Repeated drying and wetting increases soil CO₂ emissions: Microbial cell disruption a potential factor

A joint research group led by Assistant Professor Hirohiko Nagano of Niigata University's Institute of Science and Technology, Graduate Student Yuri Suzuki of Niigata University's Graduate School of Science and Technology, Professor Syuntaro Hiradate of Kyushu University's Faculty of Agriculture, and Senior Principal Researcher Jun Koarashi of the Japan Atomic Energy Agency has announced their research findings showing that repeated soil drying-wetting cycles caused by precipitation pattern changes associated with global warming substantially increase the amount of carbon dioxide (CO₂) released from the soil. Expected effects of this finding include the improved accuracy of future prediction models of the global environment. The results were published on January 16 in SOIL, a scientific journal of the European Geosciences Union.

Increasing atmospheric CO₂ concentrations are causing global warming and affecting global water cycling, thereby inducing noticeable pattern changes in precipitation around the world. Repeated soil drying and wetting cycles are linked to the decomposition of soil organic carbon. Because soil is one of the largest carbon sinks on Earth, these cycles can influence CO₂ emissions. However, the extent of this effect remained unclear.

In this study, the researchers incubated soil samples collected from forests at 10 sites across Japan for 84 days indoor under conditions simulating repeated drying and wetting associated with hanging precipitation patterns. They then evaluated the resulting changes in CO₂ emissions. The results showed that for all soil samples, the amounts of CO₂ release after repeated drying and wetting were 1.3-3.7 times higher than the amount of CO₂ released under constant soil moisture conditions. A large decrease in microbial biomass was observed under repeated drying and wetting conditions.

The decrease thus observed indicates the possibility that organic carbon newly supplied through the destruction of microbial cells due to repeated drying and wetting contributed to the increased release of CO₂. Furthermore, the percentage of increase in CO₂ release due to repeated drying and wetting was found to be higher in a soil with a higher abundance of complex components formed by active metals and organic matter in soil.

Nagano stated, "Precisely controlling moisture conditions during drying and wetting cycles over 84 days was challenging. However, the first author of this paper, Ms. Suzuki, worked diligently and patiently, including in data analysis. There are many potential directions for future research, but our primary goal is to conclusively identify the carbon sources responsible for the increased CO₂ emissions caused by drying-wetting cycles. Additionally, we aim to evaluate the universality of the mechanisms driving this process to reduce uncertainties in predicting future global environmental changes."

Journal Information
Publication: SOIL
Title: Comprehensive increase in CO₂ release by drying-rewetting cycles among Japanese forests and pastureland soils and exploring predictors of increasing magnitude
DOI: 10.5194/soil-11-35-2025