Tomato yields and quality declines due to elevated soil CO2

Controlled CO₂ release device and soil type

The experiment was conducted using a CO₂ leakage control system. The system consisted of a CO₂ release device, a pot, and a CO₂ detector. CO₂ was injected into the soil at the bottom of the pot. The pots (100 cm high × 50 cm long × 50 cm wide) were divided into two sections by a gas-permeable separator. The upper Sect. (80 cm) comprised the soil chamber for plant growth (50 cm soil thickness), and the lower Sect. (20 cm) was an air chamber that released CO₂ gas. The soil used in the test pots was collected from topsoil in surrounding farmland. The regional soil is mainly tidal cinnamon soil (pH 8.38, organic matter 15.48 g kg⁻¹, total N 0.37 g kg⁻¹, total P 0.61 g kg⁻¹, and total K 20.42 g kg⁻¹). Before sowing, 75 g of base fertilizer (10 g urea, 50 g calcium superphosphate, and 15 g potassium sulfate including N 4.69 g, P 6 g, and K 7.5 g) was applied to each pot and watered with 15 L of water.

Planting and management

The experimental site was at the Shunyi Agro-Environmental Integrated Experimental Base (40° 13′N, 116° 14′E) of the Chinese Academy of Agricultural Sciences. The local area has a temperate continental monsoon climate and is in the transition between semi-arid and semi-humid, temperate, and mid-temperate regions. The annual average temperature, precipitation, and sunshine hours are approximately 11–12 °C, 640 mm, and 2000–2800 h, respectively²¹.

Marwa tomatoes from the Netherlands were used in this experiment. The tomatoes were plastic-plate dot sowed on June 28, 2020, and seedlings were transplanted in a cultivation box on July 19, 2020. One seedling was grown in each pot for three weeks.

CO₂ leakage treatment setting

The CO₂ injection flux (hereafter, flux) was used to measure CO2 leakage intensity. The flux of natural CO₂ seepage is 1000–3000 g m⁻²d⁻¹, and the upper limit tolerance threshold for maize is 2000 g m⁻²d^[−111. Based on this data, 1500 g m⁻²d⁻¹ CO₂ was selected to inject into the soil chamber of the pots with the tomato plants. The treatments were set up as leakage throughout the growth period (Leakage treatment). In addition, there was also one control, non-leakage treatment (CK treatment). Five replicates were performed for each treatment (Fig. 1). There was a two-meter gap between each treatment to prevent interference between the pots.

In the fourth week after transplanting, the tomato plants had four or five true leaves spread out, and CO₂ began to leak out. The CO₂ released to the WL group began on August 14, 2020, and the tomato harvest on November 21, 2020. The leakage flux was calculated as follows¹¹:

Where F is the CO₂ leakage flux (g m⁻² d⁻¹), v is the CO₂ injection rate (mL min⁻¹), ρ is the density of CO₂ under atmospheric pressure (approximately 1.977 g L⁻¹), and s is the cross-sectional area of the cultivation box (0.25 m²). Hence, the CO₂ injection rate was 132 mL min⁻¹.

Measurement and analysis

The plant height was recorded weekly. Tomato fruits were harvested on November 21, 2020. The number and weight of harvested fruits were recorded, and the maximum perimeter (n = 71 fruit) was measured. After harvest, the roots and stems were pretreated with high-temperature desiccation at 120 °C and dried at 80 °C to a constant weight. The aboveground (except the fruit) and underground biomass were weighed afterward. The response characteristics of tomatoes to CO₂ leakage were evaluated by comparing the differences in biomass, production, and nutritional quality between the leakage treatments.

The tomatoes were freeze-dried and sent to a testing center at the Institute of Agricultural Environment and Sustainable Development (Chinese Academy of Agricultural Sciences) to determine their soluble sugar, titratable acid, and ascorbic acid contents. Soluble sugar content was determined using colorimetry of anthrone-sulfuric acid following the agricultural chemical analysis methods (SSC-39.2.3). Ascorbic acid (vitamin C) content was determined using the 2,6-dichlorophenol indophenol (DCPIP) Association of Official Analytical Collaboration (AOAC) method²², and the titratable acid content was determined according to the AOAC method (942.15)²³.

Tomato fruit grading and taste assessment

The tomato fruits harvested will be graded according to the Industry Standards “Grades and specifications of tomatoes (NY/T 940–2006)” and “Tomatoes grading (GH/T 1193–2021)” issued by the Ministry of Agricultural and Rural Affair of People’s Republic of China and All-China Federation of Supply and Marketing Cooperative.

The sugar acid ratio refers to the ratio of soluble sugars to titratable acids in fruits, and is one of the essential reference indicators for measuring taste. It is generally believed that the appropriate sugar acid ratio for tomatoes is 6.9–10.8²⁴.

Here, R is sugar acid ratio, dimensionless; \(\:{S}_{Soluble}\) is soluble sugar, %; \(\:{A}_{Titratable}\) is titratable acid, %.

We determined the magnitude of the effect by comparing the means from the leakage and control groups. All experimental data were analyzed using the independent sample t-test in SPSS 19.0, and significance was indicated at p ≤ 0.05.