Introduction
Vegetables, as the basal source of essential substances such as vitamins, beneficial active ingredients, cellulose, amino acids and mineral elements, are always required to contain as much of these essential nutrients to meet dietary requirements of humans, without over-the-limit of contaminants such as nitrate, nitrite, heavy metals and pesticide residues. However, vegetables are readily contaminated by nitrate and heavy metals during conventional cultivation with mass input of manure and fertilizer. In particular, off-season vegetables cultivated in protected systems with low light intensity do tend to accumulate more nitrates in tissues due to excess N fertilization driven by farmers' desire for high yieldsReference Liu, Yang, Yang, Kozai and Bot1. Over-the-limit accumulation of the harmful substances in vegetables constitutes a serious global hazard to human health. Moreover, since vegetables are usually harvested at vegetative growth stages, and the edible parts (young stems and leaves) can accumulate relatively large amounts of nitrate, vegetables have been found to be the major source of nitrate intake by humans, accounting for more than 80% of the totalReference Eichholzer and Gutzwiller2. In addition, frequent application of pesticides easily leads to accumulation of residues during the short growth period and lack of time for them to be fully degraded. For soil cultivation, together with the inputs of large amounts of nitrogen, phosphates and organic fertilizer, substantial amounts of nitrate and heavy metals are accumulated in soil and are subsequently absorbed and accumulated in vegetablesReference Zhou, Fan and Wang3–Reference Zhong, Hu and Wang5. Currently, in China and the whole world, over-the-limit levels of nitrate, heavy metals and pesticide residues in vegetables are posing a serious threat to human healthReference Zhou, Fan and Wang3–Reference Sušin, Kmecl and Gregorčič7. Globally, a serious situation was found in nitrate and heavy metal pollution in vegetables in terms of the limits issued by the different countries. In addition, apart from hygienic quality, nutritional quality of vegetables is another important issue that is of wide concern to humans. Nowadays, the quality of both the hygiene and nutrition of vegetables should be considered in the assessment of the quality of vegetables grown by various methods with respect to human health.
With the rapid development of livestock production in China, the amount of livestock wastes being generated is increasing, which gives rise to serious agricultural non-point source pollution when the waste is improperly disposedReference Zhang, Xu, Ji and Kolbe8. In China, biogas production is a good method for processing agricultural wastesReference Guo, Cai and Cheng9, Reference Guo, Niu and Cheng10. This process can maximize the utilization of biomass energy, minimize the emission of greenhouse gases and loss of N and P compounds through anaerobic fermentation. Biogas is a very important renewable energy that is used for cooking and lighting in ChinaReference Abraham, Ramachandran and Ramalingam11. Therefore, the Chinese government has encouraged the establishment of biogas projects to process the biological wastes. However, besides biogas, a great deal of by-products including biogas slurry and biogas dregs is also produced during the fermentation process. Thus, a new environmental issue has arisen. The discharge of biogas manure directly into the environment may result in serious problems, like eutrophication of surface water, over-the-limit levels of nitrate in groundwater, soil salinization, etc. Currently, improper application or disposal of biogas manure is severe in China since there are no acceptable methods for its use in agriculture with high economic benefits. However, the so-called pollutants in biogas manure, N and P, in turn, are the leading nutrients for crop production. Currently, in China, biogas manure is being explored for a win–win safe use as fertilizer for crop cultivation, particular high-quality off-season vegetable production in protected systems. Based on the constituent characteristic of biogas manure, many attempts have been made to investigate the feasibility and benefits of biogas manure on vegetables, crops and fruit trees. It is obvious that recycling of biogas manure in agriculture is multi-beneficial for both agricultural production and environmental protection. The research on biogas manure application in soilless vegetable production is reviewed here, highlighting the feasibility and benefits of biogas manure in high-quality vegetable production.
Application Methods of Biogas Manure in Agriculture in China
It has been well documented that biogas manure can be applied to agriculture as a fertilizer, protector, regulator and feed additive for pigs, functioning based on its diverse constitutes including abundant nutrients, amino acids, bioactive substances, etc. It has been proven that biogas manure could be generally applied in three ways. First, biogas slurry, as fertilizer, can support crop growthReference Guo, Cai and Cheng9, Reference Ning, Li and Pan12. In general, biogas manure provides almost all mineral nutrients for crop growth and development in quantities close to the crop requirementsReference Yang13. Second, biogas slurry can protect crops from stress. In biogas manure, bioactive substances, such as hormones, nucleic acids, monosaccharides, free amino acids, vitamins, unsaturated fatty acids, proline, linoleic acid and fulvic acid, and so on, play important roles in protecting crops from biotic and abiotic stressReference Zhu14. It has been widely acknowledged that, for instance, gibberellins stimulate seed germination and plant growthReference Brocklehurst, Rankin and Thomas15, and vitamins promote disease resistanceReference Ahn, Kim and Lee16. Third, biogas slurry makes a good animal feed, providing important nutritional substances, such as carbohydrates, amino acids, microelements, hormones and crude proteinsReference Yuan17, Reference Wang, Yuan, Guo and Chen18. In brief, biogas manure contains considerable amounts of plant nutrients and its use as a nutrient source may offer a promising win–win opportunity to improve crop production and, at the same time, to prevent adverse environmental impacts of biogas manure disposal. However, some urgent issues in the utilization of biogas slurry in crop production in China should be solved. First, there are no effective and practical methods to utilize biogas manure. Therefore, novel methods with acceptable economic benefits need to be developed. Second, the application of excessive amounts in agro-ecosystems often leads to adverse effects on soil quality, including the lack of holding capacity in different kinds of soils. Thus, investigations are needed to optimize these crucial parameters. Third, there are still some puzzles on how biogas slurry affects the growth and physiological status of crops, which need further investigations. In comparison with biogas slurry, biogas dregs are even less utilized in agriculture. Taking the feasibility, benefits and significance of the possible utilization methods in China into consideration, soilless cultivation of vegetables is a good way to use biogas manure with the advantage of optimally utilizing the functional constituents of biogas slurry. Most important, many functional substances such as amino acids can improve the nutritional and hygiene quality of vegetables to some extent, which must be controlled within the limits of the issued standardsReference Liu, Du and Yang19.
High-quality Vegetable Cultivation in China: Current Problems
Vegetables are high-value produce in China, particular during the out-of-season period. High economic benefits of vegetable cultivation under protected conditions has encouraged more and more farmers to establish solar greenhouses to produce vegetables in winter and spring. Currently, China ranks first in the world in the cultivation of protected vegetables, with up to 3.0 million hm2 under cover in 2007Reference Yang, Sun, Wei, Liu, Yang, Kozai and Bot20. However, vegetable quality (nutritional quality and hygienic quality) has declined in the past decade due to the increased fertilization levelReference Zhou, Fan and Wang3–Reference Zhong, Hu and Wang5. In 2007, the total input of fertilizers reached 51 million tons in China, and average nitrogen, particularly high in protected vegetables, has climbed up to 450 kg/hmReference Zhang21. High levels of nitrogen (urea and NH4+-N included) in soil will transform into nitrate, which is unavoidably absorbed and is substantially accumulated in vegetables to a harmful level for human health. In soilless cultivation, nitrate accumulation in vegetables is also unavoidable since nitrate/nitrogen has to be added into the nutrient solution or substrate to avoid ammonia toxicity and to balance medium pH. Moreover, low light intensity in protected systems could substantially increase nitrate accumulation in vegetablesReference Cantliffe22. Previous data indicated that nitrate as well as heavy metals in vegetables had reached a serious level according to some international or domestic standardsReference Zhou, Fan and Wang3–Reference Zhong, Hu and Wang5. In China, improving the quality of vegetables is urgently needed.
The definition of high-quality vegetables is: vegetables that do not contain over-the-limit harmful substances (nitrate, nitrite, heavy metals and pesticides) and which do not have negative effects on human health after ingestion. For soil culture, at present, it is hard to produce high-quality vegetables in the field with the high-level nitrate, heavy metal and pesticide accumulation in soils combined with continued annual heavy input. Therefore, an alternative method to produce high-quality vegetables is urgently needed. To sum up, in high-quality vegetable production, inputs of fertilizers and pesticides should be lowered in order to ensure no over-the-limit accumulation of harmful substances in vegetables. Along with yield, hygienic quality assurance and the nutritional quality of vegetables should be improved as much as possible. In modern horticulture the trend is increasingly toward more soilless cultivation, which has advantages over soil-based production (e.g. decreasing succession cropping obstacles, avoiding ground water pollution and less soilborne diseases). Currently, excessive accumulation in vegetables of nitrate, heavy metals and pesticides are the major problems existing in high-quality vegetable production in soilless cultivation.
Nitrate is naturally present in all vegetables. Green, leafy vegetables, e.g. lettuce and spinach, contain relatively high concentrations. Nitrate and nitrite contamination in vegetables is produced mainly by overuse of chemical fertilizers besides low light intensity in protected systems. High intake of nitrate can transform into nitrite and then react with various proteins in the digestive system. The product, nitrosamine, is mutagenic and carcinogenicReference Prakasa Rao and Puttanna23. In order to eliminate the above hazards, obligatory standards have been released by many countries and international organizations. For example, the World Health Organization (WHO) and Food and Agriculture Organization (FAO) established the acceptable daily intakes (ADI) for nitrate and nitrite in 1973, 3.65 and 0.13 mg/kg (body weight), respectively. That is, for a 60 kg person, he/she is allowed to eat less than 0.5 kg fresh vegetables in which nitrate concentration is over 438 mg/kg. The European Commission has set maximum limits for levels of nitrate in lettuce and spinach24. In China, the content of heavy metals, nitrite and nitrate in the tissues of high-quality vegetables' is assessed under the limits of the GB-18406-2001 standard. For example, nitrite content in vegetables is less than or equal to 4.0 mg/kg (fresh weight), while the limits for nitrate content are 600 mg/kg for fruit vegetables, 1200 mg/kg (fresh weight) for stem and root vegetables and 3000 mg/kg (fresh weight) for leafy vegetables. In the newer standard, GB-19338-2003, the above limits are changed to 440, 1200 and 2500 mg/kg (fresh weight), respectively. The contents of five heavy metals and more than 40 pesticides were limited in the above standard. According to these tolerance limits, nitrate contamination in vegetables in most regions of China is a serious problemReference Zhou, Fan and Wang3. In general, nitrate content is the principal harmful substance that is difficult to control due to the large quantity of vegetables needed and their biological characteristics for easy accumulation of nitrates. Therefore, to develop effective methods to control nitrate accumulation in vegetables is an urgent issue in China and the whole world.
High-quality Vegetable Cultivation with Biogas Manure: Feasibility Analysis
As a staple food worldwide, vegetables need special assurances on quality (nutritional quality and hygienic quality). Excessive nitrate, heavy metal and pesticide contents in vegetables are basic problems. Application of biogas manure in vegetable production could specifically help solve these tough problems. It has been verified that the application of biogas manure improves yield, nutritional quality and stress resistance. Furthermore, biogas manure has a particular role in decreasing nitrate and heavy metal contents in vegetables. Based on the above functions of biogas manure, vegetables cultivated with biogas manure using appropriate management measures (e.g. quality control and composition regulation of biogas manure) will not exceed the tolerance limits for nitrate, heavy metals and pesticide residues.
The raw materials for biogas production, animal wastes, are mainly transformed from feeds, which are mostly plant-derived. Biogas slurry therefore consists of similar kinds and ratios of mineral elements as the physiological requirements of crops since few elements are lost during anaerobic fermentation. Biogas slurry contains abundant N (0.03–0.08%), P (0.02–0.07%), K (0.05–1.4%), as well as Mg (97 mg/l), S (14.3 mg/l), Cu (36.8 μg/l), Fe (1.4 μg/l), Mo (4.2 μg/l), etc. In addition, the beneficial elements Ca, Cl, Na, Si, Co, Sr, Li, F and Ba are also contained in biogas slurryReference Yang13. Biogas slurry has a higher nutrient content than compost, and recovery rates for N, P and K exceed 90% after digestionReference Wu, Zhang and Cao25. Therefore, biogas slurry is a complete nutrient solution that basically meets the needs of vegetable growth. Similarly, dry dregs also contain abundant nutrient substances including organic matter (36–50%), humic substance (10.1–24.6%), nitrogen (0.78–1.61%), P (0.39–0.71%, P2O5) and potassium (0.61–1.3, K2O)Reference Yang13.
Partial replacement of nitrate in nutrient solution with amino acids at 20 and 30% could significantly decrease nitrate accumulation in vegetablesReference Chen and Gao26–Reference Wang, Wu, Wang, Zhu, Tao and Zhang30. However, this is hard to apply in practice due to the high cost of amino acids. More than 15 acid amides and amino acids can be found in biogas slurry in amounts of 500 mg/l.Reference Yang13, Reference Xiong31 Therefore, biogas slurry is a suitable alternative to solve this issue. Moreover, in biogas slurry, mixed amino acids are presented. Some authors have proven that mixed amino acids are more efficient in lowering nitrate content in vegetablesReference Gunes, Post, Kirkbye and Aktas27, Reference Wang, Wu, Wang, Zhu, Tao and Zhang30, Reference Xiong31. Of course, based on the complex components of biogas slurry, the effects of biogas slurry on nitrate accumulation in vegetables cannot be attributed only to the presence of amino acids; there are still some unknown mechanisms to be revealed. Shi et al.Reference Shi, Yang and Li32 suggested that biogas slurry had dual functions as a bio-fertilizer and bio-pesticide, without resistance evolution and environmental pollution. Furthermore, over 20 compounds of 120 substances in biogas slurry have the functions of anti-disease and insect preventionReference Zhang, Li, Ni, Yang, Zhang and Li33. As mentioned above, some constituents of biogas slurry could increase the resistance to abiotic stress, such as drought, chilling and so on. All the above functions are helpful in vegetable cultivation to ensure that the contents of nitrate and pesticides are under the limit. As to heavy metal content, there is a lack of sufficient data to draw a conclusion; however, the proper selection of fermentative materials could easily help control the content of heavy metals in biogas manure. In addition, biogas manure is pathogen- and vermin egg-free after the long-term fermentation processReference Yang13. Based on the above-mentioned ingredient analysis, it is concluded that biogas slurry is a feasible nutrient solution for soilless cultivation of high-quality vegetables.
High-quality Vegetable Cultivation with Biogas Manure: Benefit Analysis
In China, the studies on effects of biogas slurry on vegetable nitrate content, yield and quality have been conducted for more than two decades. To date, more than ten vegetable species, including lettuce, Chinese cabbage, rape, Malabar spinach, tomato, cucumber, garlic, green pepper, Ipmoea aquatica Forsk., bitter gourd and Gynura bicoloar, etc., have been tested and successfully grown in soil and soilless conditions supplied with biogas manure showing substantial benefits. Among the tested vegetable species, lettuce is the most common species studied. In terms of yield and quality, it is well reported that biogas manure was efficient in promoting yield and quality of vegetables. However, no comprehensive review had been published to summarize the current status of this issue in China. In this paper, studies have been integrated to provide a reference for researchers worldwide.
Yield
As early as 1985, ZhuReference Zhu34 found that the application of biogas slurry could increase the yield of Chinese cabbage. In the 2000s, more than 20 studies have been conducted focusing on the application effectiveness of biogas manure in the cultivation of vegetables. Among the previous investigations, the majority of results were obtained in pot experiments and field experiments by the soil cultivation method. The majority of results showed that biogas manure application will enhance the yield of vegetables, such as celeryReference Wang and Sun35, Chinese cabbageReference Zhu34, pakchoiReference Wang, Liu, Shen and Wu36, lettuceReference Liu, Du and Yang19, Reference Xu, Wang, Quan, Ou and Chen37, Reference Xu, Wang, Wang, Ou and Chen38, green pepperReference Zhou, Wang, Li and Zhang39, mustardReference Li, Wang, Li, Li, Liu, Xu and Zhu40, Ipmoea aquatica Forsk.Reference Lin, Lin, Liao, Luo, Liu, Yang, Kozai and Bot41 and mushroomReference Ji, Gou and Zhang42. Recently, some studies have been conducted successfully under soilless conditions. These have confirmed that lettuceReference Liu, Du and Yang19, Reference Su, Lu and Shi43, Reference Yang44, Malabar spinachReference Li, Ye and Wang45, Gynura, Gynura divaricata and Purple leaf lettuceReference Ning, Li and Pan12, cucumberReference Hao, Hong and Gao46, Reference Yuan47, pepperReference Zhou, Wang, Li and Zhang39 and Ipmoea aquatica Forsk.Reference Lin, Lin, Liao, Luo, Liu, Yang, Kozai and Bot41 can be cultivated successfully with a modified biogas slurry solution.
Nutritional quality
The content of soluble sugar, vitamin C, acidity and amino acids of vegetables are the main indices that have been examined in previous studies. In summary, there are inconsistent effects of biogas slurry on vitamin C and the soluble sugar content of vegetables. Some reports showed that vitamin C and soluble sugar in pakchoiReference Wang, Liu, Shen and Wu36, lettuceReference Su, Lu and Shi43 and cucumberReference Hao, Hong and Gao46 were increased. However, in contrast, Xu et al.Reference Xu, Wang, Quan, Ou and Chen37, Reference Xu, Wang, Wang, Ou and Chen38 found that soluble sugar and vitamin C of lettuce cultivated with biogas slurry were decreased. Similar results were presented in mustardReference Li, Wang, Li, Li, Liu, Xu and Zhu40. In addition, Zhou et al.Reference Zhou, Wang, Li and Zhang39 found no influence of biogas slurry on vitamin C and soluble sugar content in pepper, but the ratio of sugar to acid was increased. To sum up, current data about the effects of biogas manure on nutritional quality are confused, which may be caused by discrepancies in experimental materials and various experimental conditions. More investigations should therefore be conducted to clarify the effects on quality and their mechanisms.
The nitrate content is an important index used in almost all related studies to determine the nutrient quality of vegetables. Almost all results showed consistently that the nitrate content of vegetables was decreased significantly after cultivation with biogas manure, in both soil and soilless cultivation. The vegetable species reported include Malabar spinachReference Su, Lu and Shi43, lettuceReference Liu, Du and Yang19, Reference Xu, Wang, Quan, Ou and Chen37, Reference Xu, Wang, Wang, Ou and Chen38, Reference Ji, Gou and Zhang42, rapeReference Wang, Yuan, Guo and Chen18, Reference Xiong31, cucumberReference Liu, Ko, Kim and Lee29 and mustardReference Li, Wang, Li, Li, Liu, Xu and Zhu40. Also, a study showed that the content of Hg, Cr and Cd in lettuce was decreased with increasing proportion and amount of biogas manure used in soil cultivationReference Ai, Liu, Li, Chen and Ren48. Light intensity has a strong influence on nitrate accumulation in vegetablesReference Prakasa Rao and Puttanna23. Nitrate can accumulate to dangerous levels under low light conditions, when nitrate reductase is inactiveReference Cardenas-Navarro, Adamowicz and Robin49. China is the world's largest producer of vegetables grown in protected systemsReference Yang, Sun, Wei, Liu, Yang, Kozai and Bot20; thus improving the light conditions in off-season production of vegetables is an effective method to depress nitrate accumulation. Some hydroponic lettuce producers address the problem by giving their plants N-reduced nutrient solution or supplemental light prior to harvestReference Mozafar50, Reference Anderson and Nielson51. Future research should integrate these methods together, investigate the combined effects, and optimize the efficiency of lowering the nitrate level in vegetables.
Disease resistance
Many of the beneficial effects on disease resistance of biogas manures, particularly biogas slurry, have previously been demonstrated. Recently, some bio-pesticides based on biogas slurry have been investigatedReference Zhang, Li, Ni, Yang, Zhang and Li33. It is estimated that the application of biogas manure will reduce by over 20% the dosage of chemical fertilizers and pesticides, and lower by 1% the rate of pesticide residues (China National Plan on Rural Biogas Construction Projects of 2006–2010). However, the mechanisms of action of biogas manures still need to be investigated carefully. Three mechanisms of action have been revealed: (1) NH4+, acetic acid, butyric acid, propionic acid, gibberellin and indoleacetic acid in biogas slurry are effective anti-disease substances that can inhibit the occurrence of diseases and insects; (2) vitamins and amino acids increase the ability of crops to resist diseases; and (3) volatilization of methane and ethylene gas will help keep off pathogenic bacteria and insects. Jothi et al.Reference Jothi, Pugalendhi, Poornima and Rajendran52 found that tomato plants amended with biogas slurry had more vegetative growth and tended to flower and fruit much earlier when compared with the control. The nematode population in the soil decreased, thus decreasing the severity of nematode attack.
Some authors have suggested biogas manure as a feasible nutrient resource for high-quality vegetable production under soilless conditions, with the advantages of high yield and good qualityReference Ning, Li and Pan12, Reference Su, Lu and Shi43. To sum up, biogas slurry can be used as a nutrient solution, while biogas dregs can serve dual functions as solid manure and cultivation substrate for vegetable cultivation. YuanReference Yuan53 found that the mixed substrate of rough river sand and sawdust (1:1) was best for soilless culture of cucumber with 50% biogas slurry (water/biogas slurry, 1:1). In general, the original biogas slurry with high electrical conductivity (EC) was not suitable for vegetable cultivationReference Yang44. Some authors suggested that appropriate adjustments before use (e.g. dilution and the addition of fertilizer) will enhance the cultivation effectiveness. Liu et al.Reference Liu, Du and Yang19 found that biogas slurry could grow higher biomass lettuce after being diluted at 1:4 or 1:5 (v/v, biogas slurry/water). The yields of Malabar spinach in the treatment supplied with 50% biogas slurry were the highestReference Su, Lu and Shi43. Ning et al.Reference Ning, Li and Pan12 confirmed that it is feasible to utilize the biogas digested fluid as the nutrient solution in soilless plantings of Gynura (Gynura divaricata) and Purple leaf lettuce, with the dilution ratio of 1:4 of biogas digested fluid. Liu et al.Reference Liu, Du and Yang19 found that biogas slurry (1:5 dilution) could significantly decrease nitrate content in lettuce, and the added amino acids in biogas slurry would bestow other benefits. Biogas slurry was used as nutrient solution for soilless tomato cultivation, and it improved tomato yield and quality. Also Lu et al.Reference Lu, Liu and Xu54 found that biogas slurry diluted to a ratio of 1:8 (v/v) was the most suitable for tomato growth in the experiment. The relevant literature dealing with the soilless cultivation of vegetables and its efficiency is summarized in Table 1.
Table 1. Results of previous studies on the effects of biogas manure on yield and quality of vegetables grown soilless.
Vc, vitamin C.
Research Prospects
Utilization of biogas manure is a key issue for sustainable development of the biogas industry and rural eco-environment. Based on the present data, chemical fertilizer can be entirely substituted by biogas manure in soilless cultureReference Yin, Xu, Zhang, Guan, Gao, Li and Mao55. Both yield and quality of vegetables cultured by nutrient solutions modified from biogas slurry are usually improved. Furthermore, nitrate accumulation in vegetables is largely lowered, heavy metals and pesticide residues in vegetables can also be controlled. In summary, biogas slurry, after proper adjustment, is a feasible nutrient solution for high-quality vegetable production. However, ingredients of biogas manure differ with species, ratios and amount of materials for anaerobic fermentationReference Lu, Liu and Xu54. Furthermore, they could also be modified by local agronomic measures such as fertilization, spraying pesticide, soil type and climate conditions. Although ingredients of biogas manure are variable, they can be adjusted through adding mineral elements to realize the optimal components for vegetable growth. Thus, biogas slurry can be looked on as an adjustable liquid fertilizer. Many authors have suggested that biogas slurry was an excellent organic fertilizer, which could be used in soilless cultivation of vegetablesReference Liu, Du and Yang19, Reference Ning, Li and Pan12, Reference Yin, Xu, Zhang, Guan, Gao, Li and Mao55. Investigations are needed to develop regulation techniques aiming at optimizing the composition of biogas manure to increase the cultivation efficacy during pre-production, production and post-production of biogas. In addition, nutrient amendment strategies for biogas manure should be designed to balance the nutrient ratio to cater for vegetable uptake requirementsReference Ning, Li and Pan12. All in all, three kinds of regulation measures can be conducted during the production of biogas manure for vegetable soilless cultivation (Fig. 1), including material constituents, fermentative conditions and constitutive modification of biogas manure.
Figure 1. Process flow of soilless cultivation with modified biogas manure, and regulation measures.
China currently has a large number of methane tanks run by farmers, large-scale plants of livestock production and industrial enterprises. There were 18 million methane tanks used by farmers at the end of 2005 in rural areas, and 1500 were run by large-scale plants of livestock production and industrial enterprises. As designed in The Eleventh Five-year Plan of Renewable Energy Development of China, China will possess 40 million biogas farmer–users, and 4700 biogas projects for large-scale plants of livestock breeding and 1600 biogas projects for treatment of organic wastes from industry by 2010. It appears that a large quantity of biogas manure will be produced in the future. On the basis of eco-economy, biogas slurry and biogas residues re-utilization is a substantially beneficial action. The main advantages of high-quality vegetable cultivation with the modified biogas slurry, as a feasible alternative to the traditional inorganic nutrient solution, are to reduce the negative effects of environmentally toxic substances and improve the quality of vegetables. In China, it is highly feasible to combine the two developmental tendencies of increased biogas production and the need for high-quality vegetables. When this combination is applied widely in China, it is believed that non-point pollution will be reduced, even considering the ever-growing intensive livestock farming. To sum up, it is concluded that the application of biogas slurry in vegetable cultivation is a feasible pathway to agricultural production and environmental protection. China is the world's largest producer and consumer of vegetables in protected facilities; both the government and scientists should pay more attention to soilless production of high-quality off-season vegetables because, after all, public concern about vegetable quality and health is growing.
Acknowledgement
Financial support is acknowledged from the Director's Fund of the Institute of Environment and Sustainable Development in Agriculture (2009), Chinese Academy of Agricultural Sciences.
