Findings on agricultural cleaner production in the three Gorges Reservoir Area

Abstract

The ‘rural revitalization strategy’ is important to achieve sustainable development in the Three Gorges Reservoir Area (TGRA), the largest reservoir in China, located in the poverty alleviation region of the Qinling-Daba mountains, and characterized as mountainous and hilly dominated by backwards resettlements. Pig farming around the reservoir area is a key industry, accounting for 0.90% of the arable land area in the country, while the annual pig market represents a 1.37% share of the national figure. Here, 12 study sites were investigated on-site for understanding agricultural green development in the TGRA. We found two main prevailing models: one of ecological circulation (EC), based on animal husbandry and recycling. Of the 12 site, six cases of the ecological circulation model relied on pig husbandry, combined with cropping (grains/fruits/vegetables) by eco-industrial chains, such as pig-biogas-fruit (grain/vegetables), to prevent environmental pollution, while promoting agricultural economic growth by recycling fecal residues and wastewater (FSW) from pig-husbandry facilities to the fields. Our analysis predicted that a farm holding 10,000 pigs may save inorganic nitrogen and phosphorus fertilizers by as much as 74.36 and 11.15 ton·a⁻¹, respectively. On the other hand, five cases of ecological models oriented towards agritourism provided tourists with high-quality ecological products while coordinating environment protection with economic development. In addition, 11 research points applied water and fertilizer integration system for the purpose of saving water. However, lack of sufficient supporting arable land made intensive pig farming a risk of ecological degradation. Green control technologies are rarely used leading to an increase in the type and amount of pesticides. Our study has theoretical and practical significance for decision-makers to promote agricultural cleaner production (ACP).

1. Introduction

Agriculture has to feed eight billion people worldwide. However, fertilizers and pesticides have expanded steadily during the seven past decades. Thus, between 2002 and 2018, global pesticide use per hectare of cropland increased by approximately 30% and inorganic fertilizer use per hectare by approximately 23%, 13% and 56% for nitrogen, phosphorus, and potassium, respectively [1]. Moreover, crop residues and farm waste and sewage [2] pose a high, potential environmental pollution risk [3,4]. Therefore, agricultural nonpoint source pollution (ANSP) has been recognized as a key factor significantly affecting water quality [[5], [6], [7], [8]]. Consequently, although improving crop yields and the quality of agricultural products, concomitant with raising farmers income while reducing ANSP and livestock farming pollution, are all highly desirable, it is equally imperative to implement ACP by the adoption and adaptation of ecosystem-based approaches.

First of all, the design of eco-agricultural models can improve the efficiency of agricultural inputs, such as fertilizers and pesticides, water, etc., and achieve greater control of agricultural pollution from the perspective of the whole agricultural system. Integrated agricultural production schemes, e.g., rice-duck [9], rice-fish [10], and rice-crab [11] have been confirmed as effective solutions to promote ACP and control ANSP. Other ecoagriculture designs achieved by adjusting agricultural structures can improve the effectiveness of agricultural production and reduce ecological degradation [12,13]. Secondly, some ACP technologies, e.g., controlled-release fertilizers [14,15], green control technologies [16], such as bioinformation and nanopesticides [17] and ecological interception [18] are also contributing to the green development of agriculture [19]. Thirdly, research methodologies like sustainability development models [20] and assessment methods e.g., emergy [21,22], and Emergy-Data Envelopment Analysis [19], and Life Cycle Assessment [23], among others, are used for relevant research on ACP. Among ecological agricultural models, ACP technologies and research methodologies, the former designs primarily based on ecosystem principles and considers the greatest extent conservation of agricultural resources and the reduction of agricultural impacts on the environment, which should be prioritized. Nonetheless, to enhance progress towards a more sustainable agriculture, both ACP technologies and research methodologies should be considered as well.

As the largest reservoir in China, the economic development and environmental protection of the TGRA has received extensive attention from domestic and international researchers. However, inherently constrained by its natural conditions, i.e. cultivated land with a slope >15° accounted for 49.1% of the total area [24], agricultural economic development in the TGRA lags behind the national average level, as evidenced by the fact that per capita annual disposable incomes of rural households in the TGRA are lower than the national average (Table 1) [25,26]. Table 1 shows that per capita annual disposable incomes of rural households of the TGRA in Chongqing in 2020 and 2021 were lower than national average incomes by 2.92% and 2.48%, respectively, and greater gaps existed between the national average and incomes in the Key Reservoir Area.

Table 1.

Comparison of rural household incomes (yuan, RMB)

Item	Year	TGRA in Chongqing	Key Reservoir Area in Chongqing	Chongqing City	National average
per capita annual disposable income of rural households	2021	18462	17509	18100	18931
per capita annual disposable income of rural households	2020	16631	15619	16361	17131.5

Notes: The TGRA includes 19 counties (districts or cities), among which, four are located in Yichang City of Hubei Province, and the other 15 counties lie in Chongqing Municipality, namely Wanzhou, Fuling, Yubei, Ba'nan, Changshou, Jiangjin, Fengdu, Kaizhou, Wulong, Zhongxian, Yunyang, Fengjie, Wushan, Wuxi, and Shizhu. The Key Reservoir Area in Chongqing Municipality refers to eight key districts and counties of migration, namely Wanzhou, Fuling, Fengdu, Zhongxian, Kaizhou, Yunyang, Fengjie, and Wushan, where the largest proportion of cultivated land has sustained flooding (i.e., 80% of flooding losses) and approximately 80% of the population has been resettled backwards [20].

Simultaneously, the people, including some 318,900 farmers, reclaimed the sloped land to compensate for the loss of their submerged farmland, which exacerbated the conflict between people and land [27]. Water and soil have led to significant environmental problems by ANSP and intensive livestock, especially pig farming [7,28]. In addition, with the water storage of the Three Gorges Project, a narrow fluvial reservoir formed on the upper Yangtze River from Chongqing City to Yichang Hubei province, whose length is nearly 600 km, with a mean flow velocity of 0.9 m s⁻¹ from torrent before, and a significantly reduced self-cleaning ability. The increasing use of fertilizers and pesticides has resulted in serious heavy metal and nitrite nitrogen (NO₂⁻-N) pollution of capillary tributary of the Yangtz River. The contents of NO₂⁻-N in the Pengxi outfall of Yunyang reached 30 mg L⁻¹ with reduction of fish species [29]. Similarly, total phosphorus content in the upper Yangtze River (including the TGRA) increased rapidly due to the substantial increase in the use of fertilizers from 1990 to 2002 [30]. Therefore, the TGRA urgently needs to couple fast economic development with environmental protection.

The integrated technology system [7], the effects of fertilization [31], and ecological agricultural engineering [27] have been adopted to control ANSP and mitigate soil erosion with the ultimate goal of preventing further ecological degradation in the TGRA. However, these studies focused only one aspect, such as ANSP or soil erosion, while economic development was ignored. EcoAgriculture models in the area of study include: 1) circulation agriculture e.g., pig-biogas-fruit (or vegetable) and 2) adjusting agricultural structure i.e., agritourism [12,29,32,33]. What is the developmental status of ACP in the TGRA under the strategies of rural revitalization? Which models of ACP are suitable for the TGRA? How can further pollution and ecological degradation be prevented in the TGRA? Herein, we describe our investigation of the status of ACP in this study area. Our findings are expected to improve the development of EcoAgriculture in the TGRA.

2. Study area

The TGRA is located between 29°16′–31°25′N and 106°50′–110°50′E, in the upper reaches of the Yangtze River which runs in a southwest-northeast direction, with 18 two-grade tributaries, including tributaries of the Jialing River, Wujiang River, Daning River, Xiangxi River, and others (Fig. 1). The TGRA covers 19 counties (districts or cities) of Chongqing Municipality and Hubei Province in China, with a territory of 5.25 $\times$ 10⁴ km². The area is dominated by low-slope hilly topography with over 95% of the total area consisting of mountains and hills at elevations of 175–3000 m. The central portion of the TGRA consists of parallel ridges and valleys. The climate is characterized as a subtropical monsoon climate, with an average annual rainfall of 1250 mm and an average annual temperature of 17–19 °C [32,34]. The main native soils are classified as inceptisols as per the USDA soil classification system. The soil texture is silt loam, derived from purple rock [27].

Fig. 1.

Locations of the survey sites.

The current situation of agricultural development in the region is as follows.

• (1)Restricted by physical geographical factors, e.g., the proportion of mountains and hills exceeds 95%, most arable land is scattered and located on slopes >15°; priority is given to arid lands, predominantly intermediate- and low- yielding fields, with mean grain yields per unit area 2450–4050 kg hm⁻² [35] and severe soil erosion causing severe nutrient losses [36]; drought as the main obstacle to stable yields in the TGRA [13]; scarcity of agricultural infrastructure such as irrigation infrastructure and roads make cultivated land unsuitable for industrial scale farming.
• (2)Limited by human resources, i.e., most rural residents are over 60 years of age. Additionally, there is a scarcity of scientific technologies, and a limited availability and affordability of application equipment. This has resulted in excessive use of fertilizers and pesticides in crop production, leading to nonpoint source pollution and potential risks of eutrophication in the TGRA.

The output of major farm/livestock products in this area include cereal grains (maize and wheat), sweet potatoes, vegetables, rapes, meat (mainly pork, beef, and chicken), poultry eggs and fruits (principally orange). As one of the pillar industries in the TGRA, the Citrus industry comprises 35.84% of all agricultural land in the TGRA [31]. Economy in the Key Reservoir Area sites including, Wanzhou, Fengjie, Wuxi, and Wushan, among others, rely predominantly on primary industry, which has been declared out of poverty until 2020. Furthermore, agriculture in the Key Area of the TGRA is based on the traditional cereal grain-pig dualism.

3. Methodology

This study was an on-site investigation. The research was conducted covering the head, middle and tail of the Reservoir in Chongqing, for each of which, two districts and counties were selected, and two specific representative sites were selected for each district and county. Thus, altogether, six counties and 12 farms (agricultural areas/corporations) were selected (Fig. 1 and Table 2) in the TGRA. The principles of representativeness were determined by meeting the two following conditions: (1) Having large-scale pig farms, i.e., more than 500 mature pigs, or a high-value eco-friendly agricultural industry that enables rural residents to rapidly increase their income, such as blueberry farming, Goat Tripe Mushroom planting, and rural tourism, among others; (2) Small watersheds that are relatively close to the Yangtze River, at a maximum distance of 100 km.

Table 2.

Survey sites for cleaner production agriculture in the TGRA.

No.	Research points	Basins	Location	Counties/Districts	Interviewees
1	ChunKen	Houxi river (2 km)	Close to Zhongcheng Village Committee	Shibao Town Zhong County	Manager
2	MeiJianDa	Quxi river (51 km)	30°18′7″N 107°44′18″E Altitude 250 m	Bashan Town Zhong County	General Manager
3	KuiBo	Xiaojiang (19 km)	31°19′59″N 108°45′59″E Altitude 530 m	Shuikou Town Yunyang County	Owner
4	MingHao	Tangxi river (13 km)	31°2′50″N 108°49′1″E Altitude 390 m	Yunan Town Yunyang County	President of the Cooperative
5	TieFu	Meixi river (6 km)	31°5′20″N 109°29′23″E Altitude 430 m	Kangle Town Fengjie County	President of the Cooperative
6	MingYang	Yangtze River (300 m)	31°5′12″N 109°36′32″E Altitude 390 m	Baidi Town Fengjie County	Owner
7	FaFu	Meixi river (60 km)	31°19′38″N 109°27′36″E Altitude 720 m	Gulu Town Wuxi County	Owner
8	HuaTai	Daning river (40 km)	31°18′39″N 109°37′44″E Altitude 200 m	Huatai Town Wuxi County	Manager
9	Rural Impression	Shuang river (4 km)	29°36′25″N 106°55′6″E Altitude 230 m	Shuanghekou Town Ba'nan District	Owner
10	QuanHe	Nvren river (3 km)	29°39′28″N 106°54′57″E Altitude280 m	Shuanghekou Town Ba'nan District	Owner
11	HuangZhuang	Huangcheng river (5 km)	29°3′37″N 106°11′25″E Altitude 280 m	Yongxing Town Jiangjin District	Manager
12	ShengHuang	Binan river (10 km)	29°12′21″N 106°5′1″E Altitude 250 m	Wutan Town Jiangjin District	Owner

Notes: Basins refer to drainage of survey sites flowing into the Yangtze River. The numbers in parentheses show the distance from the Yangtze River.

After understanding the general situation of the eco-friendly agricultural models within the region from the Committee of Agriculture and Rural Affairs (CARAs) of each district and county, the researchers worked with the CARA professionals to identify specific research sites to survey, and were led by staff members from CARAs of every county to each study site. Additionally, the managers or owners of each farm were interviewed. During these interviews, both the researchers communication with the managers/owners and the field survey were conducted. Meanwhile, due to the short time for the investigation, only one week, any questions that were not communicated were made at any time after the research was completed.

The purpose of this study was to obtain information on the use of inorganic fertilizers and pesticides and management of water resources at the study sites, including recycling of agricultural wastes, i.e., livestock and poultry manures, straws, etc. and high-value EcoAgriculture, rather than large-scale statistical data.

4. Results

This investigation was conducted over a week in March 2018. Models of ACP were divided into two types: EC and agritourism. Our results indicated that there were six cases of EC (Table 3) and five patterns of agritourism among the 12 survey points. Furthermore, ChunKen planted citrus only, and thus belonged to neither EC nor agritourism. Promoted by the ‘Implementation Plan for Water and Fertilizer Integration (2016–2020)’ [37], except for FaFu, the other 11 survey sites used a water and fertilizer integration system for precision management of water and fertilizers.

Table 3.

Fertilizer equivalent of cases in the TGRA.

No.	Survey sites	Scales (pigs)	N (ton·a−1)	P (ton·a−1)
1	MeiJianDa	400 + 10,000	74.36	11.15
2	KuiBo	800 + 10,000	77.22	11.58
3	MingHao	150 + 3,000	22.52	3.38
4	ShengHuang	250 + 3,000	23.24	3.49
5	TieFu	350	2.50	0.38
6	MingYang	500+cattle 60	5.01	0.75
7	Rural Impression	1,000	7.15	1.07

Note: (1) The first number meant sows, and the second number indicated commodity pigs in the ‘scales (pigs)’ column; (2) N nutrients in FRW of 1 pig was equal to 11 kg, while P nutrients in that was equivalent to 1.65 kg, 100 pigs were equivalent to 30 cattle heads. Retention rates of both N and P after fermentation accounted for 65%, based on ‘technical guide for calculation on LCC of FRW’ (MARA, 2018).

Of all products, citrus from ‘ChunKen’, ‘Tiefu chun,’ and navel orange from TieFu, navel orange from MingYang and pitaya from HuaTai are certified as Green Food by the China Green Food Development Center. Similarly, slaughtered fattened hogs from MeiJianDa, KuiBo, and MingHao obtained the pollution-free agricultural products authentication granted by the China Green Food Development Center^. There was no organic products.

4.1. Ecological circulation

Models of ecological agriculture circulation refer to the application of circular economy into the agricultural ecosystem, whereby, applying ecological laws, the paradigms of traditional agricultural ‘resources-products-wastes’ are transformed into ‘resources-agricultural, production-agricultural, and wastes-renewable resources,’ to achieve the win-win-win situation of increase of agricultural economic profits, reduction of environmental pollution, and reduction of resource consumption [38], e.g., focus on product safety, control of the application of fertilizers and pesticides to minimize environmental impacts by recycling of ecological food chains or extending of agricultural eco-industrial chains.

In our cases, appropriate environmental conditions in the TGRA: (1) large elevation ranges from 175 to 2926 m, (2) humid subtropical monsoon climate with a mean annual precipitation of 1250 mm, (3) an average annual temperature from 17 to 19 °C, facilitated citrus plantation and vertical agriculture for production of maize, vegetables, potatoes, and sweet potatoes. In addition, pig husbandry has successfully expanded. Wastes and wastewater from pig farming were helpful as fine organic fertilizers for cropping, greatly reducing the need for inorganic fertilizers. On the other hand, maize, vegetables, potatoes, and sweet potatoes as feed promoted pig raising. Consequently, ecological circular agriculture focused on pig raising, e.g., pig-biogas-fruit (grain, vegetable, mulberry, etc.), like MeiJianDa, KuiBo, MingHao, TieFu. Particularly, EC proceeded with solid-liquid separation of excrements from pig houses, solid matter was returned to the field after composting, whereas liquid wastes were used to produce methane by fermentation. Cropping used fermented slurry and residual, and cooking and lighting in farms relied on biogas (Fig. 2).

Fig. 2.

Framework of ecological circulation including pigs.

As shown in Fig. 2, organic fertilizers from the composting plant were delivered by labors to slope fields with small and scattered patches. Biogas slurries were transported by pipes to the storage pool (Fig. 3 (2)) with a large volume around the wastewater treatment plant (WWTP) (Fig. 3 (1)), consequently it was pumped to the storage pools in field (Fig. 3 (3) & (4)) with a small volume for cropping. Dry manure compost (Fig. 3 (5)), transportation of biogas slurry, and plantation of Citrus are shown in Fig. 3 (6).

Fig. 3.

Transportation of biogas slurry for MeiJianDa (1. Anaerobic pools; 2. Storage pool for biogas slurry around the WWTP; 3.4. Storage pools in the field; 5. Dry-manure composting plant; 6. Citrus plantation).

At MeiJianDa, KuiBo, MingHao and ShengHuang producers prioritized pig husbandry combined with cropping. They had their own sows for an inexpensive and stable source of young pigs. In contrast, in TieFu growth was based on the expansion of navel orange cropping. To reduce pollution, the above must treat excrements. Calculated based on the ‘technical guide for calculation on land carrying capacity (LCC) of FRW’ [39], producers in MeiJianDa reduced inorganic fertilizers N and P by 74.36 and 11.15 ton·a⁻¹, respectively. Other cases which inorganic fertilizers were saved are listed in Table 3. This indicated that FRW used as organic fertilizers achieved the win-win-win situation of increasing agricultural profits, protecting the environment, and reducing inorganic fertilizers and saving energies for cooking, and lighting. Especially the unique products from KuiBo, namely, mulberry ecological pork obtained wide sales for its distinct taste by adding mulberries into pig feed. Furthermore, fed with healthy grains/vegetables to meet the standards of NY5000 in China, slaughtered fatten hogs in MeiJianDa, MingHao, and KuiBo had acquired the pollution-free agricultural product certification.

Out of 1800 mu of navel orange trees planted in TieFu, only 100 mu were irrigated with 100% fermented liquid. Comparison with utilized inorganic fertilizers (mainly compound fertilizers), fertilization was reduced to once application and pesticides were reduced every year with 100% fermented liquid in TieFu. Simultaneously, with fermented liquid and dry manures as basal fertilizers, insect pests such as starscream and diseases such peppers rust decreased significantly in ShengHuang. Thirdly, our survey revealed that, (1) glyphosate (molecular formula: C₃H₈NO₅P) residues of sprays performed five years earlier in the soil were detected by CARA in Yunyang County; (2) because of pesticide resistance of insects such as starscream, scale insects and leap to bug and so on, pesticide varieties changed frequently, and the amounts of applied pesticides grew compulsively. For example, in Citrus and peppers, pesticides are used annually seven times, which resulted in excessive heavy metal (Cr/Pb etc.) persisting in the soil; (3) detection by CARA of Yunyang County revealed that, if the total annual use of pesticides was reduced for three consecutive years, soil pollution by pesticides would decrease markedly.

Of the above six cases, only FaFu had no supporting farm but the substrate of Edible fungi, e.g., Schizophyllumcommuneh, was used as an organic fertilizer for vegetables after harvesting five times.

4.2. Agritourism

Development of agritourism fulfilled what tourists from different levels and various ages expected in terms of leisure elements they seek, such as fruits, flowers, and landscapes, resulting in education, experience, and sightseeing.

Among the five cases, HuangZhuang, QuanHe, and Rural Impression close to Chongqing City were designed elaborately, and benefited from touring urban households or financial support from the government [40]. For example, HuangZhuang took advantage of agricultural sciences, integration of knowledge on rice-duck-fish, straw return to the fields, and green control of pests to make them part of experiencing projects, furthermore, producers at this site planted colorful rice to compose TaiJi bagua (Fig. 4 (1)), aiming to inspire pleasurable awareness of ACP among tourists. Illustration of Rural Impression farmed fish in a stream in the livestock and poultry prohibiting zone in Shuanghekou Town, Ba'nan District (Fig. 4 (2)), where a variety of flowers and fruits were planted, and more than 1,000 black hogs in Mudong Town, Ba'nan District were reared to satisfy the need of travelers to experience natural products and refreshingly go back to nature.

Fig. 4.

Experiencing agritourism (1. TaiJi bagua without colorful rice in HuangZhuang; 2. Rural Impression).

In contrast, HuaTai and MingYang lay in the hinterland of the TGRA, and were both far from Chongqing City and, consequently, excluded from the core landscape and famous sceneries of the Three Gorges on the Yangtze River, wherein the number of visitors in these two sites was relatively low at the early stage of agritourism introduction. Thus, for example, in HuaTai, people planted only pitaya under greenhouse film, and water and fertilizer integration. They had many visitors (about 300 people) in the seasons of pitaya blooming and harvesting, namely, from May to October. In contrast, in MingYang, producers cultivated navel orange, rapes, and Astragalus sinicus; additionally, they raised 500 pigs in stock and approximately 60 cattle annually, thus combining cropping with animal husbandry to develop circular agriculture.

4.3. Water and fertilizer integration

Water and fertilizer integration [37] by dissolving fertilizers into water to apply with a pipeline irrigation system combined irrigation and fertilization, thereby timely and suitably meeting crop water and nutrient demands to achieve both management synchronization of water and fertilizers, and efficient technologies of water-efficient agriculture. This technology was applied in 11 survey sites, except for FaFu. As an example, in TieFu, DN 50 main pipes and DN 25 branch pipes covered an area with a radius of 50 m. This system conveyed inorganic fertilizers including nitrogen, phosphorus, and potassium and liquid organic fertilizers such as biogas slurry to ensure navel-orange appropriate water and fertilizer supply. In contrast, in HuaTai only compound fertilizers were supplied with the integration system of water and fertilizer. The apparent improvement of the efficiencies of water and fertilizers was confirmed by managers or owners.

5. Discussion

Arable lands are scattered in the hilly and low mountainous belt of the TGRA, most of which are arid lands, while few are paddy fields. Vertical farming with grains/vegetables/cash crops in the valleys and navel orange orchards on hillsides have been developed in various counties of the TGRA, with substantial planting of citruses, grapes, Chinese prickly ashes, potatoes, sweet potatoes, and rapes, among other plant species. Combined with animal husbandry (mainly pigs), FRW has increased soil fertility. Additionally, it has reduced the use of inorganic fertilizers per hectare of cropland, while gradually enhancing annual crops and fruit-tree disease resistance, and improving the quality of agricultural products. Additionally, we addressed the following questions.

• 1Was the area at each survey site enough for pig raising?

[39] enacted the ‘technical guide for the calculation of FRW based on LCC to promote FRW recycling and to protect the environment. Based on the recommended carrying capacity of various crops in the technical guide, the LCC of supporting farms was calculated and compared with the current size of existing pig holdings (Table 4).

Table 4.

LCC analysis in the TGRA.

Survey sites	Pigs (head)	Crops used for calculation	Carrying capacity (head·mu−1 in season)	farm area (mu)	LCC (head)	Difference (head)
MeiJianDa	10,400	citruses	2.3	2,500	5,750	−4,650
KuiBo	10,800	grapes	3.2	1,500	4,800	−6,000
MingHao	3,150	citruses	2.3	adequate	3,000	0
ShengHuang	3,250	peppers	3.9	600	2,340	−910
TieFu	350	navel oranges	1.2	1,800	2,160	1,810
MingYang	700	navel oranges	1.2	1,650	1,980	1,280
Rural Impression	1,000	maize	1.2	1,500	1,800	800

Note: (1) Calculation for LCC based on nutrients of nitrogen in FRW: soil nutrients level-Ⅱ; MeiJianDa, KuiBo, MingHao, and ShengHuang. Solid excrements were sold after separating liquid parts including wastewater, which was recycled completely in the farm. TieFu, MingYang, and Rural Impression recycled all FRW; percent FRW accounted for 50% of the total fertilizer used; Seasonal crop nitrogen-use-efficiency was determined at 25%. (2) Crops used calculation were selected among crops grown at each survey site prioritizing that which entailed maximum economic benefit in each case. When crops in recommended values for LCC were low priority, crops used for calculation were substituted. For example, in KuiBo, mulberries were substituted by grapes; in ShengHuang, Chinese prickly ashes were substituted by peppers. (3) + and 0 indicate adequate and inadequate LCC, respectively.

To obtain profits, at MeiJianDa, KuiBo, MingHao, and ShengHuang producers sold all their dry manures at 200 (RMB, the same below), 150, 200 and 300–500 yuan·ton⁻¹, respectively. However, in MeiJianDa producers sold only 2/3 of the dry manures. The liquid parts, especially biogas slurries with high chemical oxygen demand (COD) and high concentrations of ammonia were transported by pipes to the fields to supplement crop nutrient demands. According to the calculation of the LCC based on the ‘technical guide’ [39], for all seven farms, recycled FRW accounted for 50% of total fertilizer amounts. We found that LCC at MeiJianDa, KuiBo, and ShengHuang were not large enough to eliminate the pollution of FRW. At these survey sites, differences in LCC between current size of pig holdings and calculated values were 4,650, 6,000, and 910 heads, respectively, and the ratios of existing pigs to calculated proportions were 1.8, 2.3 and 1.4, respectively. Based on [41,42], 1 mu of arable land annually decontaminated pollution of caused by three heads of slaughtered fattened hogs; Therefore, given the size of pig holdings in each case, the total area of arable land required in MeiJianDa, KuiBo, and ShengHuang were 3,467, 3,600, and 1083 mu, respectively, despite the fact that these sites did not have a large enough LCC. Consequently, it is clear that the potential risks of contamination in the TGRA are exceedingly high. Indeed, LCC in MeiJianDa was less than 5,750 pig heads for 1/3 of dry manures recycled by the farm. However, in contrast, KuiBo, ShengHuang, MingHao, and TieFu were cooperatives with adequate land area around them to recycle all the FRW generated locally. As for MingYang and Rural Impression, which are the most benefitted by agritourism, farmers intend to use pig raising to support agritourism of people from the cities, thus LCC at these sites were large enough. In particular, 1,000 pig heads in Rural Impression were scattered fed by joining together the company, the crop production, and the farmers with sufficient relative LCC in another town of the Ba'nan District, far from the tourist destination. Additionally, commodity organic fertilizers were produced and sold in MingYang using cattle and pig manures produced locally.

Overall, farms priority of intensive pig breeding allowed for FRW recycling, although they did not have a large enough LCC, which posed high pollution potential in the TGRA. Conversely, farms focused on developing agritourism commonly supported decontamination of pollution caused by the swine industry by providing adequate arable lands for effective FRW recycling.

• 2Can heavy metals or antibiotics resistance genes (ARGs) pollute the TGRA?

To improve pig growth, antibiotics, Cu, Zn, Cd, Fe, As, and other trace elements were added to the feed during pig rearing, which later were transferred into the soil, water, and agricultural products by recycling FRW, thus posing potential risks of contamination of the soil and reduction of the quality of crops and agricultural products by long term accumulation. [27] reported that excess rates of toxic metals including Cd and As were up to 10%, while those of elements responsible for environmental pollution, such as Cu and Zn, reached 50%. Maximum Cd, As, Cu, and Zn contents in pig manures reached 147; 978; 1,747; and 11,547 mg kg⁻¹, respectively. Long-term overuse of antibiotics resulted in bacteria developing antibiotic resistance, whereby they have become increasingly difficult to kill. Antibiotic resistance genes (ARGs) were the core functional elements in antibiotic resistance bacteria, which had the ability to migrate among different microbes via horizontal gene transfer. While studying Qingcaosha Reservoir in Shanghai China for one year, [43] found that 12 ARGs were detected both in the water and sediment samples, and sulfonamide resistance genes were predominant in the reservoir, with a detection frequency of 100%. Similarly, [4] found that, among the investigated sulfonamides (SAs) in the TGRA, sulfamethoxazole (SMX) appeared as the single most toxic, in vitro and in vivo, chemical with strong environmental effects. It is noteworthy that, exposure to binary mixtures of SAs induced higher developmental toxicity and significantly disturbed the detoxification pathway in zebrafish, compared to that resulting from exposure to any single SA compound. In turn, [44] confirmed the presence of heavy metals, i.e., Cu, Cd, Ni, and Hg in water from Xiangxi River, which was instrumental in the TGRA originating primarily from agriculture.

While screening for detection of heavy metal contents in arable soils within the TGRA, we found that Cu, Zn, and Cd in soils exceeded the national standard of China due to long term use of hog dung for fertilization -- ‘Soil environmental quality Risk control standard for soil for contamination of agricultural land’ [45], and the corresponding contents in soils reached 119.18, 300.41, and 1.951 mg kg⁻¹, respectively.

In our study, organic fertilizers produced locally in ChunKen were applied for improvement of annual Citrus-tree growth by as much as 20–25 kg·tree⁻¹. Organic fertilizers included chicken, cattle, and goat manures. All these animals were fed with few puffed feeds. Besides manures, tobacco and edible fungi residues were also used to produce a total of 5000 ton·a⁻¹ of organic fertilizers. To ensure the quality and safety of green food-citrus, pig manures had been excluded from the list of organic fertilizers that could be applied to enhance yields, as it was known to include high concentrations of heavy metals. Consequently, there was a shortage of raw materials to supply for the production of organic fertilizers.

Based on literature reports and research progress reports from ChunKen, the question arose as to whether heavy metals or antibiotic resistance genes (ARGs) might potentially pollute the TGRA.

• 3What are the current concerns with respect to agriculture cleaner production in the TGRA?

Under current national Chinese policy, the continued development of urbanization, and intensification of agriculture, particularly of the swine industry, will be the major trends. This scenario will require modern facilities to improve the efficiency of agricultural production processes and to protect the environment. However, hilly-gully terrains, scattered cultivated lands, and the comparatively low level of economic development restrict the popularity of modern, intelligent agricultural machines. Although equipment for water and fertilizer integration has been applied in the TGRA, the liquid part of FRW after fermenting are transported to the fields but still need storage pools both near the WWTP and in the fields; Furthermore, matching enough land to meet LCC associated to pig raising needs to be accurately calculated. Moreover, maintenance of biogas slurry distribution networks, open storage of liquid slurry, and deficient transportation of dry manures will all challenge the sustainable development of agriculture, especially intensive livestock and poultry breeding. [3] identified seven eco-factors influencing intensive agriculture in the Baltic Sea of southern Sweden. Among such factors, financial security, expanding knowledge, and local planning were regarded as playing crucial positive roles of integration of environmental improvements within agricultural industry.

Furthermore, our study revealed that fermented liquids were recycled randomly, without determination or measurement of soil nutritional ingredients, e.g., organic matter, nitrogen and phosphorus, or harmful ingredients in fermented liquids, such as heavy metals and ARGs, which will cause potential risks of overfertilization of soils, ANSP, and, ultimately, deterioration of the TGRA.

Finally, pesticides cause both acute and long-term health impacts. Among the 12 survey points in which this research was performed, only in HuangZhuang were green pest control measures, such as the use of bio-pesticides (here: sex pheromones) and solar mosquito lamp, implemented. At all the other survey sites chemical pesticides still dominate the scene, the type and amount for each product must also be changed year by year; Although highly persistent and highly toxic pesticides, such as methamidophos are too frequently still used. A recent report indicated that pesticide use per hectare of cropland in China increased by 77.11% from 1991 to 2019 [26]. Simultaneously, glyphosate residues in soils after spraying for five years were detected by CARA of Yunyang County. [46] reported indirect effects of pesticide application on heavy metal contents in some vegetable plant species. Currently, environmentally-friendly herbicides are insufficient. To reduce labor costs, none of the 12 points adopted traditional manual weeding but all relied heavily on chemical weeding using glyphosate, which causes potential ecological risks. For example, in Chunken, pesticides (name of commodity: Ruidefeng, Bayer, etc.) are used 7 times·a⁻¹, and herbicide glyphosate is used twice as much.

6. Conclusions

A study on 12 survey sites for one week indicated that there were predominantly two models of ACP in the TGRA: EC, and agritourism. The model of ecological circular agriculture conformed to theories of circular economy, prevented environmental pollution while promoting agricultural economic growth by recycling FRM derived from pig raising to the fields, and reducing the use of fertilizers and pesticides. On the other hand, the model of agritourism met the requirements of tourists with provision of fine ecological products such as fruits, flower and landscapes, and effective coordination between environmental protection and economic development. Additionally, except for FaFu, where water resources were precisely controlled, agriculture water and fertilizer integration was applied in 11 of the 12 survey sites.

Circular agriculture is a good measure to attain a win-win-win situation of economic, growth environmental protection, and reduction of the intensity of agricultural inputs. Nonetheless, some pollutants including heavy metals and ARGs have not been properly addressed. Furthermore, as agritourism has been almost at a standstill for the past three years due to the COVID-19 pandemic, intensive pig rearing places the LCC at risk for shortage of adequate existing lands for recycling FRM. Pesticide products have changed frequently, and the amounts of pesticides applied has increased significantly due to the development of pesticide resistance by insects. Green pest control measures, such as bio-pesticides, solar mosquito lamps and mechanical weeding have been scarcely adopted in the TGRA for cost saving and technology barriers. It is necessary to further explore ACP in the TGRA.

However, statistical data regarding the development of the TGRA have not been collected, including soil and water quality. The lack of such data is a serious limitation of our study.

Author contribution statement

Houzhen Zhou: Conceived and designed the experiments; Wrote the paper.

Xiaolan Luo; Zhaoli Wang: Performed the experiments; Wrote the paper.

Shuai Wang: Contributed reagents, materials, analysis tools or data.

Yangwu Chen; Xin Li: Analyzed and interpreted the data.

Zhouliang Tan: Conceived and designed the experiments.

Funding statement

This study was funded by Chinese Academy of Sciences (CAS) Interdisciplinary Innovation Team (2019XBZG_JCTD_ZDSYS_001); CAS Science and Technology Service Network Initiative (KFJ-SW-STS-175-03); Sichuan Key R&D Program (2022YFS0452 and 2020YFS0026); The upper Yangtze River characteristic advanced industries cluster of tuber mustard construction Project (NG02.131) and Key R & D program of Chengdu Science and Technology Bureau (2022-YF09-00044-SN).

Data availability statement

Data included in article/supplementary material/referenced in article.

Declaration of interest’s statement

The authors declare no conflict of interest.

Annex.

Table 3.

Status on agriculture cleaner production in the TGRA

No.	Survey site	Models of green agriculture	Area of arable land	Farming	Fertilizers/water	Pesticides
1	ChunKen	/	10000 mu1	Citrusa	(1) Organic fertilizers produced by ChunKen, 20–25 kg per tree per year. (2) Enough inorganic fertilizers (mainly compound fertilizer) by water and fertilizer integration system.	7 times·a−1 (names of commodity: Ruidefeng, Bayer, etc.), among which herbicides glyphosate are used 2 times.
2	MeiJianDa	Ecological circulation of pig-biogas-vegetable (grain/fruits)	(1) 500 mu. (2) Additional 2000 mu for biogas slurry irrigation.	(1) Sows over 400. (2) Commercial pigsb by home-bred reached over 10000, using dry collection of pig manure technologies and visualization system. (3) Biogas provided energy for the farm and residents around it. (4) Citrus, grape and vegetables, etc.	100% manure and biogas slurry from the livestock. Beyond 80 m3 d−1 biogas slurry in summer, whereas 20 m3 d−1 in winter.	A small amount of low-toxicity pesticides.
3	KuiBo	Ecological circulation of pig-biogas-mulberry (vegetables/fruits)	1500 mu	(1) Sows 800. (2) Market pigsc exceeded 10000, using dry collection of pig manure technology. (3) Biogas provided energy for the farm and 100 families around it. (4) Mulberries, grapes, citrus and vegetables, etc.	Biogas slurry as fertilizers were directly transported to farmland by pipes, which were above 8000 m.	A small volume of low-toxicity pesticides.
4	MingHao	Ecological circulation of pig-biogas-orchard (vegetable)	Adequate, but the area was unknown	(1) Sows 150. (2) Commodity pigsd over 3000, utilizing dry manure clean-up process. (3) Biogas provided energy for the farm and 20 families surrounding it. (4) Citrus, vegetables.	Biogas slurry as fertilizers were directly transported to farmland by DN800 pipes, over 2000 m.	Qualified national farmer cooperative demonstration.
5	TieFu	Ecological circulation of pig-biogas-fruit	1800 mu	(1) Commodity pigs 350. (2) Navel orangee.	(1) Integration irrigation of water and fertilizer covering 1800 mu with DN50 pipes, then DN25 pipes reached an area with a radius of 50 m. (2) More than 100 mu used fermented liquid for 3–4 times a year. Others irrigated by compound fertilizers 3 times a year, up to 6–6.5 kg per tree.	Low residual low-toxicity pesticides 3 times per year aiming at starscream, scale insects and leap to bug, etc.
6	MingYang	Agritourism	Owned 650 mu, excess 1000 mu from the peasants near it.	(1) Pigs 500, cattle 60. Dry cleaning technique was adopted. (2) Methane gas used by the farm and a few household farmers. (3) Navel orangef, rapes and Chinese milk vetch.	Fertilization with organic fertilizers made of pig and cattle manures.	Low residual low-toxicity pesticides for starscream, scale insects and leap to bug, etc.
7	FaFu	Ecological circulation	20000 m2. Domestication and cultivation by the owner.	Edible fungi (focused on Schizophyllum commune), vegetables.	Substrate after harvesting 5 times used for organic fertilizers.	Formaldehyde (HCHO), methamidophos (C2H8NO2PS), etc.
8	HuaTai	Agritourism with catering and accommodation	45 mu	Pitayag	Irrigation system of water and fertilizer integration, fertilization with goat manure 7–8 times per year.	Non-fatal unintentional pesticides.
9	Rural Impression	Agritourism (catering, accommodation, entertainment, education and experience)	Planning 3000 mu, now 1500 mu.	(1) Fish farming with stream in livestock and poultry forbidden zone. (2) Slaughtered black pigs over 1000 head per year in Mudong Town in Ba'nan district. (3) Fruits, flower and vegetables.	Mainly farmyard manure, few fertilizers.	Applications for amenity, such as anti-mosquito liquid (diethyltoluamide, pyrethrins, etc.)
10	QuanHe	Agritourism (picking, catering)	500 mu, among which 300 mu with drip irrigation.	(1) livestock and poultry forbidden zone. (2) Cirtrus (blood orange)	(1) Compound fertilizer of potassium sulfate annually 3 kg per tree. (2) Organic fertilizers annually 40 kg per tree used once interval 1 year.	Pesticides or petroleum oil: between 4 and 6 times one year.
11	Huang Zhuang	Agritourism/rice-duck-fish or rice-green manure	Over 900 mu	(1) Fish and ducks. (2) Rapes, sorghum and rice	Straw back to field.	Green management, i.e. solar mosquito lamp, sex pheromone, etc.
12	ShengHuang	Ecological model of pig-methane-pepper circuit	Over 600 mu, irrigated by fermented liquid.	(1) Sows 250. (2) Commodity pigs 3000. (3) Biogas: 100 m3 d−1 in winter, 300 m3 d−1 in summer, used by 30 families around it. (4) Fish farms over 100 mu. (5) Visualization system. (6) Chinese prickly ash, citrus, grapes, and vegetables	(1) Organic fertilizers: fermented liquid and dry manure for peppers. (2) Compound fertilizers: annually 0.5 kg per tree.	≥7 times·a−1, twice of which after bearing Chinese prickly ashes.

Notes: Green food certifications and Pollution-free agricultural products certifications announced by China Green Food Development Center [[47], [48], [49], [50], [51], [52], [53], [54], [55]].

^aGreen Food certification (No. LB-18-18083407108A); b. Slaughtered fattened hogs from the farm obtained pollution-free agricultural products certification (No. WGH-CQ02-19000); c. Slaughtered fattened hogs from the farm obtained pollution-free agricultural products certification (No. WGH-CQ02-18000); d. Slaughtered fattened hogs from the farm obtained pollution-free agricultural products certification (No. WGH-CQ02-18000); e. Some products certified as Green Food by China Green Food Development Center (No. LB-18-18103408053A; LB-18-1405341395A); f. Certified as Green Food, (No. LB-18-18093407544A; LB-18-1311347729A); g. Green Food certification (No. LB-18-19083407560A; LB-18-19083407562A).

¹1 ha = 15 mu. Similarly herebefore.