Speedier Microbial Soil Activity Accelerates Global Warming

Increasing concentrations of atmospheric carbon dioxide (CO2) could have an even greater effect on global climate change than was previously feared. This is according to new research partly conducted at Trinity College Dublin, which suggests that microbes found in our soils break down organic matter, such as decaying plants, far quicker when atmospheric CO2 concentrations are elevated.

Soils had previously been thought of as environmental ‘sinks’, which swallowed large amounts of the carbon contained in organic matter. These sinks were believed to prevent the carbon from escaping into our atmosphere as CO2 – at least for a long time – and were thus viewed as key partners in the fight to slow global climate change. But, due to the quicker work of the microbes, this is not necessarily the case.

Dr Kees Jan van Groenigen, first author of the study just published in the leading international journal, Science, said: “These results show that nature is not as efficient in slowing global warming as we previously thought.”

Dr van Groenigen performed some of the work as Research Fellow in Botany at Trinity, having held an IRCSET-Marie Curie International Mobility Fellowship in Science, Engineering and Technology when working at the Irish institution. He is now a Research Fellow at the Center for Ecosystem Science and Society, Northern Arizona University.

To better understand how soil microbes respond to our changing atmosphere, van Groenigen and colleagues from the University of Oklahoma and University of Florida analysed results from 53 published experiments that gathered data in forests, grasslands, and agricultural fields. These experiments all measured how extra CO2 in the atmosphere affected the amount of carbon going into the soil in the form of dead plant material, and the amount that left it as CO2 after the microbes had broken it down.

Using the data from these studies, the research team performed meta-analysis, a statistical tool for finding general patterns across multiple studies, and data-assimilation, a mathematical approach to combine theoretical models with real-world data. Soil carbon input and microbial decomposition rates are difficult to measure directly in the field. But, by combining the data with a mathematical interpretation of the soil’s carbon cycle, the research team was able to estimate decomposition rates for the experiments included in their data set.

Two strong patterns emerged from the analyses: Firstly, elevated CO2 levels increased plant growth and the amount of carbon going into the soils, and secondly, the soil microbes decomposed this organic matter faster. The rate at which soil microbes decompose organic matter depends on many factors, including temperature, soil type and the amount of moisture in the soil. The researchers believe the link between CO2 and soil moisture explains part of their findings. “The higher CO2 concentrations reduce plant water use, which makes soils wetter. This in turn boosts microbial activity in dry ecosystems,” van Groenigen added.

The extra plant growth associated with elevated CO2 was viewed as one of the ways that ecosystems could slow climate change, because as plants grow more, they also ‘soak up’ more CO2 through photosynthesis. The hope was that these plants could also lock away some of the carbon in wood and soil.

However, another reason why soil microbes become more active when CO2 concentrations are elevated is that the associated increase in plant growth and subsequent soil carbon input supplies these microbes with extra energy, pumping up their metabolism. The new work therefore suggests that the extra carbon simply fuels the microbes, whose activities release CO2 into the atmosphere to counteract the cooling effects of more plant growth.

“By overlooking the effect of increased CO2 on soil microbes, models used by the IPCC [Intergovernmental Panel for Climate Change] may have overestimated the potential of soil to store carbon and mitigate the greenhouse effect,” van Groenigen concluded.