Short-term responses of Inga edulis Mart. seedlings growing under elevated CO2 and phosphorus addition: understanding potential phosphorus constraints on plant responses to elevated CO2 in the understory of a central Amazon forest

The increase in atmospheric CO₂ concentration positively affects plant carbon assimilation and carbon stock in different biomes. However, there are uncertainties regarding how plants in tropical forests, especially in the Amazon rainforest, will respond to this increase, since a large part of the soils in the region present natural low phosphorus (P) availability, which could constrain positive effects of elevated CO₂. Here, we investigated if P addition would interfere on leaf primary carbon metabolism and aboveground development responses under elevated CO₂. For that, we used 46  seedlings of Inga edulis Mart., a native leguminous nitrogen-fixing species, exposed for 10 months (November 2019 - September 2020) to CO₂ and P treatments. Plants grew in pots - half with natural P availability (-P) and half with P addition (+P) -, inside CO₂ enrichment chambers - half with ambient CO₂ (aCO₂) and half with elevated CO₂ (aCO₂ + 200 ppm; eCO₂), - in the understory of a primary forest in Central Amazonia, Manaus, Brazil.  A factorial experimental design was used, with 11-12 plants for each treatment: aCO₂-P (control), aCO₂+P, eCO₂-P and eCO₂+P. To assess the carbon metabolism, we measured light-saturated net CO₂ assimilation (A_sat), leaf respiration in the light (R_light), leaf respiration in the dark (R_dark) and photorespiration (P_R). To assess aboveground development, we measured plant height and diameter, crown height and diameter,  number of leaves and total leaf area. We found that eCO_2, regardless of P availability, significantly increased A_sat and R_light, while decreasing R_dark and A_sat:R_dark ratio, but it did not affect P_{R .} Those results suggest that seedlings indeed assimilated more carbon under eCO₂. However, irrespective of CO₂ treatment, +P significantly increased aboveground responses. Under P addition, plants showed greater height and greater crown development (higher crown height and diameter and larger leaves) compared to control or eCO₂-only. Plant diameter and number of leaves did not respond to any treatment. We did not find differences between +P seedlings under different CO₂ treatments (aCO₂+P and eCO₂+P), indicating that only P had an effect on these responses. Still, there were substantial changes on some of the aboveground responses between these treatments, particularly in total leaf area which increased 60% (aCO₂+P) and 126% (eCO₂+P) compared to control. Overall, we observed a distinguished pattern, in which eCO₂ mainly affected physiological responses, while P addition consistently affected aboveground development. The lack of response of aboveground components under eCO₂ suggests that the extra carbon assimilated was not necessarily used to aboveground development as shown by many studies. Our findings indicate that, in the short-term, eCO₂ is highly important in determining changes in plant metabolism whereas it has little impact on growth, even when nutrient limitation is alleviate. However there is still need to understand if such responses will persist in the long-term and in other species, as these processes are key factors in determining forest responses to climate change.