Environmental Impact and Nutritional Improvement of Elevated CO₂ Treatment: A Case Study of Spinach Production

Abstract

The agriculture sector is known to be the one of the major contributors to global greenhouse gas (GHG) emissions. At the same time, global climate changes have affected the agriculture sector. In order to strengthen the sustainable development of agriculture, it is important to promote environmentally friendly farming and simultaneously increase the economic value. To improve the productivity of agriculture, technical advancements have occurred. Among those, we have focused on CO₂ treatment in cultivation. We aimed to clarify the effectiveness of the elevated CO₂ treatment of spinach based on GHG emission and the economic value using the eco-efficiency score. We assumed that nutrition could represent the value of the vegetable. We measured weights, vitamin C, and CO₂ emissions of elevated CO₂ treatment and conventional production. We used life cycle assessment (LCA) to estimate CO₂ emissions. CO₂ emissions of a 100-g bouquet of spinach were estimated from agricultural inputs, farming, transport, and distribution center processes at a model spinach farm in Japan. CO₂ emission of elevated CO₂ treatment was 29.0 g-CO₂, and was 49.0 g-CO₂ for conventional production. The net weight of a bouquet of elevated CO₂-treated spinach was 1.69-fold greater than that of conventional production. Vitamin C per 100 g spinach produced via elevated CO₂ treatment was 15.1 mg, and that of conventional production was 13.5 mg on average. Finally, based on the above results, we assessed the eco-efficiency scores of the elevated CO₂ treatment and conventional production of spinach, enabling integration of the nutritional value and the environmental impact. The score showed that elevated CO₂ treatment (0.76) was 2.9-fold more efficient than conventional production (0.26). This study suggested that elevated CO₂ treatment could enhance growth and nutritional value of spinach, and further contribute to CO₂ reduction.

1. Introduction

Our concerns about global warming have stimulated the development of environmental assessment of agriculture sectors using life cycle assessment (LCA) analysis. LCA has been used since the end of the 1990s to assess the environmental impact of single crop cultivation, and has shifted to various agriculture sectors: animal production, horticulture, organic farming, etc. [1,2,3]. In Europe, it is promoted to certify environmentally-assessed products, and EPD (Environmental Product Declaration) was introduced to reduce the environmental burden of production. EPD is an independently verified and registered document that communicates comparable information about the life-cycle environmental impact of products [4,5]. It contains more than 700 products’ documents.

In order to strengthen the sustainability of agriculture, it is very important to increase both the prevalence of environmentally friendly farming the economic value of the sector. Towards this concern, ISO 14045 determined the eco-efficiency protocol in 2012. Eco-efficiency is meant to support sustainable development. It is a quantitative management tool which enables the consideration of the life cycle environmental impacts of an agricultural system alongside its product system value to a stakeholder [6,7,8]. The value of a vegetable could be enhanced by the production of the product in a manner that enlarges values such as productivity, nutrition, taste, etc. Productivity and nutrition are especially important in Japan under the recent shrinking agriculture sector and changing dietary patterns. Agriculture in Japan has been resistant to change, where small-plot holdings still dominate, the average farmer is over 66 years old, and the sector’s contribution to the economy has fallen by 25 percent since its peak in 1984. To address these factors, technological advances have been introduced to increase the productivity. For example, elevated CO₂ treatment—in which CO₂ is input higher than ambient (400 ppm)—has been recorded to yield three-to-four-fold production quantity compared to conventional farming. CO₂ is a raw material for photosynthesis, and its application to plants has been proven to improve productivity [9,10,11,12,13,14,15]. It has been introduced as a standard technique in protected horticulture, especially in the Netherlands. It is expected that CO₂—one of the major greenhouse gases—could become a useful resource in agriculture. In Japan, it has been tested in a few agricultural sectors: tomato, strawberry, rose, etc. [16,17,18]. However, it is not yet broadly applied at farms, but in 1448 ha of protected horticulture facilities, which accounts about 3.1% of the total in 2015 [19].

Regarding the changing dietary habits of Japanese people, we assumed that nutrition could be a value to attract consumers, followed by increasing economic value. The consumption of meat and excessive fats has been expanding, and the dietary eccentricity has advanced rapidly. Poor diet is more prone to disease, infection, fatigue, and poor performance. Hence, health-conscious consumers are driving demand for strains of vegetables with enhanced nutritional qualities. So-called “functional vegetables” in Japan—for example, broccoli containing three times the amount of antidotal and antioxidant elements, carrots containing a balanced content of the antioxidants lycopene and carotene, etc.—command higher prices than ordinary. Farmers are eager to comply with the above value-added vegetables.

Concerned with environmental issues in the agriculture sector, the Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan formulated a “Competitive Agriculture” policy, promoting the use of renewable energy on farms [20]. It was deemed that agriculture and fishing villages have plentiful abundance of renewable energy resources such as biomass, sunlight, and water. Our interest was the CO₂ waste exhausted from some renewable energy plants. We assumed that waste CO₂ could be a source of elevated CO₂ treatment.

From the above backgrounds, we aimed to prove the effectiveness of elevated CO₂ treatment on leafy vegetables in a protected horticulture system, where it was assumed that CO₂ would be supplied from the waste of a renewable energy plant. We estimated the eco-efficiency, which was determined per ISO 14045 [21]. We integrated the environmental impact and the nutritional quality. We chose spinach as a model leafy vegetable. First, we performed a growth experiment of spinach under different CO₂ concentrations: elevated CO₂ treatment (eCO₂ system) and conventional production (Conv system). We measured the weights and vitamin C contents of those two systems, respectively. Secondly, we conducted LCA analysis to estimate CO₂ emissions of those systems, respectively. Finally, we calculated eco-efficiency score using vitamin C contents and CO₂ emissions. The scores of those systems were compared.

2. Materials and Method

2.1. Incubator-Scale Cultivation of Spinacia oleracea

We performed the growth experiment using Spinacia oleracea as model leafy vegetable. Germinated Spinacia oleracea pots were moved into a plant incubator (Biotorn, NKsystem Co., Ltd. Tokyo, Japan) which is able to automatically control the environment of the plant. It can control the temperature, moisture, and lighting. We prepared the two different CO₂ concentration systems: ambient conventional system (Conv system, 400 ppm), and elevated CO₂ treatment (eCO₂ system, 1000 ppm). We cultivated each system three times including four samples, respectively. They were produced for 20 days. CO₂ was continuously flowed in. Incubator was set to 16 h lighting time, 22 °C temperature, 50% humidity, and 60% illumination (

Table 1. Incubator-scale cultivation of Spinacia oleracea of the conventional system and the elevated CO₂ treatment system.

	Conventional System	Elevated CO2 Treatment System
Temperature (°C)	22	22
Humidity (%)	50	50
Lighting (%)	60	60
CO2 concentration (ppm)	400	1000
Lighting time (h)	16	16

). Culture soil was mildly acidic with 170 mg/L nitrogen (N), 140 mg/L phosphorous (P), and 120 mg/L potassium (K). These conditions were kept throughout the growth experiment.

2.2. Measurement of Weight and Vitamin C

A bouquet of spinach was washed and weighted using an electronic balance (A&D Co., Ltd., Tokyo, Japan) after 20 days of cultivation. Vitamin C was measured according to the manufacturer’s procedure using a vitamin C assay kit (Shima Laboratories Co., Ltd., Tokyo, Japan). Briefly, we used a standard solution of ascorbic acid to clarify a calibration curve, showing the relationship between absorbance and vitamin C concentration due to the difference in dilution ratio. Additionally, 100 g of edible portion of spinach was homogenized with a five-fold amount of a 5.4% metaphosphoric acid aqueous solution, and centrifuged at 15,000 rpm, 10 min, 4 °C. Supernatant was measured using a spectrophotometer. The measurement was performed three times for each system. Vitamin C concentration was calculated using a calibration curve.

2.3. Life Cycle Assessment (LCA)

LCA in this study was conducted by inventory analysis. CO₂ emissions of the Conv system were calculated at the model protected horticulture system of spinach in Asakura City, Fukuoka prefecture. The model farm ranged 3 a in area and produced 720 kg spinach for 24 days.

The functional unit was 100 g of harvested spinach. The system boundary of the Conv system includes agricultural inputs, farming, transport, and distribution center processes, as shown in Figure 1. That of the eCO₂ system includes the same as the Conv system and CO₂ production. However, CO₂ emissions of CO₂ production were ignored in this study, because we set that CO₂ was supplied from the waste CO₂ of renewable energy plants.

Inventory data were acquired by interviewing the farm, and unavailable data such as transport to the central wholesale market, fuel input during farming, and electricity use at distribution center were collected from previous reports [22,23,24,25,26] (

Table 2. Inventories from production of spinach at 10 a farm to distribution center. N: nitrogen; P: phosphorous; K: potassium.

Spinach Life Cycle Inputs
	Unit	Use (per 10 a)			CO2 emissions (kg CO2 per unit)
Organic fertilizer	kg	N	P	K	N	P	K
		24.0	19.8	16.9	1.15	0.86	0.15	Adapted from [24]
Fuel	L	2.5			2.7			Fuel use adapted from [22]
Transport
	Unit	Use (per one way)			CO2 emissions (kg CO2 per unit)
Truck transport	L	5.86			2.32			Fuel efficiency [20]; Average speed adapted from [21]
Distribution Center
	Unit	Use (per day)			CO2 emissions (kg CO2 per unit)
Electricity	kWh	0.9			0.5			Electricity use adapted from [23]

). CO₂ emissions per 100 g of harvested spinach were calculated by multiplying inventories by CO₂ emission intensity units, as shown in Table 1. CO₂ emission intensity units were adapted from a government report [26]. For organic fertilizer and fuel use at the farm, CO₂ emissions per 10 a were converted to those per 100 g-spinach, by dividing those by the area and the production of the model farm. Fuel was used in agricultural machines. For transport, CO₂ emissions for a one-way trip were calculated using fuel-efficiency, 10.3 km/L, and average speed, 30.2 km/h [22,23]. Transport occurred 24 times. For the distribution center, CO₂ emissions per day was converted to 24 days. Residence time in the distribution center varied, and we assumed one day on average.

CO₂ emissions of the eCO₂ system was estimated by dividing the CO₂ emissions of the Conv system by the spinach weight ratio of eCO₂ system. Weight ratio was calculated by dividing the average weight of the eCO₂ system by that of the Conv system, which was measured as 2.2.

2.4. Eco-Efficiency Score

The eco-efficiency score was calculated by dividing vitamin C content by CO₂ emissions, which were measured in Section 2.1 and Section 2.3, respectively. We assumed that nutritional quality would represent the value of vegetable, which would possibly enhance the economic value. Among nutrients, vitamin C was chosen due to its high content in spinach and the result of questionnaire survey [27,28].

$$\mathrm{Eco}-\mathrm{efficiency}\left[\mathrm{score}\right]=\frac{\mathrm{vitamin}\mathrm{C}\mathrm{content}\left[\mathrm{g}/100\mathrm{g}-\mathrm{spinach}\right]}{{\mathrm{CO}}_2\mathrm{emission}\left[\mathrm{g}-{\mathrm{CO}}_2/100\mathrm{g}-\mathrm{spinach}\right]}$$

3. Results

The effectiveness of the elevated CO₂ treatment in improving the growth, nutritional quality, and environmental impact has been clarified. We compared weight, vitamin C content, and CO₂ emissions of the Conv system (400 ppm CO₂) to the eCO₂ system (1000 ppm CO₂). Additionally, the eco-efficiency score, integrating nutritional value and environmental impact, was compared between the two systems.

Figure 2 shows spinach weights at 20 days of growth under different CO₂ treatments. Weights were measured per bouquet of spinach. The averages of the Conv system and the eCO₂ system were 1.59 g and 2.70 g, respectively, showing 1.69 times higher weight in the eCO₂ system. t-test proved that there was a significant difference between the treatments (p = 0.005). Vitamin C concentration was estimated by the absorbance of a spectrophotometer, followed by calculation using a calibration curve (Figure 3). The average concentration of the Conv system was 13.2 mg/100 g spinach (standard deviation 4.27), and it was 15.1 mg/100 g in the eCO₂ system (standard deviation 2.27).

CO₂ emissions were estimated by the inventory calculation result of the Conv system, the model spinach farm (Figure 4). Emissions of 49.0 g-CO₂/100 g-spinach of Conv system and 29.0 g-CO₂/100 g-spinach of eCO₂ system were obtained. The CO₂ emissions of the Conv system were 1.69-fold greater than the eCO₂ system. Closely looking into the Conv system, 2 g-CO₂/100 g-spinach for organic fertilizer, 0.3 g-CO₂/100 g-spinach for fuel, 1.5 g-CO₂/100 g-spinach for distribution center, and 45.3 g-CO₂/100 g-spinach for transport were shown to be emitted. CO₂ emission during transport of spinach from the farm to the central wholesale market accounted for about 90% of the total. The reason for this is that the model farmer conducted organic farming, and they are doing pesticide-free cultivation. Agricultural chemicals are known as one of hot spots of CO₂ emission during cultivation [29,30].

The eco-efficiency of the eCO₂ system was scored as 0.76, and that of the Conv system was 0.26. This demonstrates that the eCO₂ system was 2.9-fold more efficient than the Conv system.

4. Discussion

Elevated CO₂ treatment could improve the growth and environmental impact for spinach production in a protected horticulture system by using waste CO₂ from other systems as a source. It was found that the weight of elevated CO₂ treatment for 20 days was 1.69-fold greater than ambient CO₂ system. Additionally, vitamin C was relatively higher in the elevated CO₂-treated spinach, showing 14% higher than conventional production. A previous report showed similar results to this study, which compared weights and vitamin C between elevated CO₂ treatment (900 ppm) and conventional production (350 ppm). It indicated 1.65-fold heavier bouquets and 1.08-fold more vitamin C for the elevated CO₂ treatment [29]. This tendency was also reported in other vegetables. They investigated the physiological effect of elevated CO₂ treatment on vegetables, although it differed according to species [31,32,33,34]. Ripening tomatoes and bananas showed a respiratory reduction with high CO₂ treatment [35]. Eggplants, cucumbers, podded peas, spinach, and lettuce showed C₂H₄ production and respiration increase by high CO₂ [32]. Especially, plants of C₃, to which spinach (Spinacea oleracea) belongs, showed relatively high photosynthetic rate under elevated CO₂ [13,35]. These physiological enhancements due to elevated CO₂ could possibly induce the growth and high concentration of vitamin C as shown in this study. It is expected that productivity will be improved by developing new control methods and species in the future [36,37,38].

Elevated CO₂ treatment facilities often adopt natural gas to produce CO₂ or use CO₂ exhausted from other industries. In order to further abate the environmental burden of elevated CO₂ treatment, the source of CO₂ is important. Combined heat and power (CHP) systems use natural gas as a fuel, generate electricity with a gas engine, and use exhaust heat and exhaust gas, CO₂. The electricity produced at the facility is mainly used for supplementary lighting, heat is applied to heating, and CO₂ is applied to the facility to promote growth [38]. Another example is the use of CO₂ from composting facilities. Nakano et al. showed that about 1000 ppm of CO₂ was exhausted every hour during composing tomato farm waste [39].

We would propose to use waste CO₂ from renewable energy plants. It was suggested that environmental impact could be reduced by elevated CO₂ treatment due to the growth increase, assuming that CO₂ emission to produce waste CO₂ could be ignored. It is recommended to combine a protected horticulture system with the Blue Tower process [40,41]. The Blue Tower process is a renewable energy production systems which uses biomass, and its main product is hydrogen. Nagaishi et al. estimated the CO₂ emissions of the Blue Tower process in consideration of CO₂ fertilization [42]. Results showed CO₂ emission was reduced as 36.37 g-CO₂/MJ. It was suggested that the use of CO₂ in vegetable production could reduce the total CO₂ emissions of a biomass hydrogen plant. However, the cost often hampers the use of waste CO₂ from other systems. For example, some regions in the Netherlands are using CO₂ from the oil industry, but this is not widespread due to the high cost—it costs more than natural gas [38]. In order to promote the use of waste CO₂ from renewable energy plants in protected horticulture systems, it is necessary to investigate the cost of installing and operating the plants.

Toward the sustainable development of agriculture, it is important to increase the value of vegetables while reducing environmental impact. We focused on nutrition to evaluate the value of vegetables in this study. We assumed that enhanced nutritional quality could trigger an increase in economic value. We chose to study nutritional quality as it related to CO₂ emissions. We assumed that nutrition should be an important factor, which is comparable to economic value. We assessed the eco-efficiency score, integrating nutritional value, vitamin C content, and environmental impact (CO₂ emission). Results showed that elevated CO₂ treatment was 2.9-fold more efficient than conventional production. It was deemed that elevated CO₂ treatment could be an effective production system to enhance the vegetable nutritional quality and reduce environmental burden. However, there are still challenges. The growth and vitamin C increase should be verified at the field scale, because the growth experiment in this study was performed at the laboratory scale using a plant incubator. By integrating nutritional quality and environmental burden, eco-efficiency could be a useful environmental communication tool. It is expected that it should give incentives to producers and customers to raise the environmental awareness, and further produce value-added vegetables.

5. Conclusions

It is suggested that elevated CO₂ treatment could promote the growth and nutritional quality of spinach. Further, it could contribute to CO₂ abatement in the agriculture sector due to the use of waste CO₂ and enhanced growth. The eco-efficiency score indicated that elevated CO₂ treatment of spinach could increase nutritional quality while reducing environmental burden. We believe that eco-efficiency, integrating nutrition and environmental impact, could be useful to give incentive to customers and farmers, raising environmental awareness for customers and inspiring farmers to produce value-added vegetables. This study offers important preliminary data to further strengthen the long-term sustainability of the agriculture sector, integrating environmental impact and the nutritional quality of vegetables.