Species-Dependent Response of Brassica chinensis L. to Elevated CO₂ Gradients Influences Uptake and Utilization of Soil Nitrogen, Phosphorus and Potassium

Employing elevated CO₂ (eCO₂), similar to increasing atmospheric CO₂ in a greenhouse, is a common practice used to increase vegetable crop yields. However, the responses of nutrient availability, nutrient uptake and use efficiency in leafy vegetables to eCO₂ remain largely unknown. The plant biomass production, nitrogen (N), phosphorus (P) and potassium (K) contents, nutrient uptake, and soil enzymatic activities of three Brassica chinensis varieties of ‘Longpangqing’, ‘Heimeiren’ and ‘Qingjiangbai’ were thus assessed under ambient CO₂ (C0, 420/470 ppm, day/night) and eCO₂ (C1, C0 + 33.3%C0; C2, C0 + 66.7%C0; and C3, C0 + 100%C0) for 4 months. Biomass production was significantly enhanced in these three tested B. chinensis varieties along with eCO₂ gradients, with the highest biomass production under C3. Moreover, the eCO₂ significantly increased plant N and K uptake, P and K use efficiency, soil saccharase, urease, and phosphatase activities, but decreased plant P content, soil available N and K, and catalase activity. Under eCO₂, ‘Qingjiangbai’ exhibited higher biomass production and P and K uptake, without a decrease in soil available N and P. Consequently, more external fertilizer supplementation (especially N, followed by K) is required to maintain or promote Brassica chinensis yield under eCO₂, while these management strategies must be carefully considered for different crop varieties.