Initial responses of fine root dynamics of understory plants to elevated CO2 in a Central Amazon rainforest
Authors: Martins Nathielly , F. Lugli Laynara , J. Valverde-Barrantes Oscar , Takeshi Bruno , Pires Maria , G. Menezes Juliane , Sales Pereira Iokanam , Guedes Alacimar , R. Ferrer Vanessa , R. Santos Yago , U. Neves Gabriela , C. M. Moraes Anna , Caroline Miron Ana , P. Hartley Iain , J. Norby Richard , A. Quesada Carlos , Fuchslueger Lucia ,   AmazonFACE team
Published: March 2022

&lt;p&gt;In large parts of the Amazon rainforest low soil phosphorus availability may prevent the stimulation of forest growth in response to elevated atmospheric CO&lt;sub&gt;2&lt;/sub&gt; (eCO&lt;sub&gt;2&lt;/sub&gt;). 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.&lt;/p&gt;&lt;p&gt;We tested our hypothesis in the first CO&lt;sub&gt;2&lt;/sub&gt; enrichment experiment in Central Amazonia at a low soil phosphorus site, increasing CO&lt;sub&gt;2&lt;/sub&gt; levels by 200 ppm relative to CO&lt;sub&gt;2&lt;/sub&gt; ambient (aCO&lt;sub&gt;2&lt;/sub&gt;) 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.&lt;/p&gt;&lt;p&gt;In contrast to our hypothesis, we observed a reduction in fine root productivity (&lt;1mm diameter), from 0.038 &amp;#177; 0.01 mg cm&lt;sup&gt;2&lt;/sup&gt; day&lt;sup&gt;-1 &lt;/sup&gt;under aCO&lt;sub&gt;2&lt;/sub&gt; to 0.013 &amp;#177; 0.004 mg cm&lt;sup&gt;2 &lt;/sup&gt;day&lt;sup&gt;-1&lt;/sup&gt; after 12 months of eCO&lt;sub&gt;2.&lt;/sub&gt; On the other hand, the fine root biomass stock (&lt;2mm diameter) increased at 5-10 cm from 0.86 &amp;#177; 0.18 at aCO&lt;sub&gt;2&lt;/sub&gt; to 1.74 &amp;#177; 0.65 mg&lt;sup&gt;-1&lt;/sup&gt; cm&lt;sup&gt;2&lt;/sup&gt; with eCO&lt;sub&gt;2&lt;/sub&gt;, but there was no effect of eCO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2 &lt;/sub&gt;(SRL: 4.66 &amp;#177; 1.08 and 9.58 &amp;#177; 2.12 cm mg&lt;sup&gt;-1&lt;/sup&gt;; SRA: &amp;#160;0.63 &amp;#177; 0.18 and 1.0 &amp;#177; 0.25 cm&lt;sup&gt;2&lt;/sup&gt; mg&lt;sup&gt;-1&lt;/sup&gt; with aCO&lt;sub&gt;2&lt;/sub&gt; and eCO&lt;sub&gt;2&lt;/sub&gt;, 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&lt;sub&gt;2 &lt;/sub&gt;in the litter by 20.0 % (aCO&lt;sub&gt;2&lt;/sub&gt;: 66.16 &amp;#177; 10.4; eCO&lt;sub&gt;2&lt;/sub&gt;: 79.39 &amp;#177; 20.8 nmol mg&lt;sup&gt;-1 &lt;/sup&gt;dry root h&lt;sup&gt;-1&lt;/sup&gt;) and soil by 45.61% (aCO&lt;sub&gt;2&lt;/sub&gt;: 97.42 &amp;#177; 30.76; eCO&lt;sub&gt;2&lt;/sub&gt;:141.86 &amp;#177; 34.04 nmol mg&lt;sup&gt;-1 &lt;/sup&gt;dry root h&lt;sup&gt;-1&lt;/sup&gt;).&lt;/p&gt;&lt;p&gt;Our initial results suggest that understory plants intensified the investment in fine root dynamics in litter layer as response to eCO&lt;sub&gt;2&lt;/sub&gt; (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&lt;sub&gt;2&lt;/sub&gt;, our results reinforce the importance of this mechanism to mobilize inorganic P. Our results provide an initial understanding of nutrient mechanisms acquisition under eCO&lt;sub&gt;2&lt;/sub&gt; in a tropical forest, which can be incorporated into ecosystem models to allow more reliable predictions of forest productivity under eCO&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;