The role of elevated CO2 and phosphorus addition in aboveground biomass and functional traits of Inga edulis Mart. seedlings in a Central Amazon understory

The increase in atmospheric CO₂ may lead to greater carbon uptake by plants and a subsequent increase in forest biomass stock. However, there are no direct field-based studies that assess this hypothesis in the Amazon forest, where the naturally low phosphorus (P) availability in the soil might reduce the assumed increase in biomass stock. Here, we evaluated the response of Inga edulis Mart. seedlings, a native N-fixing tree species, to elevated CO₂ and P addition in a set of open-top chambers (OTCs) with a CO₂ enriched atmosphere of 200 ppm above the ambient concentration, installed in the understory of a forest in the central Amazon. Inside each OTC (four controls with ambient CO₂ - aCO₂, and four treatments with +200ppm CO₂ - eCO₂), six pots were installed with seeds of I. edulis, three in natural soil and three in fertilized soil with P concentration of 600mg/kg, resulting in 12 pots for each treatments (aCO₂; eCO₂; eCO₂+P; +P). After two years (November 2019 - September 2021), we measured the total aboveground biomass, stem diameter at the base (mm), plant height (cm), stem density (g.cm^-3), leaf area (cm²), specific leaf area (SLA, cm².g^-1), thickness (µm), stomatal density, and chlorophyll content index (CCI). Irrespective of +P, SLA was significantly lower under eCO₂, but none of the other parameters analysed showed a strong response to eCO₂. Under +P treatment the number of leaves, leaf area, height, and total biomass increased significantly compared to control, but increased less than under eCO₂+P. Height was also higher under +P when compared to eCO₂. However, eCO₂ in combination with P addition (eCO₂+P) had major effects on aboveground biomass and leaf area, leading to a significant and strong increase of total leaf area (377%), aboveground plant biomass (373%), number of leaves (187%), total height (164%), and also a decrease in the CCI compared to controls and eCO₂ (except for the number of leaves, which was different only compared to the control), suggesting that the response to eCO₂ is limited by P availability. No significant changes were observed in stomatal density, leaf thickness, stem density, and stem diameter. Our findings suggest a directional shift of photosynthetic tissue under eCO₂ towards greater resource conservation through higher investment of dry mass per area (lower SLA), increasing protection against herbivores, pathogens, and mechanical damage. Despite the total biomass also increased under +P (203%), the biomass allocation to leaves and trunk resulted in a greater accumulation of biomass under eCO₂+P (373%). Nevertheless, the eCO₂+P supply provided a clear change in the resource use strategy, with plants investing mainly in light-capture-related traits, and more resistant leaves. These results showed an effect of eCO₂ when P limitation was alleviated, indicating, that in the future the role of this ecosystem as a carbon sink may be highly limited by the availability of P. However, we still need to understand whether these responses from seedlings in an understory forest will be effectively maintained in the canopy, through the full-ecosystem AmazonFACE experiment.