Soil moisture an emerging factor in soil CO₂ emission in boreal regions

Soil moisture may exert more influence on soil carbon dioxide (CO₂) emissions than temperature in moisture-deficient subarctic forests¹. The finding, reported in the Russian Journal of Ecology, may help reduce the uncertainty associated with quantifying CO₂ efflux from soils in the boreal region and its potential role in climate change.

“We need to understand how climate change and increased anthropogenic activity will shift the equilibrium in boreal regions, which are highly sensitive to external stress,” says Anastasia Makhnykina from the research team at the Siberian Federal University and the Russian Academy of Sciences in Krasnoyarsk, Russia. “Our data demonstrates that soil moisture may regulate the temperature sensitivity of the soil CO₂ emission process.”

Soil CO₂ emissions are a complex and important part of the global carbon cycle; they are produced from respiring plant roots and heterotrophic microorganisms. In a warming climate, the total CO₂ flux from soil is predicted to increase, and boreal regions, once a major carbon sink thought to store up to a third or more of the world’s land-based carbon, could instead become a significant source of atmospheric CO₂.

“The climate change response from these autotrophic and heterotrophic constituents needs to be determined; however, we cannot neglect the water limitation in boreal regions, especially during the summer season,” says Makhnykina.

Makhnykina and her colleagues collected data from soils of the Middle-Taiga forests of Central Siberia over five summer seasons to examine the relationship between soil CO₂ emissions, temperature and moisture. They took measurements five to six times each month, which required the team to reside in the forests throughout the summer season.

“The field conditions are out of your control, and they can really affect your data,” says Makhnykina. “To better understand our data, we collected additional soil samples to measure the carbon and nitrogen concentrations and quantify the amount of roots.”

The researchers were able to determine the exact soil moisture content that regulates the CO₂ emission rates for different forest types within the boreal zone. This value dictates the additional growth or decline in soil CO₂ emissions. They intend to combine the data with tree-ring dating data, atmospheric CO₂ fluxes above the tree canopy, and carbon pools in different central Siberian ecosystems, to help analyze and predict the future condition of boreal regions. The team has already used the new findings on soil moisture to modify the exponential CO₂ emission model.

“Soon we plan to add one more CO₂ flux to represent the accumulation of carbon and its relation to photosynthesis, and subsequently estimate its influence in the subordinate forest layer of the boreal region,” Makhnykina says.