Global Warming — Increased CO2 alters carbon cycle
The level of carbon dioxide (CO2) in our atmosphere is rising, which is a main contributor to global warming, as we are all familiar. But wait! Plants grow by using CO2. So, in theory, as CO2 in the atmosphere increases, plants grow more, removing CO2 from the air and slowing down climate change. Problem solved? Unfortunately, as it turns out, it doesn’t quite work out in our favor.
While it is true that the largest carbon store is found in the soil, which is replenished by plant biomass, and that the increased CO2 in the atmosphere stimulates plant growth, another player comes into the equation: microbial decomposition.
Carbon (C) is constantly cycled between the atmosphere and the earth. Plants use CO2 in the air to grow, using C as their building blocks. Eventually, plant matter integrates into the soil, as plants shed leaves or die and turn into logs, and C is stored in the ground. In the soil, microbes decompose the organic matter, releasing C back in the air as CO2. The amount of C that is stored in the soil, thus, depends on the balance between the input and the loss of C.
A recent study used models to calculate the levels of plant production (C input that adds C in the soil and removes CO2 from the atmosphere), microbial respiration (C turnover that removes C from the soil and adds it back to the atmosphere), and the soil C stock.
The researchers found that the plants grow better by +19.8% when the atmospheric CO2 is high. However, the C turnover rate is also increased by +16.5%. Overall, the two components counteract each other, and the C store in the soil remains steady. This means that the plant growth alone cannot effectively remove CO2 from the atmosphere.
The authors of the study remark that the increase in the C turnover rate is unexpected. Note the word rate. It makes sense that the amount of biomass decomposition rises as the plant growth is enhanced, since there will be more material (or food) for microbes to work with. But if the process simply increases due to the enrichment of organic matter in the soil, the speed of microbial activities should remain the same. In the study, however, the rate of decomposition significantly increased, indicating that the rich atmospheric CO2 is affecting the soil C turnover in some other ways. An explanation for this phenomenon is still uncertain, but possible reasons include priming and altered soil moisture. The increase in the C store leads to more availability of simpler C compounds that can be readily digested by microbes, leading to the loss of C reserves in the soil that are normally protected. The CO2 enrichment allows plants to use water efficiently, and more moisture is retained in the soil, a condition favorable to microbes.
So where does this lead us? The sure way to negate global warming, perhaps, is to reduce our C emission, rather than to rely on plants.
The report was published in Science, which can be accessed from here.