Initial responses of fine root dynamics of understory plants to elevated CO2 in a Central Amazon rainforest

In large parts of the Amazon rainforest low soil phosphorus availability may prevent the stimulation of forest growth in response to elevated atmospheric CO₂ (eCO₂). 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.

We tested our hypothesis in the first CO₂ enrichment experiment in Central Amazonia at a low soil phosphorus site, increasing CO₂ levels by 200 ppm relative to CO₂ ambient (aCO₂) 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.

In contrast to our hypothesis, we observed a reduction in fine root productivity (<1mm diameter), from 0.038 ± 0.01 mg cm² day^-1 under aCO₂ to 0.013 ± 0.004 mg cm² day^-1 after 12 months of eCO_2. On the other hand, the fine root biomass stock (<2mm diameter) increased at 5-10 cm from 0.86 ± 0.18 at aCO₂ to 1.74 ± 0.65 mg^-1 cm² with eCO₂, but there was no effect of eCO₂ 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 eCO₂ (SRL: 4.66 ± 1.08 and 9.58 ± 2.12 cm mg^-1; SRA:  0.63 ± 0.18 and 1.0 ± 0.25 cm² mg^-1 with aCO₂ and eCO₂, 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 eCO₂ in the litter by 20.0 % (aCO₂: 66.16 ± 10.4; eCO₂: 79.39 ± 20.8 nmol mg^-1 dry root h^-1) and soil by 45.61% (aCO₂: 97.42 ± 30.76; eCO₂:141.86 ± 34.04 nmol mg^-1 dry root h^-1).

Our initial results suggest that understory plants intensified the investment in fine root dynamics in litter layer as response to eCO₂ (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 eCO₂, our results reinforce the importance of this mechanism to mobilize inorganic P. Our results provide an initial understanding of nutrient mechanisms acquisition under eCO₂ in a tropical forest, which can be incorporated into ecosystem models to allow more reliable predictions of forest productivity under eCO₂.