Effect of Elevated Carbon Dioxide on the Nutritional and Yield Constituents of Kale and Spinach: A Meta-Analysis

The meta-analysis examines the effects of eCO₂ on the growth, yield, and nutritional composition of two widely consumed leafy vegetables: kale (Brassica oleracea) and spinach (Spinacia oleracea). Following the Collaboration for Environmental Evidence (CEE) guidelines, we systematically reviewed studies that reported on the impacts of eCO₂ on these crops and conducted a meta-analysis to quantify the overall as well as sub-group responses via moderator analyses. A random-effects model was used to calculate effect sizes (Hedges' g), and confidence intervals (CI) were set at 95%. Our results reveal that eCO₂ significantly increased the biomass of spinach (g = 1.21, p < 0.01, CI [0.88, 1.54]) and kale (g = 0.97, p < 0.05, CI [0.65, 1.29]). However, the analysis also detected a significant decrease in protein content in both crops under eCO₂ conditions (spinach: g = -0.76, p = 0.03, CI [-1.10, -0.42]; kale: g = -0.61, p = 0.04, CI [-0.95, -0.27]). Additionally, calcium and magnesium concentrations declined in kale (g = -0.55, p = 0.05, CI [-0.89, -0.21]), and spinach showed a stronger reduction in nutrient content overall. The variability in response across different CO2 concentrations and exposure times further underscores the complexity of eCO2 effects. These findings suggest that while eCO₂ may enhance crop yields, it also leads to a dilution of essential nutrients, raising concerns about the trade-offs between productivity and nutritional quality. The study concludes that targeted breeding programmes, strategic precision agriculture, sustainable agricultural practices and policy interventions will be critical in mitigating the negative nutritional effects of eCO₂ to ensure food security in a changing climate.