Extreme drought alters methane uptake but not methane sink in semi-arid steppes of Inner Mongolia

Authors: WEN, F. - University Of Chinese Academy Of Sciences; Biederman, Joel; HAO, Y. - University Of Chinese Academy Of Sciences; QIAN, R. - University Of Chinese Academy Of Sciences; ZHENG, Z. - University Of Chinese Academy Of Sciences; CUI, X. - University Of Chinese Academy Of Sciences; ZHAO, T. - University Of Chinese Academy Of Sciences; XUE, K. - Chinese Academy Of Sciences; WANG, Y. - Chinese Academy Of Sciences

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2023
Publication Date: 1/7/2024
Citation: Wen, F., Biederman, J.A., Hao, Y., Qian, R., Zheng, Z., Cui, X., Zhao, T., Xue, K., Wang, Y. 2024. Extreme drought alters methane uptake but not methane sink in semi-arid steppes of Inner Mongolia. Science of the Total Environment. 915. Article 169834. https://doi.org/10.1016/j.scitotenv.2023.169834.
DOI: https://doi.org/10.1016/j.scitotenv.2023.169834

Interpretive Summary: Although carbon dioxide is the most commonly-discussed greenhouse gas, there is growing concern about methane, which causes 25 times more warming than an equivalent amount of carbon dioxide. Across Earth’s extensive dryland ecosystems, methane is consumed by soil microorganisms, and this process is a key component of carbon cycle models. However, there are limited field measurements about how dryland soil methane consumption responds to changes in hydroclimate, such as droughts of increasing duration. This laboratory experiment measured methane uptake by microorganisms in soil pots harvested from 16 semiarid grassland sites across Inner Mongolia, China. When exposed to an extreme-duration drought of 30 days, all soils continued to absorb methane from the atmosphere, although the rate usually declined as the soils dried out. When a higher atmospheric concentration of methane was applied, as is expected globally in the future, methane uptake rates increased. Collectively, these results imply that dryland soils can continue to take up atmospheric methane even under changing hydroclimate, and that methane uptake rates may increase, partially compensating for rises in global atmospheric methane concentrations.

Technical Abstract: Frequent extreme drought events related to climate change may alter the generally accepted bell-shaped relationship between methane (CH4) uptake and soil water content (SWC). This relationship is widely incorporated into carbon cycle models, but it is primarily postulated from studies only focusing on single sites and/or over a moderate range of SWC. Meanwhile, it remains unclear how the CH4-SWC relationship may differ under extreme drought or across multiple ecosystems contributing to a regional carbon cycle drought response. In this soil mesocosm study, we explored the dynamic changes in CH4 fluxes during an initial wet period followed by a prolonged dry period (three-month drought, corresponding to a 100-year recurrence interval) in soils collected from 16 different Eurasian steppe sites across the Inner Mongolia plateau, China. A bell-shaped relationship was observed between SWC and CH4 uptake in soils from 11 sites, while a novel W-shaped relationship was found in soils from the other five sites. All soils functioned as persistent CH4 sinks, even when SWC was less than 5%. Across the regional dataset, the duration and magnitude of peak CH4 uptake mainly depended on nitrogen, phosphorous and total soil organic content. The relative abundance of methanotrophs, rather than the community composition of methanotrophs, regulated CH4 fluxes measured when wet soil gradually dried. Under drought conditions, experimentally increased CH4 concentration counteracted the possible depression of CH4 uptake related to physiological water stress on methanotrophic soil microbes. These findings suggest that for some steppe sites, extreme drought alters the expected bell-shaped relationship of CH4 uptake with SWC. Furthermore, the results imply that a future increase of global atmospheric methane concentration will partly offset the depression of CH4 uptake under severe drought. Considering the expansive extent of the Eurasian steppes and similar semiarid grasslands globally, these findings are of critical importance to predicting the global carbon cycle under climate change, especially extreme drought.