Effect of elevated CO2 in a late-stage leaf litter decomposition process in the understory of Amazonian forest: the role of plant root and microbial interaction on nutrient availability

More than 60% of the Amazonian rainforest grows on old and weathered soil with low availability of important rock-derived nutrients like phosphorus (P), and efficient nutrient recycling is the main source of nutrients to maintain forest productivity. Thus, the effect of the elevated CO₂ atmospheric concentrations (eCO₂) on tree productivity (i.e., fertilization effect) may depend on the capacity of plants to access currently unavailable nutrients or increase nutrient acquisition efficiency. In some Amazon regions, the high root proliferation in the litter layer, where roots intercept newly mineralized nutrients before they are leached into the soil, is an important mechanism. These roots can also influence nutrient mobilization directly by exuding phosphatase enzymes to hydrolyze organic P without releasing carbon or indirectly by exuding labile carbon (i.e., glucose, sucrose) that can be used as energy for the microbial community to increase the decomposition and nutrient release from leaf litter. 

In an Open-Top Chamber experiment in a lowland understory forest in the Central Amazon, we investigated how elevated CO₂ influences plant-root-microbe interactions during a late-stage (i.e., after one year) leaf litter decomposition. We found that under eCO₂ leaf litter mass loss did not change. However,  we observed that under eCO₂, higher root net production in the leaf litter decreased litter mass loss. This may suggest that increased root exudates under eCO₂ influence microbial litter decomposition. Furthermore, we observed a decrease in microbial biomass carbon (C) and an increase in the ratio of enzymes responsible for degrading C, nitrogen (N), and P, normalized by microbial biomass C. This could suggest microbial C and nutrient limitation, which means that the plant root exudates under eCO₂ were not benefiting microbial growth, and they needed to invest energy in maintenance and resource acquisition. The lack of change or decrease in mass loss under eCO₂, even with a possible microbial C limitation, may be related to the stage of the litter decomposition process and the more recalcitrant C fractions available,  or antagonistic interaction between plant and microbial community. Nevertheless, we found a significant decrease in leaf litter P concentration under eCO₂ without changing litter decomposition. Still, the decrease in the inorganic microbial P may suggest that C microbial investment did not result in a microbial P mobilization, and probably trees directly took up this available P, indicating that eCO₂ intensifies the P competition between plants and microbes. 

Our results suggest that under eCO₂, trees may change the microbial stoichiometry to increase resource acquisition, and the shift in the competition for P between plants and microbes may be the key factor in controlling plant P mobilization in a late-stage decomposition process. This suggests that plant-microbial interaction may be an important strategy for increasing nutrient availability in scenarios under elevated CO₂ atmospheric concentrations, possibly directly impacting the Amazon forest productivity and resilience to climate change.