Amazon forest's carbon sink strength depends on plant nutrient efficiency at the root-soil interface

The impact of enhanced atmospheric CO₂ (eCO₂) concentrations on the Amazon forest's capacity to continue acting as a carbon (C) sink largely depends on soil nutrient availability. In particular, phosphorus (P) and the ability of plants to balance the extra CO₂ with the additional nutrient demand play an important role in highly weathered tropical soils. We hypothesize that plants may allocate the extra C belowground to different nutrient acquisition mechanisms, thereby alleviating nutrient limitation. Potential changes in nutrient acquisition mechanisms include an increase in fine root productivity, adjustments in root morphological traits, and investment in arbuscular mycorrhizal fungi symbioses that will enhance nutrient foraging capacity. Additionally, the plant community could increase root labile C exudation, which can be utilized by the microbial community as an energy source, leading to increased extracellular enzyme production and enhanced nutrient mineralization. Furthermore, it is important to highlight the litter layer as a significant nutrient source, and root mats growing in the litter layer allow roots to intercept newly mineralized nutrients before they reach the soil.

Here, we increased atmospheric CO₂ by ~300 ppm in situ, in a P-depleted Amazonian forest understory using open-top chambers, which not only increased plant C assimilation but also promoted aboveground biomass growth. Therefore, our primary goal was to better understand the mechanisms and adaptations at the root-soil interface that facilitated this positive CO₂ fertilization response. Our results show that in the litter layer, eCO₂ did not change net root productivity, but increased specific root length, indicating an enhanced foraging strategy. In contrast, in soil, eCO₂ caused a decrease in root productivity, but an increase in arbuscular mycorrhizal fungi colonization, which may represent an alternative foraging strategy for plant communities. Simultaneously, eCO₂ induced a significant decrease in the soil enzyme C and P stoichiometry, and a decline of the soil organic P fraction. One year later, a decrease in leaf litter P was observed under eCO₂, which suggests that the adaptations of litter-based fine roots to eCO₂ may have longer-term consequences for litter P recycling.

Taken together, our experiment provides in situ evidence that eCO₂ promotes different root responses along the litter-soil continuum, which may alter P availability and intensify competition between plant roots and soil microorganisms. Such multiple spatial adaptations in root P acquisition strategies may strongly regulate plant-soil belowground dynamics and need to be considered to better understand the resilience of the Amazon forest to future climate change.