Effects of Different Densities of Carbon Dioxide Generation Bags on Cucumber Growth and Yield

Abstract

Carbon dioxide (CO₂) is one of the important factors affecting vegetable yield in controlled environments. This study used cucumber as an experimental material to investigate the effects of hanging different amounts of CO₂ generation bags (CGBs) on the growth of temperature-loving vegetables under facility soil cultivation. CGBs of three different densities were set up: no application (TC), eight bags/265 m² (T1), and sixteen bags/265 m² (T2). The results showed the following: (1) Hanging CGBs at different densities significantly impacted indoor CO₂ concentration. Light, temperature, and humidity also affected CO₂ concentration to a certain extent. (2) The application of CGBs improved cucumber growth, photosynthesis, and quality-related indexes, resulting in a 28.9% increased yield compared to the control group. (3) The economic benefits of CGB application in each group were analyzed, revealing the economic benefits of high-density CGB cultivation on solar greenhouse cucumber. This study explored a low-cost and effective CO₂ generation application mode.

1. Introduction

As an important pillar of China’s agricultural industry, the vegetable industry supports the daily dietary needs of thousands of households. In 2022, 2.34 million hm² of vegetables had been planted nationwide [1]. Despite the high proportion of solar greenhouses and plastic greenhouses, the output of vegetables per unit area is still low. This is partially attributed to a poor ability to regulate the environment, leading to suboptimal crop growth. Among many environmental factors, CO₂ is a key factor affecting plant yield and is the carbon source of plant photosynthesis. Therefore, studying the regulation of the CO₂ environment holds significance in optimizing the growth and yield of greenhouse vegetables.

CO₂ concentrations in the atmosphere have increased from 287 μL/L in 1850 to 412 μL/L in 2020 [2]. Its concentration is expected to reach 700 μL/L by the end of the 21st century. The rise in global CO₂ concentration has an impact on crops [3]. For agricultural production, the increase in CO₂ concentration exerts positive effects, as CO₂ enrichment promotes plant morphogenesis and growth. Notably, high CO₂ concentrations have been shown to promote plant growth and yield [4]. Previous studies have reported the positive effects of CO₂ enrichment on plant height, leaf area, stem diameter, fresh weight, and specific leaf weight [5]. The photosynthetic rate of plant leaves increases, promoting the transportation of photosynthetic products to the root system, thereby promoting root development and improving water and nutrient absorption capacity. Additionally, CO₂ enrichment increases the growth rate of vegetables [6], enhances the assimilating ability of nitrate in plant roots, and improves nitrogen absorption efficiency [7,8].

Regulating CO₂ concentration to a suitable level has a positive impact on crop growth. Currently, the most commonly used CO₂ supplement method in facility agriculture in China is natural ventilation [9]. However, this method leads to heat loss and lower greenhouse temperatures in winter. Moreover, to improve air supply efficiency, many scholars have proposed and studied some facilities and equipment for CO₂ application, such as fermentation apparatus [10], small CO₂ generators that burn natural gas to produce CO₂ [11], augmentation devices that use zeolite to absorb surrounding CO₂ and release it near crops [12], and CO₂ rotating adsorption wheels [13]. Despite the development of these CO₂ application systems, they are rarely seen in China’s large number of solar greenhouses and plastic greenhouses. The slow adoption could be due to the expensive cost of the storage equipment and precision control system. On the other hand, the price of vegetables in China’s market is generally low, and the output per unit area is not high, limiting the economic income from planting fruits and vegetables. Considering the high investment and low income, the above-mentioned CO₂ replenishment methods in greenhouses are not financially feasible.

Cucumber is one of the most widely planted horticultural vegetable crops in China [14], and due to its huge cultivation area, distribution degree, and economic benefits, the study of cucumber can better reflect the application effect of CGBs in facility vegetable cultivation. Therefore, this study addresses the problem of CO₂ supplementation in greenhouse-cultivated cucumber by exploring a cost-effective method (hanging CGB). The hypothesis of hanging CGBs with different densities on the greenhouse CO₂ environment and the differences in cucumber growth, yield, quality, and economic benefits was explored. It provides a scientific reference for the rational and effective application of CGBs in greenhouses.

2. Materials and Methods

2.1. Experiment Site and Materials

The experiment was conducted from October to December 2023 in a soil wall Chinese solar greenhouse in Guoguan Village, Yangling (34°15′ N, 108°04′ E), Shaanxi Province. The cucumber variety was Jinyou No. 1 (produced by Tianjin Kerun Cucumber Research Institute). CO₂ was supplemented by hanging CGBs from Guosheng Biofertilizer Limited. The length of the experimental greenhouse was 100 m, with a span of 8 m, a ridge height of 3.5 m, and a back wall height of 2.8 m.

CGBs consist of two different components, namely the main material and the catalytic slow-release material, where the main material is ammonium bicarbonate, and the primary components of the catalytic slow-release material are humic acid, citric acid, and other weak acids. Prior to use, they are packaged separately in two small bags to prevent any reactions and the release of carbon dioxide. During the application process, the individual packages are opened, and the slow-release material is sprinkled into the main material and mixed evenly. The bags are then opened and hung in the greenhouse, where ammonium bicarbonate reacts with the acid to produce carbon dioxide, which is released into the greenhouse. CGBs continuously release CO₂ to the plants in the greenhouse until the reactants are completely consumed. After a certain reaction time, the raw materials are depleted, and the effect of CO₂ supplementation ends, leaving the remaining reaction residues in the bag. Figure 1 illustrates the usage scenarios of CGBs.

2.2. Experimental Design

Taking the different application densities of CGBs as the test factors, three compartments were designed for the experiment: no additional application (TC), no CGBs were used for air replenishment at all; at the low-density application level (T1), 8 CGBs were hung evenly over the 265 m² experimental area; at the high-density application level (T2), 16 CGBs were hung evenly over the 265 m² experimental area. Each CGB was hung at a height of 1.8 m in the greenhouse. About 1500 cucumbers were used as the total number of plants designed in the greenhouse; the cucumber stubble was planted only once during the winter growing and was not replicated. The entire solar greenhouse was divided into 3 compartments for CGB application density tests. Each compartment was partitioned with plastic film. In each compartment, the number of cucumber plants was about 500. A schematic diagram of the test site is shown in Figure 2.

CGBs were applied from the fruiting stage of the plants (when more than half of the plants were in the fruiting stage), and the CGBs were replenished and replaced every 20 days. The CGBs were continuously applied until the end of harvesting. The cucumber cultivation used in the experiment was based on soil cultivation and combined with mulching the ground surface with plastic film. Drip irrigation belts are used to irrigate crops in all compartments. Pruning, forking, and hanging vines are carried out according to conventional field management methods.

The CGB purchase expenditure and output improvement data of each region during the experiment period were counted, and the economic benefits of yield improvement were calculated by using the data of the agricultural wholesale market platform, and the arithmetic average of the unit area was taken as the unit area income after offsetting the economic benefits of output improvement and CGB purchase in each compartment.

2.3. Measurement Items and Methods

2.3.1. Environmental Parameters

The environmental parameters of each compartment were monitored and recorded by using CO₂ concentration sensors (measurement range 0~2000 μL/L, accuracy ±50 μL/L, Langhande Company, Jinan, China), temperature and relative humidity (RH) sensors (temperature measurement range −20~80 °C, accuracy ±0.5 °C, RH measurement range 0~100%, accuracy ±3%, Langhande Company, Jinan, China), and light sensors (measurement range 0~60 klux, accuracy ±5%, Full Scale, Langhande Company, Jinan, China). The illuminance value was converted into solar radiation using the conversion relationship 108 lux = 1 W/m² [15].

The CO₂ sensors were arranged in the center of each test compartment at a suspension height of 1.8 m. One temperature and humidity sensor and one solar radiation sensor were arranged in the center of the T1 test compartment at a suspension height of 1.8 m. The same new plastic film with consistent light transmittance was used in all three compartments of the greenhouse. Since the test areas are all in the same greenhouse, all three compartments of the greenhouse are made of new plastic films with the same light transmittance, and the T1 compartment is located in the middle of the greenhouse. Based on this approximation, the temperature, humidity and solar radiation levels in the center of the greenhouse are considered representative of the overall environmental level of the greenhouse. Therefore, a temperature, humidity, and light measurement point is set up in the center of the greenhouse to represent the overall environmental conditions of the greenhouse.

2.3.2. Pattern Indicators

A total of 15 plants of uniform growth were selected from each compartment (45 cucumber plants in total), and plant heights and stem diameters were measured every 14 days using a tape measure and vernier calipers.

2.3.3. Photosynthetic Indicators

On the 40th day after the addition of CO₂, the net photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), stomatal conductance (Gs), and transpiration rate (Tr) of functional leaves (from the top of the plant to the bottom of the 7th leaf) were measured using the LI-6800 photosynthetic analyzer (LI-COR, Lincoln, NE, USA). Meanwhile, the chlorophyll content (SPAD value) of functional leaves was determined using a SPAD-502 chlorophyll meter (Fangke, Weifang, China). In each compartment, 15 cucumber plants with uniform growth in the middle of the region (45 cucumber plants in total) were selected to measure their photosynthetic indexes.

2.3.4. Quality Indicators

In the experiment, cucumbers were harvested every 4 days after entering the harvest period, and the harvest data were recorded. A total of 15 plants were selected in each compartment (45 cucumber plants in total); cucumber fruits with uniform size and straight fruit shape were picked, and the transverse diameter, longitudinal diameter, and fruit stalk length of the fruits were measured with rulers and vernier calipers. After harvesting, the ascorbic acid content of cucumber fruits under different treatments was determined by the molybdenum blue colorimetric method [16]. Finally, the soluble protein content of cucumber fruits was determined by the Coomassie brilliant blue G-250 staining method [17]; ascorbic acid and protein are measured on fresh weight basis.

2.3.5. Yield Indicators

Data from five harvests were collected within 30 days following the addition of CGBs, and 15 plants with uniform growth from each compartment were selected for weighing. Only the number of fruits per plant when the harvest criteria are met is counted, and the average number of fruits in each area is calculated. The weight of single fruits was measured using a small electronic scale (range of 0.1 g~3 kg, accuracy ± 0.1 g, Dr. Me, Yancheng, China) to count the yield of each plant for harvest, and the yield of each compartment was calculated according to the planting density.

2.4. Data Processing and Analysis

The experimental data were plotted using Excel 2016 software, and the difference significance analysis (p < 0.05) of cucumber parameters was conducted using IBM SPSS Statistics 26.0 software. Pearson correlation analysis (p < 0.05) was used to analyze the correlation between CO₂ concentration differences and other environmental parameters. Microsoft Excel 2016 software was used to draw the environmental suitability scoring model, AHP analytic hierarchy process was used to calculate the weights of each index, and finally Microsoft Excel 2016 software was used to calculate the comprehensive evaluation value according to the TOPSIS method. In this study, a one-way ANOVA was used to test for significant differences between groups of data, with the Dunnett method used to determine significant differences between groups, which were labeled using alphabetic marking.

To ensure that the effect of CGB concentration in the correlation analysis is not affected by the subsequent replacement CGBs, the greenhouse environmental parameters of the first batch of CGBs (October 24 to October 31) were monitored. Linear correlation analysis was performed on the average greenhouse temperature, average RH, cumulative solar radiation, and the difference in CO₂ concentration between the T2 area and TC area from 10 a.m. to 2 p.m. during the day.

The analytic hierarchy process (AHP) can measure the relative importance of each index to construct a judgment matrix. In this study, the AHP was used to comprehensively evaluate the greenhouse environment, and the comprehensive suitability of each day was divided into three index layers: daily average temperature suitability (C₁), daily average RH suitability (C₂), and cumulative solar radiation suitability (C₃). Based on the R² level of correlation CO₂ concentration difference C₁, C₂, and C₃, the judgment matrix was constructed, as shown in Table 1.

The results of the weight calculation are displayed in Table 2. The coefficient calculated from the consistency test was less than 0.10, indicating good consistency, and showing that the established judgment matrix was reliable and reasonable.

After obtaining the weight indicators of each environmental parameter, the environmental indicators were uniformly converted into maximum indicators, and the positive matrix was standardized. The Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method was employed to form a weighted judgment matrix, and the ideal solution of the index was calculated and evaluated. Due to the unit, order of magnitude, or trend differences between various environmental parameters, the extreme value standardization method was used for the dimensionless transformation of the environmental data. The data were normalized to [0, 1] and calculated according to Equation (1) [18].

(1)$U_{in}={\displaystyle \frac{X_{in}-X_{imin}}{X_{imax}-X_{imin}}}$

where $U_{in}$ represents the normalized membership function value of the nth index of the ith parameter, $X_{in}$ indicates the original measured value of the nth index of the ith parameter, $X_{imax}$ denotes the maximum value of the ith index in the environmental parameter, and $X_{imin}$ represents the minimum value of the ith index in the environmental parameter.

3. Experiment Results

3.1. Effect of CGBs of Different CO₂ Densities in the Greenhouse

3.1.1. Variation of CO₂ Concentration in Typical Weather

Figure 3 shows the difference in solar radiation and temperature in a greenhouse on a typical sunny and cloudy day. On a typical sunny day (1 November 2023), the light and temperature conditions in the greenhouse were ideal, with a high overall solar radiation intensity in the greenhouse, reaching a maximum of 619 W/m². The temperature rose from around 10 a.m. to over 30 °C and lasted until around 5 p.m. On this typical sunny day, the average daytime temperature in the greenhouse reached 35 °C. In contrast, on a typical cloudy day (4 November 2023), the intensity of solar radiation in the greenhouse after sunrise was low, with a maximum value of only 145 W/m² and a short duration. The temperature in the greenhouse did not exceed 30 °C throughout the day, and the average daytime temperature was only 19 °C.

Figure 4 shows the difference in the CO₂ replenishment effect of hanging CGBs in a solar greenhouse under typical sunny and cloudy conditions. On a sunny day (1 November 2023), in the morning, the CO₂ concentration in the greenhouse decreased rapidly and remained at 500 μL/L. At about 11 o’clock, the CO₂ concentration in the T1 and T2 test areas continued to rise, and the highest concentration in the T2 area reached 854 μL/L, whereas the highest concentration in the T1 area was 706 μL/L. The CO₂ concentration in the two recharge zones was about 200–300 μL/L higher than that in the TC zone during the day. The high CO₂ enrichment effect lasted for nearly 5 h, until it gradually weakened and returned to the baseline level after 17 o’clock. On a cloudy day (4 November 2023), the CO₂ concentration in the greenhouse also decreased rapidly after sunup, but the difference between the CO₂ concentration in the T1 and T2 areas and the TC area was smaller than that on sunny days. The highest concentration in the T2 area was only 687 μL/L, while the highest concentration in the T1 area was 591 μL/L. The CO₂ concentration in the two recharge zones was only about 150–250 μL/L higher compared to the TC area during the day. Moreover, the high CO₂ enrichment effect lasted only for about 3 h and began to weaken after 3 pm and gradually returned to the natural level.

3.1.2. The Daily Average CO₂ Concentration in Solar Greenhouses Varies Continuously

Figure 5 illustrates the environmental parameters of the solar greenhouse in the first 20 days after the start of the gas replenishment test (the first batch of CGB application cycle) and the difference of CO₂ concentration between T2 compared with the TC area from 10 a.m. to 2 p.m. during the day. Taking the single environmental parameter of cumulative solar radiation as an example, a daily cumulative solar radiation of less than 1 MJ/m² (5 November 2023 and 10 November 2023) resulted in a small difference in the daily average CO₂ concentration between the T2 areas and the TC area, indicating that low levels of solar radiation affect the gas production of the CGBs. However, on 3 November 2023, the daily cumulative solar radiation was 1.05 MJ/m², and the difference in the CO₂ concentration between the T2 area and the TC area reached 227 μL/L. On 29 October 2023, the highest cumulative solar radiation level was recorded (7.59 MJ/m²), and the difference in CO₂ concentration between the T2 area and the TC area reached 332 μL/L, indicating that high levels of solar radiation improve the gas production of the CGB.

3.1.3. Correlation Analysis of CO₂ Concentration Changes

The data in Figure 6 revealed a correlation between CO₂ concentration difference and greenhouse microclimate environmental factors in the T2 area and the TC area. The difference in the daily average CO₂ concentration was positively correlated with the average temperature and cumulative solar radiation from 10 a.m. to 2 p.m. in solar greenhouses, while negatively correlated with RH from 10 a.m. to 2 p.m.

The R² level of the CO₂ concentration difference in different environmental parameters was evaluated in the correlation analysis. The results indicated that the suitability of cumulative solar radiation was slightly higher than that of daily average RH, and the suitability of daily average RH was slightly higher than that of daily average temperature suitability.

The R² parameters in the correlation analysis were all at a low level. Therefore, a single environmental parameter cannot completely explain the difference in CO₂ concentration. To further analyze the comprehensive relationship between the increase in CO₂ concentration and greenhouse environmental parameters in the experimental area, the suitability of the environmental parameters in the greenhouse was classified according to production experience, including temperature [19], RH [20], and cumulative solar radiation [21], as shown in Table 3. Hence, comprehensive evaluation indicators were established.

The data in Figure 7 show that the correlation coefficient between the environmental suitability score and the difference in CO₂ concentration is much higher than that between single environmental parameters and the difference in CO₂ concentration. This finding indicates that the comprehensive suitability score of environmental factors can reflect the effect of CGBs to a certain extent compared with the evaluation of a single environmental parameter. Maintaining a high overall environmental score in the greenhouse and replacing CGBs regularly and reasonably are key factors in ensuring the effectiveness of the CGBs.

3.2. Effects of Different Densities of CGBs on the Growth, Yield, and Quality of Cucumber, and Economic Benefits

3.2.1. Effect of Different Densities of CGBs on Cucumber Plant Height and Stem Diameter

According to the significance analysis results shown in Figure 8, the plant height of cucumber in the T1 and T2 areas treated with CGBs was significantly higher than that in the TC area. The T2 area had the highest CO₂ concentration and reached the highest average plant height of 205.3 cm among all groups. The plant height difference between the T2 group and the TC group increased gradually with plant growth, suggesting that hanging CGBs significantly promoted the growth of cucumber plant height.

In addition, the stem diameter in the T1 and T2 areas was significantly higher than that in the TC area. In the T2 area, which had the highest CO₂ concentration, the stem diameter reached a maximum of 1.07 cm, while the stem diameter growth in the TC area was relatively slow. These results indicated that the increase in CO₂ concentration promoted the stem diameter growth of cucumber. The difference in stem diameter between T2 treatment and TC treatment gradually widened as the plants grew, further proving that hanging CGBs significantly enhanced the stem diameter growth index of cucumber.

3.2.2. Effect of Different Densities of CGB on Photosynthetic Parameters of Cucumber

Figure 9 shows the significant difference in leaf chlorophyll content between TC treatment and T1 and T2 treatments in the experiment of cucumber application of CGBs. The chlorophyll content of cucumber showed an upward trend with the increase in CO₂ concentration.

In Figure 10, the net photosynthetic rate of T1 and T2 treatments was slightly higher than that of TC, but the difference was not significant, and the net photosynthetic rate was 2.65 and 2.69 μmol/(m²·s) higher than that of the TC group, respectively. However, significant differences in the intercellular CO₂ concentrations were found among the treatment groups. The T1 and T2 concentrations of the supplemental treatment were significantly higher than those of TC, with intercellular CO₂ concentrations of 326, 479, and 686 μL/L, respectively. The stomatal conductance of T2 was significantly lower than that of TC and T1 among different treatments, and the stomatal conductance level of T2 was only 1.12 μmol/(m²·s). However, no significant difference in the stomatal conductance level was observed between T1 and TC. Additionally, the transpiration rate of TC was higher than that of T1 and T2. The transpiration rate of T2 was the lowest, only 0.52 mmol/(m²·s). According to the analysis of various parameters, the net photosynthetic rate and intercellular CO₂ concentration in the T1 and T2 areas were higher than those of the TC group. Furthermore, the stomatal conductance and transpiration rate were reduced by the gas supplementation treatment.

3.2.3. Effects of Different Densities of CGB on the Yield and Quality of Cucumber

According to the cucumber fruit quality indexes shown in Table 4, the transverse diameter in the T2 treatment group was significantly higher than that of the TC group. In addition, the longitudinal diameter and fruit stalk length of the T1 and T2 treatments were also significantly higher than those of the TC group. These results showed that hanging CGBs led to larger transverse diameter and longitudinal diameter in cucumber fruits, while also increasing the length of the fruit stalk. In addition to the appearance quality, the ascorbic acid and protein contents of T2 treatment were significantly higher than those of the TC group, indicating that high-density hanging CGBs could enhance the quality indexes of cucumber fruits.

According to the cucumber yield results shown in Table 5, the application of CGBs had a significant effect on cucumber yield. The per weight of a single cucumber in the T2 area was 31.2 g higher than that in the TC area, showing a weight increase of 17.46%. Moreover, the T2 group exhibited a 0.28 kg higher yield per plant compared to the TC group, corresponding to an increase of 28.87%. Compared to the TC group, the T1 and T2 areas demonstrated a 13.57 and 22.78 t/hm² increase in total yield, respectively. The T2 area increased by 28.89% compared with the control TC. These results showed that the application of hanging CGBs significantly increased cucumber yield.

3.2.4. Effects of Different Densities of CGB on the Economic Benefits of Cucumber

The relevant yields obtained under different experiments were calculated, and the economic benefits of CGB application were evaluated. As displayed in Table 6, these results confirmed that the gas recharge benefit of T2 treatment was the highest, demonstrating that hanging CGBs at high density can yield larger gas recharge benefits.

4. Discussion

The use of CGBs can increase the CO₂ concentration in the greenhouse to a certain extent. However, the air replenishment effect of this method has limited controllability, and suitable environmental conditions such as temperature, humidity, and light are required to achieve a higher concentration of CO₂, so the environmental conditions on sunny days can produce better CGB effects than on cloudy days. Compared with the use of CGBs, the CO₂ concentration in the greenhouse can be stabilized at about 800 μL/L by burning natural gas to supplement CO₂ [6], and the CO₂ concentration can be maintained at 1000–1500 μL/L by compost fermentation [10]. In contrast, the maximum CO₂ concentration of CO₂ supplementation with CGBs reached about 1000 μL/L and had a relatively short duration. Moreover, a single cycle of CGBs does not maintain a high concentration of CO₂ for a long time, and it needs to be replenished and replaced regularly.

Environmental factors have a significant impact on the use of CGBs, as evidenced by the difference in CO₂ concentrations between typical sunny and typical cloudy days. Under typical weather conditions, the change in CO₂ concentration showed the same trend as the change in environmental parameters in the greenhouse. Under high temperatures and high solar radiation intensity, the CGBs were consumed to produce more CO₂. Since these two typical weather times are only 3 days apart, it is unlikely that the gas production of the CGBs will vary greatly due to the consumption of gas-producing substances. However, due to the significant differences in environmental parameters, the maximum CO₂ concentrations in the T1 and T2 areas differed by 115 and 167 μL/L, respectively. These findings indicated that the differences in environmental parameters under different weather conditions significantly affect the gas production of CGBs. Higher temperatures and stronger light can increase the reaction rate of the substrate in CGBs, resulting in higher CO₂ concentrations, and conversely, excessive humidity will reduce the amount of CO₂ produced by CGBs. Therefore, CGBs will work better when used in greenhouses with a more comfortable environment. By constructing environmental score, we provide a method and idea for evaluating and improving the CO₂ replenishment effect of CGBs.

Similar to the effect of other types of aeration on crops, hanging CGBs can promote the growth and development of crops and improve their growth indicators. Additionally, the photosynthetic parameters of crops showed that the increase in CO₂ concentration effectively increased the net photosynthetic rate and intercellular CO₂ concentration of crop leaves, and reduced the transpiration rate and stomatal conductance, and improved the quality indexes of crops to a certain extent. In summary, the effect of gas supplementation was similar to that obtained in previous experiments [22,23,24].

At present, CGBs are difficult to be accurately controlled in use, and are only suitable for more extensive culture mode. There will be substrate residue in the bag after use, which needs to be recycled, which limits the practicability of CGBs. On the other hand, CGBs require no installation, floor space, or upfront investment. Compared to the traditional CO₂ replenishment method, the cost of a CO₂ replenishment remote control device using a liquefied gas cylinder is about 20,000 yuan [25], while the CGB input is only 16 yuan per square meter. Therefore, the use of CGBs has the advantages of ease of use, low space requirements in the planting area, and low cost, which is more acceptable to farmers. In addition, due to its convenience, it has important application and promotion value in environments where it is difficult to add CO₂ supplementation facilities, such as small greenhouses, glass greenhouses, and solar greenhouses.

In the future, we will consider starting from the direction of environmental regulation and substrate utilization. By coupling CGBs with ventilation, and combining the form of reactive substrate topdressing, we try to regulate its use effect and improve the utilization efficiency of CGBs.

5. Conclusions

In this study, CGBs of different densities were applied in the greenhouse, and the aeration effect of CGBs was evaluated from three aspects: indoor CO₂ concentration, cucumber growth, and economic benefits, and a comprehensive score of environmental factors was established based on the influence of environmental parameters to reflect the aeration effect of CGBs. CGBs exhibit different CO₂ concentrations depending on their suitability to the environment. Correlation analysis showed that the environmental score was positively correlated with the increase in CO₂ concentration in the high concentration aeration area. In addition, the application of CGBs had a positive effect on the growth, photosynthesis, and quality indexes of cucumber. Furthermore, the yield of cucumber with CGBs increased by 28.9% compared with the control, and the application of CGBs had a positive effect on the economic benefits of cucumber.

Author Contributions

Conceptualization, Y.C. and H.S.; Data collection, Y.L., S.Z. and J.H.; Figures, S.Z.; Methodology, S.G.; Resources, Y.C. and H.S.; Supervision, Y.C.; Writing—original draft, Y.L.; Writing—review and editing, Y.L., S.Z., Y.C., J.H. and H.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Components and applications of CGBs.

Figure 2. Schematic diagram of the layout of CGBs and sensors. No application (TC), 8 bags/265 m2 (T1), and 16 bags/265 m2 (T2).

Figure 3. Variation of environmental parameters in typical weather in a Chinese solar greenhouse.

Figure 4. Changes in CO2 concentration in typical weather in a Chinese solar greenhouse.

Figure 5. Continuous variation of environmental parameters and CO2 concentration. Note: T represents the average temperature, CS represents the cumulative solar radiation, RH represents the average RH, and CO2 represents the difference in CO2 concentration between the T2 compartment and the TC compartment.

Figure 6. Correlation analysis of CO2 concentration difference.

Figure 7. Correlation analysis of CO2 concentration difference and suitability score.

Figure 8. Changes in plant height and stem thickness in cucumber CGB test. TC—no application, T1—8 bags/265 m2, and T2—16 bags/265 m2. Note: According to Duncan’s multiple range test, different letters indicate significant differences at p [less than] 0.05.

Figure 9. Histogram of chlorophyll content in cucumber. Note: According to Duncan’s multiple range test, different letters indicate significant differences at p [less than] 0.05. No application (TC), 8 bags/265 m2 (T1), and 16 bags/265 m2 (T2).

Figure 10. Histogram of cucumber leaf photosynthetic indexes. Note: According to Duncan’s multiple range test, different letters indicate significant differences at p [less than] 0.05. No application (TC), 8 bags/265 m2 (T1), and 16 bags/265 m2 (T2).

Table 1

AHP judgment matrix.

	C1	C2	C3
C1	1/3	1	1/5
C2	1	3	1/3
C3	3	5	1

Table 2

Weights of each environmental parameter.

C1	C2	C3
0.10313	0.29148	0.60538

Table 3

Classification table of environmental parameter factors affecting the suitability of CGBs in greenhouses.

Environmental Parameters	Suitable	Suboptimal	Inappropriate
Average temperature (°C)	20 ≤ T ≤ 25	10 ≤ T < 20, 25 < T ≤ 33	T < 10, T > 33
Average RH (%)	50 ≤ RH ≤ 70	40 ≤ RH < 50, 70 < RH ≤ 85	RH < 40, RH > 85
Cumulative solar radiation (MJ/m2)	6 ≤ S ≤ 8	4 ≤ S < 6, 8 < S ≤ 10	S < 4, S > 10

Table 4

Fruit quality indexes of cucumber based on different CO₂ concentrations.

Treatment	Transverse Diameter (cm)	Longitudinal Diameter (cm)	Fruit Stalk Length (cm)	Ascorbic Acid (mg/100 g)	Protein (mg/g)
TC	3.02b	26.7b	5.93b	14.23b	1.88b
T1	3.21ab	30.05a	7.62a	15.64b	1.91b
T2	3.32a	32.03a	8.73a	19.7a	1.98a

Note: According to Duncan’s multiple range test, different letters indicate significant differences at p < 0.05.

Table 5

Cucumber yield with different CO₂ concentrations.

Treatment	Cucumber Per Weight(g)	Number of Cucumbers Per Plant (Units)	Cucumber YieldPer Plant (kg)	Cucumber Yield Per Square Metre (kg/m2)
TC	178.6b	5.44a	0.97b	2.23b
T1	199.8a	5.71a	1.14a	2.62a
T2	209.8a	5.98a	1.25a	2.88a

Note: According to Duncan’s multiple range test, different letters indicate significant differences at p < 0.05.

Table 6

Analysis of the economic benefits of applying different concentrations of CGB.

Treatment	IncreasedProduction(kg/m2)	Expenditures(Yuan/m2)	Replenishment Benefits (Yuan/m2)
TC	0	0	0
T1	2.34	0.54	8.82
T2	3.90	1.08	14.52

Note: The cucumber price data in this study mainly originated from the national key agricultural product market information platform. The wholesale price of cucumber in the Yangling area in March 2024 was about 4 yuan/kg.

References

1. Li, T.L.; Qi, M.F.; Meng, S.D. Sixty Years of Facility Horticulture Development in China: Achievements and Prospects. Acta Hortic. Sin.; 2022; 49, pp. 2119-2130. [DOI: https://dx.doi.org/10.16420/j.issn.0513-353x.2022-0700]

2. Goosse, H.; Barriat, P.Y.; Brovkin, V.; Klein, F.; Meissner, K.; Menviel, L.; Mouchet, A. Changes in atmospheric CO₂ concentration over the past two millennia: Contribution of climate variability, land-use and Southern Ocean dynamics. Clim. Dyn.; 2022; 58, pp. 2957-2979. [DOI: https://dx.doi.org/10.1007/s00382-021-06078-z]

3. Tursun, A.Ö.; Jabran, K.; Gürkan, H.; Telci, I. Effects of elevated temperature and carbon dioxide concentrations on aromatic compounds of Stevia rebaudiana. Sugar Tech; 2021; 23, pp. 941-948. [DOI: https://dx.doi.org/10.1007/s12355-020-00932-w]

4. Mochizuki, Y.; Murakami, S.; Kobayashi, T.; Worarad, K.; Yonezu, Y.; Umeda, H.; Okayama, T.; Inoue, E. Local CO₂ application within strawberry plant canopy increased dry matter production and fruit yield in summer and autumn culture. Int. J. Fruit Sci.; 2022; 22, pp. 675-685. [DOI: https://dx.doi.org/10.1080/15538362.2022.2103063]

5. Esmaili, M.; Aliniaeifard, S.; Mashal, M.; Ghorbanzadeh, P.; Seif, M.; Gavilan, M.U.; Carrillo, F.F.; Lastochkina, O.; Li, T. CO₂ enrichment and increasing light intensity till a threshold level, enhance growth and water use efficiency of lettuce plants in controlled environment. Not. Bot. Horti Agrobot. Cluj-Napoca; 2020; 48, pp. 2244-2262. [DOI: https://dx.doi.org/10.15835/nbha48411835]

6. Singh, H.; Poudel, M.R.; Dunn, B.L.; Fontanier, C.; Kakani, G. Effect of greenhouse CO₂ supplementation on yield and mineral element concentrations of leafy greens grown using nutrient film technique. Agronomy; 2020; 10, 323. [DOI: https://dx.doi.org/10.3390/agronomy10030323]

7. Bloom, A.J.; Kasemsap, P.; Rubio-Asensio, J.S. Rising atmospheric CO₂ concentration inhibits nitrate assimilation in shoots but enhances it in roots of C3 plants. Physiol. Plant.; 2020; 168, pp. 963-972. [DOI: https://dx.doi.org/10.1111/ppl.13040] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31642522]

8. Du, B.; Shukla, M.; Yang, X.; Du, T. Enhanced fruit yield and quality of tomato by photosynthetic bacteria and CO₂ enrichment under reduced irrigation. Agric. Water Manag.; 2023; 277, 108106. [DOI: https://dx.doi.org/10.1016/j.agwat.2022.108106]

9. Yuan, R.; Nie, L.Y.; Hao, X.Y.; Sun, S.; Zong, Y.Z.; Li, P. Effects of elevated [CO₂] on photosynthesis and relevant physiological characteristics of pepper (Capsicum annuum L.). Chin. J. Ecol.; 2017; 36, pp. 3510-3516. [DOI: https://dx.doi.org/10.13292/j.1000-4890.201712.013]

10. Karim, M.F.; Hao, P.; Nordin, N.H.; Qiu, C.; Zeeshan, M.; Khan, A.A.; Wu, F.; Shamsi, I.H. CO₂ enrichment using CRAM fermentation improves growth, physiological traits and yield of cherry tomato (Solanum lycopersicum L.). Saudi J. Biol. Sci.; 2020; 27, pp. 1041-1048. [DOI: https://dx.doi.org/10.1016/j.sjbs.2020.02.020]

11. Li, Y.J.; Wang, S.J.; Zhu, H.C.; Xie, J.; Yu, Z.J. Comparison on increasing effect and economic benefit of three kinds of CO₂ increasing equipment for celery. Agric. Eng.; 2015; 5, pp. 55-59.

12. Wang, A.; Wang, H.; Lv, J.; Liang, X.; Ying, H.; Wang, L.; Yu, J.; Xiao, F.-S.; Shi, K. Zeolite enhances leaf photosynthesis and fruit quality in tomato through greenhouse CO₂ enrichment. J. Clean. Prod.; 2024; 444, 141276. [DOI: https://dx.doi.org/10.1016/j.jclepro.2024.141276]

13. Tang, C.; Gao, X.; Shao, Y.; Wang, L.; Liu, K.; Gao, R.; Che, D. Investigation on the rotary regenerative adsorption wheel in a new strategy for CO₂ enrichment in greenhouse. Appl. Therm. Eng.; 2022; 205, 118043. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2022.118043]

14. Wang, Z.; Wang, S.; Bian, T.; Wang, T.; Fu, H.; Sun, Z. Revealing the Response of Cucumber Soil Microbial Community Composition and Function to Nitrogen Addition in Northern Chinese Greenhouses. Horticulturae; 2024; 10, 1090. [DOI: https://dx.doi.org/10.3390/horticulturae10101090]

15. Stevens, R.; Buret, M.; Garchery, C.; Carretero, Y.; Causse, M. Technique for rapid, small-scale analysisof Vitamin C levels in fruit and application to a tomato mutant collection. J. Agric. Food Chem.; 2006; 54, pp. 6159-6165. [DOI: https://dx.doi.org/10.1021/jf061241e]

16. Petre, S.N.; Pele, M.; Draghici, E.M. Influence of perlite and jiffy substrates on cucumber fruitproductivity and quality. J. Agric. Sci.; 2015; 7, pp. 185-196. [DOI: https://dx.doi.org/10.5539/jas.v7n8p185]

17. Mao, J.X. Conversion between solar irradiance and illuminance. Henan Agric. Sci.; 1995; 1, pp. 11-12.

18. Zheng, Y.; Yang, Z.; Wei, T.; Zhao, H. Response of tomato sugar and acid metabolism and fruit quality under different high temperature and relative humidity conditions. Phyton-Int. J. Exp. Bot.; 2022; 91, pp. 2033-2054. [DOI: https://dx.doi.org/10.32604/phyton.2022.019468]

19. Li, B.S.; Wang, Q.; Liu, W.Q.; Shao, M.J. Effects of substrate-embedded cultivation on root-zone temperature and growth and yield of tomato in solar greenhouse. Shandong Agric. Sci.; 2021; 53, pp. 24-28. [DOI: https://dx.doi.org/10.14083/j.issn.1001-4942.2021.02.005]

20. Liu, F.H.; Guo, S.B.; Wang, D.; Huang, B.; Cao, Y.F. Construction and verification of an external morphology, substance accumulation, and distribution model of tomatoes in greenhouses. Trans. Chin. Soc. Agric. Eng.; 2022; 38, pp. 188-196.

21. Li, L.; Li, W.J.; Ma, D.X.; Yang, C.; Meng, F. Prediction model of transpiration of greenhouse tomato based on LSTM. Trans. Chin. Soc. Agric. Mach.; 2021; 52, pp. 369-376.

22. Pan, T.; Ding, J.J.; Qin, G.G.; Wang, Y.; Xi, L.; Yang, J.; Li, J.; Zhang, J.; Zou, Z. Interaction of supplementary light and CO₂ enrichment improves growth, photosynthesis, yield, and quality of tomato in autumn through spring greenhouse production. HortScience; 2019; 54, pp. 246-252. [DOI: https://dx.doi.org/10.21273/HORTSCI13709-18]

23. Wang, X.Y. Response of Cucumber and Cherry Tomato Growth to CO₂ Enrichment and LED Light Supplement in Solar Greenhouse. Bachelor’s Thesis; Ningxia University: Ningxia, China, 2022; [DOI: https://dx.doi.org/10.27257/d.cnki.gnxhc.2022.001237]

24. Han, X.T.; Sun, Y.; Chen, J.Q.; Wang, Z.; Qi, H.; Liu, Y.; Liu, Y. Effects of CO₂ enrichment on carbon assimilation, yield and quality of oriental melon cultivated in a solar greenhouse. Horticulturae; 2023; 9, 561. [DOI: https://dx.doi.org/10.3390/horticulturae9050561]

25. Sheng, S.; An, H.Y.; Liu, W. Influence of CO₂ increasing technology on the growth of lettuce in solar greenhouse. J. Chin. Agric. Mech.; 2017; 38, pp. 52-55. [DOI: https://dx.doi.org/10.13733/j.jcam.issn.2095-5553.2017.01.011]