Spatiotemporal variations of water, land, and carbon footprints of pig production in China

Pig production not only consumes large water and land, but also emits substantial greenhouse gases. Scholars have used environmental footprint, including water, land, and carbon footprints, to assess the environmental impacts of pig production. However, there is a lack of simultaneous analysis of water, land, and carbon footprints for pig production, particularly in China, the leading pig breeding and consumption country in the world. In this study, we used environmental footprint analysis to develop a water-land-carbon footprint model for pig production system from the life cycle perspective. We also examined virtual water, land and carbon flows embedded in trade. The life cycle of pig production mainly involves feed crop cultivation, feed production, and pig breeding. Then we analyzed the water, land, and carbon footprints of the pig production in China from 1990 to 2018. The findings indicated that both the total water footprint and the total land footprint increased, whereas the total carbon footprints fluctuated over the years. All the unit footprints generally decreased. Feed crop cultivation had a significant impact on the footprints. Regarding the spatial characteristics in China, all the three total footprints were generally higher in the southeast compared to the northwest. While the unit footprints in different provinces exhibited spatial heterogeneity, primarily attributed to the different feed consumptions. Overall, the trade of feed crop shifted virtual water, land and carbon flows from the north to the south, reducing the three total footprints on the whole. To achieve a resource-saving and eco-friendly pig production in China, we proposed some potential recommendations, including improving feed crop cultivation techniques, reducing feed conversion ratio, implementing precision feeding, and managing manure. We hope not only mitigate the environmental impacts of China’s pig production, but also provide references for achieving agricultural sustainability in other regions of the world.


Introduction
Global livestock husbandry contributes to 22% of the total evapotranspiration from global agricultural lands and 42% of the total agricultural consumptive water use allocated for human use (Heinke et al 2020).In addition, global livestock production accounts for almost 80% of all agricultural land, with feed production taking up approximately one-third of total cropland (FAO 2021).Livestock also contributes to 18% of total greenhouse gas (GHG) emissions (FAO 2006(FAO , 2022)).As one of the largest livestock sectors globally (Govoni et al 2022), the pig sector accounts for over one-third of the meat products in the world, and primarily contributes to the environmental impacts arising from livestock (Zhu andChen 2018, Pexas et al 2020).For example, pig production contributes to 18% of global anthropogenic GHG emissions, making it the second largest source of GHG emissions after beef production (Philippe and Nicks 2015).
Therefore, scholars have been paying significant attention to the environmental issues caused by pig production in recent years.Environmental footprint makes important contributions in environmental sustainability (Vanham et al 2019, Matuštík andKoc ˇí 2021).Environmental footprints are resource use and emissions oriented, combined referred to as pressure oriented (Vanham et al 2019).They are indicators that can be used to evaluate the pressure of human activities on the environment (Wu et al 2021).The environmental footprint method was extended from the ecological footprint method proposed by William Rees and his student Wackernagel in the 1990s (Wackernagel and Reels 1996).Since then, scholars have applied this concept to calculate water footprint (Hoekstra 2003), energy footprint (Larson et al 2012), carbon footprint (Hammond 2007, Liu et al 2020), and have implemented modeling methods in different social fields and industries, such as crop production (Adewale et al 2018, Bajan and Mrówczyńska-Kamińska 2020, Chen et al 2021a, Huang et al 2022), livestock breeding (Luo et al 2015, Wang et al 2015, Lynch 2019, Dai et al 2021), aquaculture (Hammer et al 2022), and industrial production (Heidari andAnctil 2022, Zhao et al 2022).The studies evaluating carbon footprints of pig production have also been conducted in numerous countries, such as Canada (Vergé et al 2016), Argentina (Arrieta and González 2019), Spain (Noya et al 2016), and Italy (Bava et al 2017).Given the significant water consumption of pig production, previous studies have also analyzed water footprint from various perspectives, such as carrying capacity (Yu et al 2012), trade (Zhuo et al 2019), farming scale (Xie et al 2020), and water use efficiency (Zhang et al 2022).The study of carbon emissions from pig farming is often combined with land use (Meul et al 2012, Monteiro et al 2019).In fact, simultaneous study of various environmental footprints is necessary (Dalin andRodríguez-Iturbe 2016, Wiedmann andLenzen 2018).Consequently, an increasing number of studies are being conducted to integrate water, land and carbon footprints in agriculture (Fan et al 2020) and other livestock production such as sheep, chicken and cattle (Ridoutt et al 2014, Ibidhi et al 2017).While these studies seldom simultaneously analyze water, land, and carbon footprints for pig production.
Additionally, the stages of feed crop cultivation, feed production and farm breeding contribute to water consumption, land use, and carbon emissions (Chen et al 2021b, Arrieta et al 2022).Analyzing water, land and carbon footprints of pig production from a life-cycle perspective would provide a more comprehensive and objective assessment.Thus, it is imperative to simultaneously analyze the water, land, and carbon footprints of pig production from a life cycle perspective.The assessment of environmental footprint is based on the life cycle thinking (Li et al 2021a).Life cycle assessment (LCA) is also a life-cycle approach aiming to assess the overall environmental impact of all emissions, resource use and production of the product through its life cycle from cradle to grave (ISO 2006).Currently, the LCA of livestock is mainly conducted from cradle to farm gate considering a system from breeding phase to growing fattening phase (Pirlo et al 2016, Makara et al 2019), or from cradle to consumption considering a system from breeding phase to consumption phase (Noya et al 2017a, Liu et al 2021a, Mazzetto et al 2023).The life cycle of pig farming mainly often refers to the entire process from growing feed crops to farming (McAuliffe et al 2016, Zheng et al 2021).Therefore, we can integrate footprint analysis with LCA, which means analyzing the water, land, and carbon footprints from a cradle-to-farm gate perspective.
It also should note that feed supply and demand are frequently adjusted through inter-provincial and international trade, resulting the transfer of the environmental burdens from importing regions to feedcultivating regions.This water and land consumptions and carbon emission resulting from the virtual embedding in trade are called virtual water, land and carbon (Chapagain and Hoekstra 2003a, 2003b, Wu et al 2018a).Some studies on crop trade have calculated virtual water (Zhuo et al 2016, Ali et al 2017, Wu et al 2018a, 2019, Ji et al 2022), virtual land (Ali et al 2017, Wu et al 2018a, Govoni et al 2022) and virtual carbon (Wu et al 2018a).Though some researchers have utilized virtual concept to analyze the environmental footprints in livestock production (Zhuo et al 2019, Li et al 2021b), they focus on virtual water flows or water footprint, with limited attention given to other virtual flows or footprints, such as carbon and land.Hence, the research question is how to simultaneously analyze all water, land, and carbon footprints including virtual footprints through the life cycle of pig production.
China has experienced rapid development in its pig husbandry sector, and become the leading pig breeding and consumption country in the world (Yuan et al 2018).As the world's largest consumer of pork, China accounts for 44.7% of global pork consumption (FAO 2020, Hu et al 2023).Wheat, corn, and soybean, the main components of feed, are supplied and consumed differently across the provinces of China.Soybean is mainly imported from Brazil, the United States and Argentina, which are the top three soybean-producing countries, collectively accounting for 80% of global soybean production (FAO 2020, GAC 2020).There are also significant spatial variations in pig production among the provinces of China.Pig production is typically concentrated in densely populated regions, such as the coastal, the central, and the eastern areas of southwestern China (Xie et al 2020).
The objectives of this study are as follows: (i) establishing a water-land-carbon footprint model of pig production from cradle-to-farm gate life cycle perspective; (ii) quantifying the water, land, and carbon footprints through the life cycle of pig production in China from 1990 to 2018; (iii) comparing the interprovincial distributions of water, land and carbon footprints of pig production in China in 1990 and2018, respectively;(iv) quantifying the virtual water, land and carbon flows of the pig production industry in China in 2018; (v) proposing potential recommendations to reduce the environmental footprints of pig production in China.

Methods
From the life cycle perspective (Suh and Huppes 2009), we used environmental footprint (Arrieta and González 2019, Xie et al 2020, Arrieta et al 2022, Govoni et al 2022) to analyze the water, land, and carbon footprints of pig production in China.

System boundary
In this study, we established a water-land-carbon footprint model for pig production.We calculated two metrics of footprints (Xie et al 2020, Lin et al 2021, Zhang et al 2022).One is the total water, land, and carbon footprints of pig production in China (m 3 , m 2 , and kg CO 2 eq), the other is the unit water, land, and carbon footprints per kilogram live weight (LW) of pig (m 3 kg −1 LW, m 2 kg −1 LW, and kg CO 2 eq kg −1 LW), which is calculated by the total footprints divided by the LW of pig.We set the system boundary from cradle-to-farm gate, focusing on primary production, which encompassing feed crop cultivation, feed production, and pig breeding (figure 1).Enteric fermentation and subsequent slaughter processing were excluded, due to their small proportions and data availability (Arrieta and González 2019, Liu et al 2021a, Chen et al 2021b).

Feed crop cultivation
We mainly considered the activities of cultivating the feed crops including wheat, maize, and soybean.Because the ingredients of pig feeds in China are generally similar, mainly including maize, wheat bran, and soybean cake (PDNDRC 1999-2021, Luo et al 2015, Hui et al 2016).According to national statistics (CFIA 2014), we considered these three feed ingredients account for 54%, 16%, and 16% of the total feed, respectively.We did not take into account the feed additives accounting for only small part of feed.The water consumed in feed crop cultivation includes feed crop evapotranspiration (green water), and consumption of ground and surface waters (blue water).The use of agricultural materials including fertilizers, pesticides, agricultural films, diesel fuel, and electricity for irrigation contributes to the GHG emissions (Chen et al 2021b).We excluded the material uses and emissions associated with the production of these agricultural materials.The land mainly refers to the cultivated area of the feed crops.

Feed production
Then the feed crops are processed, consuming water and emitting GHG emissions from electricity and natural gas use (Wu et al 2018b).We have not considered the land occupied by factories and equipment in this process, due to its relatively small occupation compared to that in other processes.We assumed that all feed crops were processed and feeds were firstly consumed nearby.Given that some feed crops for farms breeding are transported from different Chinese provinces and abroad, we analyzed the virtual water, land and carbon footprints, taking transportation cost into account.

Pig breeding
In pig breeding, land is occupied by farm.Water is used for pig drinking and farm service.GHGs are emitted from electricity use, coal use, and manure management (Philippe and Nicks 2015, Uwizeye et al 2020).It should also note that coal is used to keep pig and feed warm (SDPCCPD 1991(SDPCCPD -2019)).Details about the system boundary is illustrated in the supplementary information.

Footprints estimations
Water footprint refers to the actual amount of water resources needed to maintain the consumption of human products and services (Hoekstra 2003).Here, we regarded the water footprint as the sum of the green water footprint (the volume of the precipitation consumed in the crop production process) and the blue water footprint (the surface and groundwater consumed in crop production) (Hoekstra et al 2011, Mekonnen andHoekstra 2011).Land footprint refers to the amount of the land needed to produce a product expressed in area per unit of product (Borucke et al 2013).Carbon footprint represents the CO 2 equivalent of GHGs emitted by a product through its life cycle (Wiedmann and Minx 2008).
The calculation scheme of water-land-carbon footprints of China's pig production is shown in figure 2. In feed crop cultivation, water footprint is the sum of green water footprint and blue water footprint.Land footprint refers to the cultivated land areas for feed crops.Carbon footprint is the GHGs emitted from the use of agricultural materials in cultivation.We examined the three virtual footprints associated with water consumption, land use, and carbon emission resulting from international and interprovincial trades of crop feeds.Since land was not taken into account in feed production, we only considered the water footprint associated with water consumption, as well as the carbon footprint resulting from the use of electricity and natural gas.In pig breeding, the three footprints are attributed to the specific material uses mentioned above.Further information regarding the calculations and data for the footprints are provided in the supplementary information.

Water footprint
Figure 3 shows the unit water footprint gradually declined from 2.57 m 3 kg −1 LW in 1990 to 1.78 m 3 kg −1 LW in 2015, then rebounded to 1.81 m 3 kg −1 LW in 2018.The feed crop cultivation, decreased from 2.48 m 3 kg −1 LW in 1990 to 1.72 m 3 kg −1 LW in 2018, contributed approximately 97% of the water footprint of pig production.This is mainly attributed to the feed consumption and the water footprint of cultivating the feed crop.The amount of pig feed decreased from 1990 to 2005 and then gradually increased.The consumption of drinking water and service water in the breeding process decreased by 37% from 1990 to 2018.The main reason is that pigs became heavier in fewer feeding days with the development of feeding techniques.The average daily weight gain of pigs increased during these years.The total water footprint increased by 69% from 91.12 Gm 3 in 1990-153.60Gm 3 in 2018.Despite a downward trend in the unit water footprint, the annual pig slaughter increased by 124%.
Figures 4(a) and (b) shows the total water footprints across all the 31 provinces of China's main land in 1990 and 2018.Generally, the southeast exhibited a higher total water footprint compared to the northwest.This can be attributed to the dense population and the higher pork demand in the southeast.In 1990, Sichuan had the highest total water footprint, followed by Shandong and Hunan.In 2018, the total water footprints were higher in Hunan and Hubei.Tibet had the lowest total water footprint in both 1990 and 2018.As central China is the region with large pork farming and the fastest growth of total water footprint, Hubei and Hunan increased by 224% and 139%, respectively.They also have to supply pork to the eastern coastal cities.In contrast, the total water footprints in the more economically developed regions of Beijing, Shanghai, Jiangsu and Zhejian decreased by 2%-72%.This decline can be attributed to both crop yields and pig stocks decreased with the rapid urbanization of these cities.
The unit water footprints of pig production exhibited spatial heterogeneity across different provinces (figures 4(c) and (d)).In 1990, Shaanxi had the highest unit water footprint, followed by Guizhou and Shandong.While Heilongjiang had the lowest unit water footprint.In 2018, Heilongjiang had the highest unit water footprint, followed by Hubei and Shaanxi.In contrast, Xinjiang, Beijing, and Sichuan had significantly lower unit water footprints.The spatial distribution of unit water footprint was generally consistent with the distribution of feed consumed (figure 4(e)).Overall, the unit water footprint decreased in most provinces, except for Inner Mongolia and Heilongjiang, which increased by 24% and 50%, respectively.

Land footprint
Feed crop cultivation primarily contributed to the land footprint of the pig production (figure 6). Figure 5 shows that the unit land footprint of feed crops decreased from 5.34 m 2 kg −1 LW in 1990 to 3.30 m 2 kg −1 LW in 2018.From 1990 to 2018, the yields per acre of wheat, corn, and soybean in China increased by 68%, 32%, and 29%, respectively.
The total land footprint increased in most regions except for Beijing, Shanghai, Jiangsu, Zhejiang and Sichuan.In 1990, Sichuan and Tibet contributed the highest and the lowest total land footprints, respectively (figures 6(a) and (b)).In 2018, Hunan had the highest total land footprint, while Tiber had the lowest.However, the unit land footprint of pig production generally decreased in most regions except for Heilongjiang, Anhui, Fujian, and Hubei (figures 6(c) and (d)).In 1990, Shaanxi had the highest unit land footprint, while Guangdong had the lowest.In 2018, Shaanxi and Shanghai had the two highest unit land footprints, respectively.

Carbon footprint
The unit carbon footprint of pig production has gradually decreased from 3.33 kg CO 2 eq/kg LW in 1990 to 1.28 kg CO 2 eq kg −1 LW in 2018.Figure 7 shows farm accounted for the half of the unit carbon footprint in 1990, followed by feed cultivation (29%) and manure management (19%), and feed production (2%).In 2018, the contribution of farms to the unit carbon footprint had decreased to 9%, while feed cultivation and manure management accounted for 39% and 45%, respectively.This is primarily attributed to the structure change of fuel power consumption on farms.Due to the higher manure transportation costs and the implementation of strict standards for organic fertilizer application, farm gradually replace coal with cleaner energy, such as biogas (Yuan et al 2018, Sun et al 2022).Sun et al (2022) showed that a pig farm with an annual output of 10 000 pigs could reduce 504t CO 2 eq annually by implementing a biogas project.Despite a 124% increase in annual pig slaughtering, the total carbon footprint decreased from 1.18 × 10 11 kg CO 2 eq to 1.09 × 10 11 kg CO 2 eq.
In both 1990 and 2018, Sichuan had the highest total carbon footprint, while Tibet had the lowest one, respectively (figures 8(a) and (b)).In 1990, Shanxi, Guangdong, and Sichuan had the three highest unit carbon footprints, while Shanghai had the lowest one.In 2018, Beijing and Gansu had the two highest unit carbon footprints.In contrast, Henan, Guangdong, and Guangxi had the three lowest (figures 8(c) and (d)).Generally, the southeast region with dense population had a lower unit carbon footprint compared to the northwest region.Figure 8(e) shows the spatial unit carbon footprints of the stages of pig production were similar to those of the whole pig production system.

Virtual water, land and carbon flows
Figure 9 represents the virtual water, land and carbon flows in 2018.It shows that Brazil, Henan, and Heilongjiang had the three largest transfer-out water footprints.Conversely, Hunan, Hubei, and Henan had the largest transfer-in water footprints.It should note that Henan Province is a major grain production region in China.The productions of the three main feed crops, including rice, wheat, and soybean, are at the forefront of crop production in China.The number of breeding pigs in Henan was significant higher than in other provinces (NBSC 2022).Moreover, Henan possesses limited water resources and exports a substantial quantity of virtual water to other provinces (Chen et al 2017).Heilongjiang, Shandong, and Inner Mongolia contributed the three largest transfer-out carbon footprints.While Hunan, Yunnan, and Sichuan had the highest transfer-in carbon footprints.Brazil, Heilongjiang, and Henan had the three highest transfer-out land footprints.While Hunan, Hubei, and Henan had the three highest transfer-in land footprints.
Brazil, Heilongjiang, and Henan were the main suppliers of feed.While Hunan, Henan, and Sichuan had the highest demand for feed.The water and land footprints of producing crops in exporting provinces had less impact on virtual water and land flows.This is due to the fact that areas with high water and land footprints of producing crops often had low crop yields and export volumes.
In general, the feed crops transported from the northern provinces to the majority of southern provinces had a lower water footprints compared to the local water footprints.The water consumption was transferred from the southern provinces to the northern provinces, moving from water-rich areas to water-poor areas.The land footprint is strongly correlated with crop yield, implying that the higher yields of feed crops can effectively decrease the land footprint.The carbon footprint of crop production is primarily determined by the amount of fertilizer applied.Therefore, importing fodder crops from regions with a high carbon footprint of crop production will increase the local carbon footprint.Although Brazil has a high supply of soybean, the carbon footprint associated with its production is lower than the domestic average.This is likely attributed to the significantly lower carbon footprint from fertilizer application on soybean in Brazil.Qiang et al (2013)' study showed that the carbon footprints from fertilizer application on soybean in Brazil and China were 19 kg CO 2 /t and 63 kg CO 2 /t, respectively.The low carbon footprint transferred to Henan was also mainly can be attributed to the fact that the crops used for pig feed in Henan are mainly produced domestically and supplied by Brazil.(Bava et al 2017), and China (Liu et al 2021a), arranging from 2.4 kg CO 2 eq kg −1 to 6 kg CO 2 eq kg −1 (table S11).These differences arise primarily from their different system boundaries, feed compositions, energy consumptions in different farming patterns, and carbon emission factors.Furthermore, our study has showed that manure management and feed cultivation together contributed almost half of the unit carbon footprint in recent several years.While Liu et al (2021a)'s study conducted between 2016 and 2017 revealed that feed cultivation contributed to almost half of the total carbon footprint, followed by pig breeding and manure management.Chen et al (2021b)'s study showed a similar trend of the total carbon emission from the pig production in China from 2000 to 2016, initially declining and subsequently increasing.These are consistent with this study.The water footprint of this study also differs from that of other studies (table S10).This differences can be attributed to variations in feed conversion efficiencies and water footprints of crop production (Mekonnen and Hoekstra 2012a).The land footprint in this study was similar to the results of the European study (Sporchia et al 2021), and lower than that of the Australian study (Wiedemann et al 2018) (table S12).

Discussion
Govoni et al (2022) have assessed both internal and external land and water consumptions in the pig feeding of the world in 2018.They found that China used the largest water which was 201.4 km 3 , including 181.2 km 3 of green water and 20.2 km 3 of blue water.This value is higher than our result in 2018, which was 153 km 3 of total water footprint.They also showed that the China's demand for pig feed required 39.4 Mha of agricultural land, which is also higher than our study (28.2Mha).These can be attributed to two reasons.Firstly, Govoni et al (2022) primarily used the Global Livestock Environmental Assessment Model developed by the Food and Agriculture Organization of the United Nations (FAO) (2018).The model encompasses more stages along livestock supply chains, including feed production, processing and transport, herd dynamics, animal feeding and manure management, as well as animal products processing and transport.The data is also international, and may have some variations compare to our national data.In addition, Govoni et al (2022)'s study determined a significantly different pig diet composed of energy sources (70%), of which cereals comprising the largest share (including grains from wheat, maize, barley, millet, rice, and sorghum).Additionally, cassava, and sugarcane tops, and other energy crop by-products are sometimes added.
Ji et al (2022) have assessed the inter-provincial virtual water flows associated with pork production and consumption in China for the years 2008 and 2017.This is different from our study including the virtual water flows from both inter-provincial and international crop trade.Both studies indicated that Heilongjiang and Henan were the major exporters of feed-related virtual water flows, whereas Hunan had the highest imports of feed-related virtual water flows.Furthermore, many studies on China's pig production focus on only one or two footprints.Here, we have analyzed water, land, and carbon footprints of China's production to provide a more objective and comprehensive assessment for China's sustainable development.

Recommendations
This study shows that the feed crop cultivation had the highest water and land footprints and the high carbon footprint.Therefore, it is necessary to reduce resource consumptions and GHG emissions in feed crop cultivation.In agriculture, various strategies can be implemented to enhance productivity, such as optimizing piglet selection, improving breeding technique, enhancing soil quality, reducing the use of chemical fertilizers, and increasing yields with limited  have suggested implementing a policy that combines planting and breeding practices to reduce carbon emissions (Li et al 2021b).
The government can also implement a range of macroeconomic policies.Reducing water footprint and land footprint can be achieved through market supply and demand.For example, Liu et al (2021b) illustrated that the export of agricultural products exported from regions with high water and land use efficiency can enhance sustainability.Additionally, their research showed that agricultural trade contributed to water and land depletions in the five central Asian nations.Zhuo et al (2019) emphasized that the expansion of pork industry would inevitably increase the demand for local feed production demand and the subsequent consumption of water resource.Given that soybean is the primary ingredient in pig feed and a significant contributor to environmental impacts (Noya et al 2017b, Liu et al 2021a), it is possible to reduce its export and substitute it with alternative protein-rich ingredients such as feed-use amino acids (Garcia-Launay et al 2014, Monteiro et al 2016).Applying carbon tax policies to agriculture is challenging due to the complexity of the carbon footprint.Nevertheless, the government can implement various economic incentives to encourage agriculture to meet below carbon emission standards, optimize land use through macro-regulation means, and cultivate crops suitable for specific soils to reduce fertilizer application, etc.Further simulation analysis can help identify more effective policies.
The Ministry of Agriculture and Rural Affairs released the National Pig Production Development Plan in 2016 (MOA 2016).The plan divided the primarily pig production areas in China into key development regions, namely Henan, Chongqing, Sichuan, Hainan, Guangxi, Hebei, and Shandong.They supply pork to coastal cities with lower water, land and carbon footprints.The potential growth areas include Liaoning, Jilin, Inner Mongolia, Yunnan, Guizhou, and Heilongjiang.When developing pig farming in these areas, the farming technology that improves the feed conversion efficiency should be implemented, similar to the traditional pig-raising provinces.Additionally, we should not ignore the fuel consumed in the northeast region, where pigs should be kept warm in winter.It should take the initiative to adopt clean energy for heating.

Limitations and future research
Accessing some statistical data before 2000 is challenging, requiring estimation using data from other years.Except for national data, we collected some data from international database and studies conducted in other regions.Furthermore, we assumed that the structure of feed components remained constant during these years due to data availability, despite demonstrating variations in China's pig from 1990 to 2018.Some studies have been analyzing the effects of different pig diets (Nakamura and Itsubo 2019, Benavides et al 2020, Hu et al 2023).In the future, we will investigate more farms and collect more data sources to improve the data quality.
We did not divide the pig grow stage (e.g.rearing of sows, piglets and breeding boar), and the breeding scales, expect for drinking water, average feeding days, and service water, which were used to differentiate between extensive breeding and intensive breeding.In fact, some studies have analyzed the environmental impacts of pig production considering the grow stages (Qian et al 2018, Liu et al 2021a) and breeding scales (Wang et al 2016, Cheng et al 2022).These studies will help us to compare the environmental impacts of pig productions with different grow stages and breeding scales.
As shown in supplementary information, we calculated the total water footprint by combining green water footprint and blue water footprint, both of which were obtained from the Water Stat Dataset (Mekonnen and Hoekstra 2012b).To simplify the analysis and directly compare the water footprints, we did not analyze them individually.Some studies have shown that the green water footprint typically accounts for the largest proportion of the total water footprint.For instance, Xie et al (2020) and Govoni et al (2022) found that the green water footprint of pig feeding in China accounted for 82% and 90% of the total water footprint, respectively.
Currently, agricultural production, diet change, COVID-19, Russo-Ukrainian war, and food loss highlight the challenges to food security, food trade and its virtual environmental footprints (Wang et al

Conclusions
The results show that all the three unit footprints of China's pig production generally decreased from 1990 to 2018.The water and land footprints primarily originated from feed cultivation.The carbon footprint primarily originated from feed cultivation and manure management.However, both the total water and land footprints increased during these years with the increasing pork consumption.The total carbon footprint fluctuated down due to change of the energy structure.
Spatially, all the three total footprints in the southeast were higher than those in the northwest.While the unit footprints in different provinces exhibited spatial heterogeneity, primarily due to the different feed consumptions.As feed crops are predominately cultivated in the north, the virtual water, land and carbon flows were generally shifted from the north to the south, resulting in a transfer of the burden of water and land consumptions from the south to the north.
Based on these findings, it is imperative to enhance the sustainability of pig production from various aspects, including crop cultivation, feed consumption, and farm breeding.This study hopes to establish a research foundation and serve as a reference for agricultural development in other regions worldwide.Furthermore, future research will

Figure 1 .
Figure 1.System boundary of the water-land-carbon footprint model of pig production system.

Figure 2 .
Figure 2. Calculation scheme of water-land-carbon footprints of China's pig production system.

Figure 3 .
Figure 3.The variation of unit water footprint through its life cycle and the total water footprint of pig production in China during 1990-2018.

Figure
Figure Provincial total water footprint (a), (b); unit water footprint (c), (d) of pig production in China for 1990 and 2018; provincial unit water footprint through its life cycle and feed consumed per pig in China for 1990 and 2018 (e).

4. 1 .
Comparisons with other studies Many scholars have analyzed the carbon emissions of pig production.Some studies have quantified the different unit carbon footprints of pig production in countries, such as United States (Tallaksen et al 2020), Argentina (Arrieta and González 2019), Italy

Figure 5 .
Figure 5.The change of unit land footprint through the life cycle and total land footprint of pig production in China during 1990-2018.

Figure 6 .
Figure 6.Provincial total land footprint (a), (b); unit land footprint (c), (d) of pig production in China for 1990 and 2018; provincial unit land footprint through its life cycle and feed consumed per pig in China for 1990 and 2018 (e).

Figure 7 .
Figure 7.The variation of Chinese unit carbon footprint through the life cycle and total carbon footprint of pig production in China during 1990-2018 (a); Percentage of carbon footprint of each stage of pig production (b).

Figure 8 .
Figure 8. Provincial total carbon footprint (a), (b); unit carbon footprint (c), (d) in China for 1990 and 2018; unit carbon footprint of the province through the life cycle of pig production (e).

Figure 9 .
Figure 9. Spatial virtual water footprint (a), virtual land footprint (b), and virtual carbon footprint (c) of pig production in China in 2018.The left side of the circle represents the southern cities, and the right side is the northern cities.The range connected to the nodes is transfer-out footprints, which indicate the footprints of feed crop cultivation in the province, including those used in the province and exported to other provinces.The range pointed by the arrow is transfer-in footprints, which indicate the footprint of consumed feed crop by pigs breeding in the province, including both in-province and imported crops.The line indicates the flow of the footprint.
Availability of disaggregated greenhouse gas emissions from beef cattle production: a systematic review Environ.Impact Assess.Rev. 76 69-78 Mackenzie S G, Leinonen I, Ferguson N and Kyriazakis I 2016 Can the environmental impact of pig systems be reduced by utilising co-products as feed?J. Clean.Prod.115 172-81 Makara A, Kowalski Z, Lelek Ł and Kulczycka J 2019 Comparative analyses of pig farming management systems using the life cycle assessment method J. Clean.Prod.241 118305 Matuštík J and Koc ˇí V 2021 What is a footprint?A conceptual analysis of environmental footprint indicators J. Clean.Prod.285 124833 Mazzetto A M, Falconer S and Ledgard S 2023 Carbon footprint of New Zealand beef and sheep meat exported to different markets Environ.Impact Assess.98 106946 McAuliffe G A, Chapman D V and Sage C L 2016 A thematic review of life cycle assessment (LCA) applied to pig production Environ.Impact Assess.Rev. 56 12-22 Mekonnen M M and Hoekstra A Y 2011 The green, blue and grey water footprint of crops and derived crop products Hydrol.Earth Syst.Sci. 15 1577-600 Mekonnen M M and Hoekstra A Y 2012a A global assessment of the water footprint of farm animal products Ecosystems 15 401-15 Mekonnen M M and Hoekstra A Y 2012b WaterStat water footprint statistics (available at: https://wbwaterdata.org/ dataset/waterstat-water-footprint-statistics) Meul M, Ginneberge C, Van Middelaar C E, de Boer I J M, Fremaut D and Haesaert G 2012 Carbon footprint of five pig diets using three land use change accounting methods Livest.Sci.149 215-23 MOA (Ministry of Agriculture and Rural Affairs of the People's Republic of China) 2016 National pig production development plan (2016-2020) (available at: www.moa.gov.cn/nybgb/2016/diwuqi/201711/t20171127_5920859.htm)(Accessed 22 November 2021) (in Chinese) Monteiro A N, Garcia-Launay F, Brossard L, Wilfart A and Dourmad J Y 2016 Effect of feeding strategy on environmental impacts of pig fattening in different contexts of production: evaluation through life cycle assessment J. Animal Sci.94 4832-47 Monteiro A, Wilfart A, Utzeri V J, Lukac ˇet N B, Tomažin U, Costa L N, Čandek-Potokar M, Fontanesial L and Garcia-Launay F 2019 Environmental impacts of pig production systems using European local breeds: the contribution of carbon sequestration and emissions from grazing J. Clean.Prod.237 117843 Nakamura K and Itsubo N 2019 Carbon and water footprints of pig feed in France: environmental contributions of pig feed with industrial amino acid supplements Water Resour.Ind. 21 100108 NBSC (National Bureau of Statistics of China) 2022 China Statistical Yearbook (China Statistics Press) (in Chinese) Noya I et al 2016 Carbon and water footprint of pork supply chain in Catalonia: from feed to final products J. Environ.Manage.171 133-43 Noya I, Aldea X, González-García S, Gasol C M, Moreira M T, Amores M J, Marín D and Boschmonart-Rives J 2017a Environmental assessment of the entire pork value chain in Catalonia: a strategy to work towards circular economy Sci.Total Environ.589 122-9 Noya I, Villanueva-Rey P, Gonzalez-Garcia S, Fernandez M D, Rodriguez M R and Moreira M T 2017b Life cycle assessment of pig production: a case study in Galicia J. Clean.Prod.142 4327-38 OECD/FAO 2019 OECD-FAO Agricultural Outlook 2019-2028 (Paris/Food and Agriculture Organization of the United Nations) (https://doi.org/10.1787/agr_outlook-2019-en)PDNDRC (Price Department of the National Development and Reform Commission) 1999-2021 National Agricultural Products Cost-benefit Compilation (China Statistics Press) (in Chinese)