The Tibetan plateau
Nicolas Marino/mauritius images GmbH/Alamy
Climate change is causing winter temperatures to rise faster than summer temperatures, especially in high altitude regions. This “asymmetric” warming could disrupt the large amount of carbon stored in soils by modifying microbial activity more than expected.
The planet’s soils store more carbon than any other ecosystem other than the oceans, and could store a lot more if better managed. But soil carbon is threatened by climate change. Researchers expect that higher temperatures will increase the amount of soil carbon lost to the atmosphere as greenhouse gases, largely because of changes in the behavior of soil microbes. However, the magnitude of this warming response remains unclear.
Ning Ling at Lanzhou University in China and colleagues heated soils in an experimental grassland on the Tibetan Plateau to test how different warming patterns might change microbial activity. Some soils were kept at room temperature, while others were exposed to a “symmetrical” warming of 2°C throughout the year. A third group was exposed to warming of 2.5 to 2.8°C in winter and 0.5 to 0.8°C the rest of the year, a more realistic simulation of actual warming patterns.
After a decade of this treatment between 2011 and 2020, the researchers tested the microbial activity of samples from different soils. They focused on two metrics in particular: growth rate and an indicator of how organisms use carbon, called carbon use efficiency. This was turns out to be a major determinant the quantity of organic carbon stored in soils.
“When a microbe eats carbon, it can do one of two things: it can break it down to produce energy and breathe that carbon in as CO2, or it can use it to create new structures bodily”, explains Daniel Rath at the Natural Resources Defense Council, a nonprofit environmental organization based in New York. A higher growth rate means that microbes use more carbon, and greater carbon use efficiency means that more of that carbon is transformed into body structures, rather than breathed in as CO2, he said.
Ling and his colleagues found that both modes of warming significantly reduced microbial activity. Soils subjected to symmetrical warming saw their growth rate decrease by 31 percent and their carbon use efficiency decrease by 22 percent compared to soils exposed to ambient temperatures. Under asymmetric warming, this effect was even stronger, with growth rate reduced by 58 percent and carbon use efficiency reduced by 81 percent compared to soils exposed to ambient temperatures. They attributed these differences to factors including a change in the nutrients available to the microbes.
“Their results suggest that soil carbon storage will likely decline, reducing the ability of terrestrial ecosystems to sequester carbon and degrading soil effectiveness for natural solutions to climate change,” says Yiqi Luo at Cornell University in New York.
Rath says the fact that current models don’t account for asymmetric warming means we are likely underestimating soil carbon losses due to climate change. However, he says the results can only apply to soils in cold ecosystems and that more research is needed to understand exactly what these changes in microbial activity mean for carbon. For example, despite the significant change in microbial activity, the total amount of carbon stored in the soil did not change during the experiment.
