The productivity of rainforests growing on highly-weathered tropical soils is expected to be limited by phosphorus (P) availability 1 . Yet, controlled fertilisation experiments have failed to demonstrate a dominant role for P in controlling tropical forest net primary productivity (NPP). Recent syntheses have demonstrated that responses to N addition are as large as to P 2 , and adaptations to low P availability appear to allow NPP to be maintained across major soil P gradients 3 . Thus, the extent to which P availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterised by soils even more depleted in P than where most tropical fertilisation experiments have previously taken place 2 . Thus, we established the first P, nitrogen (N), and base cation addition experiment in an old growth Amazon rainforest, with the site's low soil P content representative of ~60% of the basin. Here we show that NPP increased exclusively with P addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of P limitation of NPP suggests that P availability may restrict Amazon forest responses to CO2 fertilisation 4 , with major implications for future carbon sequestration and forest resilience to climate change.

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<p>In large parts of the Amazon rainforest low soil phosphorus availability may prevent the stimulation of forest growth in response to elevated atmospheric CO<sub>2</sub> (eCO<sub>2</sub>). One strategy of plants could be to increase the relative allocation of the extra C belowground to their root systems to enhance nutrient acquisition and alleviate the potential phosphorus limitation, but little is known about the responses of tropical lowland forest species. We hypothesized that in tropical understory plants will trigger a first a fast upregulation of fine root phosphatase activities, followed by changes in fine root productivity and adaptions of morphological parameters, such as specific root length (SRL), specific root area (SRA) and root tissue density (RTD) to enhance phosphorus mobilization, increase its availability and exploit a larger soil and litter volume.</p><p>We tested our hypothesis in the first CO<sub>2</sub> enrichment experiment in Central Amazonia at a low soil phosphorus site, increasing CO<sub>2</sub> levels by 200 ppm relative to CO<sub>2</sub> ambient (aCO<sub>2</sub>) concentrations using open top chambers (OTC) in the forest understory. We monitored potential root phosphatase activity, root productivity, and morphological traits in the soil with ingrowth cores (0-15 cm) and in the litter layer, as well as root biomass stocks in 0-5 and 5-10 cm of depth.</p><p>In contrast to our hypothesis, we observed a reduction in fine root productivity (<1mm diameter), from 0.038 ± 0.01 mg cm<sup>2</sup> day<sup>-1 </sup>under aCO<sub>2</sub> to 0.013 ± 0.004 mg cm<sup>2 </sup>day<sup>-1</sup> after 12 months of eCO<sub>2.</sub> On the other hand, the fine root biomass stock (<2mm diameter) increased at 5-10 cm from 0.86 ± 0.18 at aCO<sub>2</sub> to 1.74 ± 0.65 mg<sup>-1</sup> cm<sup>2</sup> with eCO<sub>2</sub>, but there was no effect of eCO<sub>2</sub> on fine root biomass in the litter layer. However, roots growing in the litter layer significantly increased their SRL and showed a strong tendency of higher SRA in response to eCO<sub>2 </sub>(SRL: 4.66 ± 1.08 and 9.58 ± 2.12 cm mg<sup>-1</sup>; SRA:  0.63 ± 0.18 and 1.0 ± 0.25 cm<sup>2</sup> mg<sup>-1</sup> with aCO<sub>2</sub> and eCO<sub>2</sub>, respectively), but we did not observe changes in root morphological parameters in the soil, only a tendency towards decreasing RTD. Moreover, we found a strong trend towards an increase in potential root phosphatase activity with eCO<sub>2 </sub>in the litter by 20.0 % (aCO<sub>2</sub>: 66.16 ± 10.4; eCO<sub>2</sub>: 79.39 ± 20.8 nmol mg<sup>-1 </sup>dry root h<sup>-1</sup>) and soil by 45.61% (aCO<sub>2</sub>: 97.42 ± 30.76; eCO<sub>2</sub>:141.86 ± 34.04 nmol mg<sup>-1 </sup>dry root h<sup>-1</sup>).</p><p>Our initial results suggest that understory plants intensified the investment in fine root dynamics in litter layer as response to eCO<sub>2</sub> (e.g., increase in SRL and potential root phosphatase activity) Furthermore, with a potential increase in root phosphatases exudation (litter and soil) in the first year with eCO<sub>2</sub>, our results reinforce the importance of this mechanism to mobilize inorganic P. Our results provide an initial understanding of nutrient mechanisms acquisition under eCO<sub>2</sub> in a tropical forest, which can be incorporated into ecosystem models to allow more reliable predictions of forest productivity under eCO<sub>2</sub>.</p>

<p>The increase in atmospheric CO<sub>2</sub> 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 CO<sub>2</sub>. Here, we investigated if P addition would interfere on leaf primary carbon metabolism and aboveground development responses under elevated CO<sub>2</sub>. For that, we used 46  seedlings of <em>Inga edulis</em> Mart., a native leguminous nitrogen-fixing species, exposed for 10 months (November 2019 - September 2020) to CO<sub>2</sub> and P treatments. Plants grew in pots - half with natural P availability (-P) and half with P addition (+P) -, inside CO<sub>2</sub> enrichment chambers - half with ambient CO<sub>2</sub> (aCO<sub>2</sub>) and half with elevated CO<sub>2</sub> (aCO<sub>2 </sub>+ 200 ppm; eCO<sub>2</sub>), - 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: aCO<sub>2</sub>-P (control), aCO<sub>2</sub>+P, eCO<sub>2</sub>-P and eCO<sub>2</sub>+P. To assess the carbon metabolism, we measured light-saturated net CO<sub>2</sub> assimilation (A<sub>sat</sub>), leaf respiration in the light (R<sub>light</sub>), leaf respiration in the dark (R<sub>dark</sub>) and photorespiration (P<sub>R</sub>). 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<sub>2, </sub>regardless of P availability, significantly increased A<sub>sat</sub> and R<sub>light</sub>, while decreasing R<sub>dark </sub>and A<sub>sat</sub>:R<sub>dark </sub>ratio, but it did not affect P<sub>R </sub><sub>.</sub> Those results suggest that seedlings indeed assimilated more carbon under eCO<sub>2</sub>. However, irrespective of CO<sub>2 </sub>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 eCO<sub>2</sub>-only. Plant diameter and number of leaves did not respond to any treatment. We did not find differences between +P seedlings under different CO<sub>2</sub> treatments (aCO<sub>2</sub>+P and eCO<sub>2</sub>+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% (aCO<sub>2</sub>+P) and 126% (eCO<sub>2</sub>+P) compared to control. Overall, we observed a distinguished pattern, in which eCO<sub>2</sub> mainly affected physiological responses, while P addition consistently affected aboveground development. The lack of response of aboveground components under eCO<sub>2</sub> 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, eCO<sub>2</sub> 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.  </p>