon sequestration capabilities can help reduce carbon emissions. For example, increasing the planting area of beans and C4 crops (such as corn, sunflower, etc.), not only fixes nitrogen and reduces the amount of nitrogen fertilizer used, but also increases carbon fixation through photosynthesis and reduces carbon emissions.

Secondly, developing precision agriculture and reducing agricultural inputs from the source are helpful. We should aim to vigorously popularize precision fertilization, pesticide application, and irrigation technologies; moderately reduce the amount of agricultural film, and increase the recycling of agricultural film. This study confirms that carbon emissions from agricultural inputs are the main source of carbon emissions in the planting industry in Beijing. Agricultural inputs include fertilizers, pesticides, agricultural films, irrigation, and mechanical diesel used for plowing. Smart agriculture helps with the precise input of agricultural chemicals, avoiding resource waste and reducing carbon emissions. Using soil testing data and agronomic big data can intelligently provide precise fertilization plans for growers based on the nutrient status of different plots and the growth requirements of crops, avoiding the excessive use of fertilizers, and reducing carbon emissions from chemical fertilizer inputs and N₂O of farmland. We should establish a monitoring and early warning mechanism for pests and diseases, carry out precise prevention and control measures, and actively promote biological prevention and control technology to reduce pesticide investment. We should optimize the types of crops planted, appropriately reduce the input of agricultural film, and introduce advanced residual film recycling machinery to improve recycling and utilization rates. At the same time, smart irrigation water-saving technologies, such as drip irrigation and sprinkler irrigation, are promoted in various directions to reduce the waste of water resources and carbon emissions caused by improper water management.

Thirdly, we should improve the comprehensive utilization of agricultural waste. The direct carbon emissions of N₂O from farmland come from nitrogen-containing fertilizers, organic fertilizers, and straw returning. The effective utilization of organic fertilizers and straw resources can reduce agricultural carbon emissions. We should carry out the resource-based treatment of waste, such as crop straws and livestock manure. For example, silage and ammonification treatment of straw can be used as feed, or biogas can be generated through anaerobic fermentation for energy use. This not only reduces greenhouse gas emissions generated by the decomposition of waste in the natural environment, but also produces clean energy, realizing the recycling of resources.