Introduction
In China, tomatoes are often grown in greenhouses, in order to provide early-ripening fruit that meet the demands of consumers. However, the quality of greenhouse tomato, as indicated by characteristics such as colour and flavour, as well as content of ascorbic acid and carotenoids, is usually found to be poor. Many complaints about poor quality of tomato fruit have been made in the past few years (Baldwin, Scott, Shewmaker, & Schuch, 2000), and consumers demand products with better flavour (Baldwin et al., 2000; Causse, Buret, Robini, & Verschave, 2003).

Several studies have reported that lower carbon dioxide (CO2) is one of the primary factors affecting the quality of greenhouse tomato.

It is considered that the optimal concentration of CO2 for plant growth is 800–1000 µL L-1 (Kläring, Hauschild, Heibner, & Bar-Yosef, 2007; Jin, Du, Wang, Condon, Lin, & Zhang, 2009). However, the CO2 concentration in greenhouse is only 100–250 µL L-1 during the daytime due to the hermetic conditions, which impair plant growth (Kläring et al., 2007). Jin et al. (2009) recently proposed a new strategy of CO2 enrichment by composting crop residues and animal manures directly in the greenhouse. It is profitable for farmers as it requires only low-cost inputs, and avoids the possible environmental problems caused by burning and practices of disposal of these agricultural by-products. The application of this CO2 enrichment technology increased the production of five leafy and stem vegetables, including celery, leaf lettuce, stem lettuce, oily sow thistle, and Chinese cabbage (Jin et al., 2009).
To our knowledge, CO2 is the substrate for photosynthesis and its concentration impacts on plant growth. Previous studies have demonstrated the beneficial influence of CO2 on photosynthetic rate, plant growth and crop yield (Ainsworth & Long, 2005; Sanz-Sáez, Erice, Aranjuelo, Nogués, Irigoyen, & Sánchez-Díaz, 2010). A report studied the effect of CO2 enrichment on quality of tomatoes (Islam, Matsui, & Yoshida, 1996).

Carbon dioxide released through various human activities is the main greenhouse gas causing global warming. Its concentration in the atmosphere rose from the preindustrial level of 280 ppm (IPCC 2007) to the present level of 395.15 ppm in August2013(www.esrl.noaa.gov/gmd/ccgg/trends/). Atmospheric CO2 is expected to reach 700 ppm by the end of the century according to the Intergovernmental Panel on Climate Change (IPCC) under emission Scenario A1B (Carter et al. 2007). Increasing CO2 concentration in the atmosphere focused scientists’ interest on studying the response of crop plants to CO2 enrichment. Studies under controlled environment have shown that CO2 fertigation enhances photosynthetic rate (PN) and the yield in both C3 and C4 crops (Kimball et al. 2002, Reddy et al. 2010). It is also observed that the increased intercellular CO2 concentration leads to decrease in stomatal conductance (gs), increased PN, and water-use efficiency (WUE) at EC (Ainsworth and Long 2005, Leakey et al. 2009, Zhao et al. 2011, Li et al. 2013). Crop water use is a critical issue for crop production and increased WUE may represent one of the most significant plant responses to EC (Rogers et al. 1994). However, screening genotypes for high leaf transpiration (E) efficiency at projected, future CO2 concentrations could be more efficiently accomplished under the EC concentrations (Bunce 2012). The studies suggest that many crops, notably C3 crops, may respond positively to increase in atmospheric CO2 in the absence of other stressful conditions (Long et al. 2004). It is evident that the steady increase in atmospheric CO2 influences the
overall physiology, growth, and development of crops. Tomato (L. esculentum Mill.) is one of the most consumed vegetables in the world and is rich in dietary nutrients and antioxidants. It is a good source of bioactive compounds, including carotenes (lycopene, ?-carotene), ascorbic acid, and phenolic compounds. High amount of ascorbic acid has been reported by many workers (Leonardi et al. 2000, Stewart et al. 2000, Kaur et al. 2013). Being a C3 plant, it has shown positive response to a range of EC concentrations. An increase in PN was found in cv. Virosa grown under EC from 500 to 2,000 ppm of CO2 (Nilsen et al. 1983). Significantly higher PN, reduced gs, and increased leaf area was observed in two species, ‘Vedettos’ (L. esculentum) and LA1028 (L. chmielewskii), grown at 900 ppm of CO2 (Yelle et al. 1990). Lower E (Behboudian and Lai 1994) and the increased yield have been reported (Nilsen et al. 1983, Yelle et al. 1990, Reinert et al. 1997). The changes in physiology, phenology, growth, and the yield of crops lead to changes in quality of the produce. The EC effects on physiology and quality of fruits and vegetables have been summarized by Moretti et al. (2010). In general, high CO2 has been reported to influence the fruit quality by affecting content of antioxidants, ascorbic acid, and sugars (Tajiri 1985, Islam et al. 1996, Idso et al. 2002, Wang et al. 2003). High ascorbic acid and sugar contents in tomato fruits were reported by Islam et al. (1996), when EC was used at different maturity stages. However, some of the studies reported that enhancements in antioxidant substance contents were very low in tomato under high CO2 concentrations (Barbale 1970, Madsen 1971, 1975; Kimball and Michell 1981). As higher intake of flavonoids, ascorbic acid, and carotenoids have been reported to reduce the risks of many degenerative diseases (Agarwal and Rao 2000).

It is considered that the optimal concentration of carbon dioxide (CO2) for plant growth is 800–1000 µL L-1. However, the CO2 concentration in greenhouses is approximately 100–250 µL L-1 during the daytime due to hermetical conditions, which cause the CO2 concentration to be suboptimal for growth (Kläring et al., 2007). Various strategies have been developed to increase CO2 concentrations within greenhouses. These include ventilation, direct gas injection, and chemical production by mixing sulfuric acid and ammonium bicarbonate (Linker et al., 1999; Kläring et al., 2007). However, these measures have significant shortcomings. The maximum concentration of CO2 by ventilation is only 350 µL L-1, and the temperature in the greenhouse may be decreased, especially in winter. The two latter methods, which are expensive and difficult to operate, have not commonly been adopted by farmers. Therefore, CO2 deficiency has become a limiting factor for vegetable production in greenhouses. We propose a new strategy of CO2 enrichment by composting crop residues and animal manures (CRAM) directly in the greenhouse. Economic benefits from higher yields and quality of vegetables may encourage farmers to adopt this technique. Approximately 22% of total Chinese crop residues, estimated to be nearly 0.6 Gt y-1 (National Bureau of Statistics of China, 2005), were burnt in the field (Cao et al., 2006) causing carbon (C) emissions of 5.5 × 107 t y-1. Compositing offers an alternative and productive use of these crop residues that would significantly reduce C emissions into the atmosphere. Composting crop residues also requires the addition of nitrogen (N). This can be achieved with the use of animal manure that would otherwise require disposal. There are 2.75 billion tons of animal manures produced in China every year, and about 55 to 220 million tons were directly discharged to water bodies (Gao et al., 2006). The indiscriminate discharge of manures from high-density livestock operations and individual farms cause large quantities of P and N to flow into water bodies and eventually lead to eutrophication (Zvomuya et al., 2006; Kleinman and Sharpley, 2003; Tabbara, 2003). In areas with high livestock densities, manure production exceeds the needs of crops to which the manure is applied (Carpenter et al., 1998). This reflects the serious status of P pollution from manures and the urgent need for effective strategies to be employed to reduce incorrect manure disposal. Composting facilitates the utilization of currently unused manure, giving it a value to food production which will act to decrease harmful discharges of manures into water bodies. In order to use composting to increase CO2 concentrations within greenhouses, the optimum fermentation conditions for composting were studied by Du et al. (2004). The results showed that the optimal initial C : N ratio of substrate, temperature, water content, and initial pH for CO2 production by bio degradation of a mixture of rice straw and pig manure were 40:1, 50_C, 70% (w/w), and 6.0–7.0, respectively. Among these, the C : N ratio is the most important factor influencing the fermentation (Du et al., 2004). In the present research, the effects of a simple but efficient composting unit on increasing CO2 concentrations in greenhouses and subsequent vegetable yield and quality were investigated. The CRAM mixture was inoculated with a mixture of three species of fungi (Panusconclmtw zj3, Trichoderma viride zj2, and Aspergillas niger zj1) to increase CO2 production through accelerated fermentation. The pH of CRAM material was regulated to 6.5–7.0 with pickled vegetable juice (pH 3) every 2 weeks.

Moreover, substituting fertilizers and peat with digestate and compost, and applying CO2 and excess heat in the greenhouse, will add to the overall environmental performance (Halmannand Steinberg, 1998).

The functioning and composition of ecosystems are changing due to the effect of humankind, which is producing modifications in the climate and global biochemistry (IPCC, 2014). Thus, the atmospheric CO2 has been increased by the burning of fossil fuel in the last two centuries, stimulating an acceleration of global climate change (IPCC, 2014). In principle, C3 plants increase their rates of photosynthesis and growth as much as 35% under elevated CO2 (eCO2) (Russell et al., 2014), as these plants are generally not photosynthetically saturated at current CO2. However, despite the improvement in yields, the efficiency of use of CO2is reduced at eCO2, due in part to the reduction of photosynthetic stimulation after a certain time of exposure, associated with various aspects of a phenomenon known as “acclimatization” or “down-regulation”(Reich et al., 2006). Several studies indicate that the acclimatization under eCO2 is a consequence of an insufficient sink capacity caused by a limiting N supply (Rogers and Ainsworth, 2006), where?
inhibition of photorespiration may play a crucial role (Bloom et al.,2010). Effects of the N input (as NO3?or NH4+) on photosynthesis were observed, especially in relation to stomatal conductance and the intercellular partial pressure (Ci) of CO2: plants supplied with NH4+showed higher rates of assimilation and stomatal conductance than those receiving N exclusively as NO3?(Bloom et al.,2002). Thus, the growth of many plant species is affected by the form of nitrogenous nutrition (Fernández-Crespo et al., 2012), the photosynthetic response to eCO2 also depending on the balance between the supply and the new demand for N, the distribution of biomass, and the source-sink balance (Sanz-Sáez et al., 2010).

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