Global implications of a low soil moisture threshold for microbial hydrogen uptake

Area of Science:

• Environmental Science
• Microbiology
• Geochemistry

Background:

• Anthropogenic hydrogen (H2) emissions affect Earth's radiative balance.
• Soil microbial H2 uptake is the largest, most uncertain component of the global H2 budget.
• Soil moisture is a key regulator of this microbial H2 sink, but its precise influence is poorly understood.

Purpose of the Study:

• To investigate the impact of soil moisture on microbial hydrogen oxidation.
• To determine the lower limits of soil moisture for H2 uptake by soil microbes.
• To refine global hydrogen budget simulations by incorporating new data on soil moisture thresholds.

Main Methods:

• Laboratory experiments measuring H2 oxidation rates across a range of soil moisture levels in temperate and arid soils.
• Genome-resolved meta-omics to identify microbial taxa responsible for H2 oxidation and their adaptation to dry conditions.
• Global simulations to assess the impact of revised soil moisture thresholds on regional H2 uptake contributions.

Main Results:

• Microbial H2 oxidation was observed at extremely low water potentials (-70 to -100 MPa), significantly drier than previously assumed in models.
• Specific desiccation-adapted microbial taxa in temperate soils were identified as key contributors to H2 uptake along the moisture gradient.
• Revised, drier moisture thresholds increased the modeled contribution of arid/semi-arid regions to soil H2 uptake by 4-7 percentage points.

Conclusions:

• Soil microbes remain active and contribute to hydrogen uptake under extreme drought conditions.
• The expansion of arid regions and the ecophysiology of H2-oxidizing microbes are critical factors influencing the global H2 budget.
• Accurate quantification of soil H2 uptake, especially under dry conditions, is essential for understanding Earth's radiative balance and climate change.