A recent 2014 study carried out by Joe Quirk and his team investigated the carbon-energy flux hypothesis, which posits that underground weathering is largely influenced by aboveground plant productivity. Annually, 20% of carbon created by plants through photosynthesis is allocated to the fungi living beneath the forest floor. This amount of energy equates to six times more electricity produced by fossil fuels, and drives much of the biochemistry taking place in the soil depths. This study reveals that plants in elevated CO2 environments allocate more plant sugars to underground symbiotic fungi, which in turn produce more enzymes to sequester soil nutrients. This ancient, symbiotic interaction may ultimately increase soil erosion and nutrient leaching as CO2 becomes more readily available to plants during this modern human era, also known as the Anthropocene.
Although CO2 levels are the center climate change debates, they have been much higher in Earth’s past. Now the rate of change which closely coincides with human activities has never been higher, so yea, we are without a doubt responsible for global climate change. Right now, Earth’s atmosphere has around 400 ppm (parts per million) of CO2. Ectomycorrhizal fungi (ECM) made their way onto Earth’s ecological scene when atmospheric CO2 was much higher during the Cretaceous. ECM evolved beside the diversification of angiosperm trees, when CO2 levels were between 1100 and 1700 ppm! For this reason, these researchers wanted to look at the enzymatic activities of ECM in the elevated CO2 conditions that these organisms first evolved with.
To assess the difference of ECM activities in present vs. Cretaceous CO2 levels, these researchers grew five tree species in two controlled environmental conditions that represented both present and past CO2 quantities. Three of the trees used associate with the more ancient arbuscular mycorrhizal fungi (AMF), while the other two tree species form ECM relationships. The results of this experiment show that significantly more sugars are allocated to ECM in elevated CO2 concentrations. Interestingly, the sugars allocated from AMF mutualists where not significantly increased with the enhanced CO2 conditions.
By analyzing the degree of calcium silicate dissolution, the researchers were able to link fungal enzymatic activity to increased carbon allocation in the separate CO2 treatments. ECM fungi had nearly a 2-fold increase of mineral weathering in the elevated CO2 conditions. Another study in 2009 carried out a similar study, but instead tested Pinus sylvestris seedlings grown at CO2 concentrations of 700 ppm vs. 350 ppm and also found enhanced mineral weathering. This 2009 P. sylvestris study showed that ECM enzymatic activity increased by 270% in the elevated CO2 treatment!
Most of us know the ecological implications of elevating Earth’s CO2 concentration. These recent studies show yet another potentially negative aspect associated with the increased CO2 emissions of the Anthropocene. With more CO2 available in Earth’s atmosphere, plants can create more sugar. This seemingly positive interaction results in the allocation of more sugars to underground fungal mutualists, that in turn produce more degradative enzymes that enter the environment. With CO2 emissions continuing to escalate, we shouldn’t be surprised when the compositions of our future forest floors change in the wake of increased mineral weathering.