Multiple plant root strategies improve phosphorus acquisition under elevated CO2 in the Amazon rainforest

The impact of elevated atmospheric CO2 concentrations on forest productivity depends on the capacity of plants to balance the additional CO2 with the demand for additional nutrients. One hypothesis states that plants may allocate the extra carbon belowground in producing and maintaining fine roots to alleviate nutrient limitation. In the Amazon basin, where approximately 60% of the forest is on old and weathered soil, the litter layer is an important nutrient source. In some regions, root mats growing in the litter layer can be observed, where the roots intercept the newly mineralized nutrients before they reach the soil and may bind to the mineral matrix. To improve their nutrient uptake capacity, trees can either modify their root morphology to a do-it-yourself strategy, increasing root length and branching intensity or alternatively, they can outsource the same function by investing in symbioses with mycorrhizal fungi. Additionally, fine roots can stimulate microbial decomposition of recalcitrant substrates (e.g., wood debris) by exuding low molecular weight organic compounds (LMWO) and increasing P mobilization by phosphatase activity without changing decomposition. These strategies could also vary depending on the growing depth of roots due to the different physical conditions between the organic upper and mineral layers. However, little is known about the role of trait differences in roots under higher CO2 concentrations. To increase our understanding of belowground responses of understory plants to elevated CO2 concentrations, we set up an Open-Top Chamber experiment in a lowland forest in the Central Amazon. We observed that under eCO2, root productivity did not change in the litter layer but showed a decreased pattern in the soil layer. Moreover, plants intensified root foraging in the litter layer by increasing their specific root length more than threefold under elevated CO2. In contrast, roots in the soil mineral layer followed an outsourcing strategy by increasing arbuscular mycorrhizal colonization by 117%. In addition, our results showed a decrease in the organic P in litter without a change in C decomposition under higher CO2 concentrations, suggesting a direct P mobilization. Our results suggest that plants may plastically adjust resource acquisition strategies to increase nutrient uptake efficiency and be able to directly affect P mobilization from the litter layer. We conclude that this ability of plants to adapt their P acquisition strategies in response to eCO2 by tackling different sources within the litter-soil continuum and maximizing nutrient acquisition represents an important mechanism to support the CO2 fertilization effect and might affect the resilience of the Amazonian rainforest to climate change, and thus global carbon balance.