The impacts of CO2 enrichment on the plant resources utilization efficiency of Schima superba seedlings in subtropical China

The resources (light, nitrogen and water) utilization efficiency of plant is a key indicator reflecting the adaptive ability of plant to environment. CO₂ enrichment would increase photosynthesis substrate supply and nutrient absorption in plants，and may also change the utilization efficiencies of light (LUE), nitrogen (NUE) and water (WUE) and their trade-offs relationship. However, the knowledge regarding how the LUE, NUE and WUE of woody plant change in the context of CO₂ enrichment is still weak. In order to understand the impacts of CO₂ enrichment on the LUE, NUE and WUE of Schima superba and their trade-offs, one-year-old container seedlings of S. superba were grown with ambient air (AA treatment), 550 ppm of CO₂ concentration (E1-CO₂ treatment), 750 ppm of CO₂ concentration ( E2-CO₂ treatment) and 1000ppm of CO₂ concentration (E3-CO₂ treatment) using open top chambers. In the growing season, we regularly examined the net photosynthetic rate, stomatal conductance, transpiration rate, nitrogen concentration and photosynthetic pigment concentration of S. superba leaves. In addition, the different organ biomass, leaf area, soil nitrate and ammonium nitrogen concentrations were also simultaneously examined. The results demonstrate that three CO₂ enrichment treatments significantly increased the LUE and NUE of S. superba leaves at the end of June, while the leaf nitrogen concentration and soil nitrate nitrogen significantly decreased under both the E2-CO₂ and E3-CO₂ treatments compared with those under the AA treatment. In contrast, only the E1-CO₂ treatment significantly increased the LUE and NUE of S. superba leaves at the end of August. The NUE of S. superba leaves under both the E2-CO₂ and E3-CO₂ treatments were significantly higher than that under the AA treatment at the end of October. With regard to the WUE of S. superba leaves, there were no significant differences between the four treatments. At the end of October, the total biomass of S. superba under the E1-CO₂ treatment was significantly higher than that under both the AA and E3-CO₂ treatments, while the total biomass of S. superba under the AA treatment was not significantly different from that under both the E2-CO₂ and E3-CO₂treatments. During the experiment, the LUE, NUE, stomatal conductance, and transpiration rate of S. superba leaves were significantly and positively related to each other. The LUE also had a significantly positive correlation with specific leaf weight. Furthermore, the NUE was significantly and positively correlated with the total biomass and the ratio of underground and aboveground biomass. Meanwhile, the NUE was significantly and negatively correlated with the chlorophyll a concentration, chlorophyll b concentration, carotenoid concentration, leaf nitrogen concentration, soil ammonia nitrogen and nitrate nitrogen concentration. The WUE was significantly and negatively related to the stomatal conductance, transpiration rate and total biomass. CO₂ enrichment may enhance both the LUE and NUE of S. superba seedlings, whereas the impacts of CO₂ enrichment on the LUE and NUE of S. superba seedlings varied with time. S. superba seedlings would appear photosynthesis acclimation with the persistently high CO₂ enrichment.