Assistant Professor University of Georgia Athens, Georgia, United States
Background/Question/Methods Climate change-related factors such as increased temperature, decreased precipitation and fire frequencies, and elevated CO2 levels (eCO2) have been shown to drive woody species expansion into dryland areas previously dominated by grasses. In drylands, woody shrubs tend to be C3 plants, and grasses tend to be C4 plants. Woody encroachment of these C3 shrubs is often accompanied by an increase in bare soil patches and spatial redistribution of soil resources, which directly impacts plant community dynamics and can ultimately lead to desertification and decline in ecosystem functioning. A previously overlooked factor in woody encroachment is the role that feedbacks between plants and their soil microbes (Plant Soil Feedbacks aka PSFs) play in range expansion capabilities.
To determine the effects of eCO2 on Larrea tridentata and Bouteloua eriopoda and the effect on their plant-soil feedbacks , I conducted a growth chamber experiment with two chambers set to eCO2 (600 ppm) and two chambers set to ambient CO2 (400 ppm). The full experiment included 2 plant species x 2 soil sources x 2 sterility treatments (sterile/live microbes) x 2 CO2 treatments x 10 reps = 160 pots. The plants were grown from seed for 8 months in the chambers before harvest.
For Bouteloua eriopoda, ANOVA results for a mixed model showed that sterility treatment, soil source, and CO2 treatment significantly interacted with each other to affect biomass. Post hoc analysis showed that Bouteloua eriopoda plants grew larger (by 0.2315 mg on average) in their own soil vs. Larrea tridentata soil when the plants were in autoclaved soil, at eCO2 (P=0.0119). Larrea tridentata showed a similar significant three-way interaction between soil source, sterility, and CO2, but post hoc analyses did not show significant contrasts.
These results are in opposition with what we expected to see. Previous research has shown that Bouteloua eriopoda has a strong positive plant-soil feedback with itself, so it was surprising that it performed better without its microbes at eCO2. Another unexpected result was that Larrea tridentata reacted similarly to Bouteloua eriopoda; while not statistically significant, the pattern of doing better in its own soil, but only without its live microbes was present in Larrea tridentata as well. These results suggest the potential of abiotic soil factors to promote positive PSFs in both species at eCO2. Importantly, all of these observations show that eCO2 will change the way these species typically perform and interact with their respective soil microbiomes.