One of the things that scientists rely on to accurately predict climate change is the amount of carbon sequestered underground. Carbon dioxide in the air leads to increased global warming, exacerbating climate change. When plants have a lot of access to carbon dioxide, they photosynthesize more. Scientists have assumed for a long time that this led to a high concentration of carbon sequestered under the ground. As plants took in this carbon dioxide, they transformed it into compounds and organic structures such as roots and leaves, which would add to the amount of carbon stored underground. However, a recent study published in Nature suggests that scientists may have overestimated the amount of carbon stored underground.

The Difficulty in Estimating Carbon Stores

Experiments have been done in the past to determine how much carbon is stored in an ecosystem when there is a rise in carbon dioxide levels. The term for carbon being absorbed by these ecosystems is known as "The CO₂ Fertilization Effect." When we zoom down to the scale of leaves, more CO₂ means more photosynthesis. However, this only works in isolation. The effect of the overall CO₂ fertilization effect has been extrapolated from these isolated experiments. In larger ecosystems, the processes have become diluted because of the connected processes involved. For a long time, it was a matter of figuring out what was happening while not having a clear idea of the entire picture.

When we consider soil carbon levels, there's no precise way of measuring its change over a large area. Studies haven't been very clear about what they've found either, muddying the waters further. The meta-analysis mentioned in Nature estimates that carbon sinks remained relatively constant in forested environments, yet those values changed in non-forest locales. After careful study, the authors concluded that the best explanation for this comes from taking above-ground plant biomass into consideration. Thus, they concluded that soil-carbon stocks were inversely related to above-ground biomass stores. The relationship was discovered by looking closely at experiments where no additional nutrients had been added. The conclusion the authors drew was that plant nutrient acquisition was directly responsible for this relationship.

The Role of Mycorrhizal Soil Fungi

According to the journal Science, a slight increase in above-ground plant-biomass happens in enriched CO2 environments with plants associated with a particular type of fungi, arbuscular mycorrhizae (AM). The AM-associated plants significantly benefit from the fungi's interconnected hyphae (branching filaments that help with plant growth). These hyphae are crucial in nitrogen uptake from the soil. Unfortunately, AM isn't very efficient in "mining" nitrogen from the earth. They have a very low uptake efficiency when compared with other mycorrhizal families. The lower availability of nitrogen limits the growth of plant biomass. Another type of plant that associates with a family of fungi known as ectomycorrhizae (ECM) shows much more above-ground plant-biomass as they grow. ECM is more efficient in fixing nitrogen, but there is a strange side effect to this process. The nitrogen mining by ECM leads to a decomposition of organic compounds within soil.

When the team examined the different fungal-dependent plants, their conclusions were astounding. AM-associated plants lead to a larger store of carbon underground thanks to their flawed method of mining nitrogen. On the other hand, ECM-associated plants lead to a larger volume of above-ground plant biomass at the cost of less underground storage of carbon. The team sees an extrapolation to an ecosystem, suggesting that this tradeoff results in more or less the same volume of carbon sequestered in plants as is in the soil. Unfortunately, this isn't the assumption that most Earth-system models use. The traditional models assume that rising CO₂ levels will lead to an increase in soil carbon. This modeling methodology suggests that our current levels of estimation of CO₂ sequestration are abysmally low. In addition, these earth models ignore the limiting factor of nitrogen availability in plant growth. Revisions may reveal that our estimations for climate change may be too conservative.

Some Limitations to Consider

The studies examined by the meta-analysis had a significant skew towards temperate regions. However, the authors didn't find that the change in the region had a significant enough impact on results to warrant separate consideration. The authors did note that further studies were needed in tropical areas to address the discrepancy. It's known that tropical ecosystems are a vital carbon sink. Unfortunately, they have fewer studies dedicated to them than temperate regions. Tropical underground processes are much harder to deal with, hence the low volume of tasks within the area. Nevertheless, the authors provide a solid framework for developing studies within this environment.

These studies typically span a few years - not nearly long enough to get a picture of how carbon dioxide levels impact plant life spans. Elements such as species composition within the study area and soil-carbon turnover over time are neglected in short-term studies. However, understanding the mechanisms behind these processes make it possible to use modeling to extrapolate based on test data. This new model should allow scientists to get a better grasp of carbon sinks in the coming decades.