Microbial responses to soil cooling might explain increases in microbial biomass in winter

Area of Science:

• Soil Science
• Microbiology
• Biogeochemistry

Background:

• Microbial biomass in temperate, boreal, and arctic soils often increases in winter and decreases in spring.
• This seasonal microbial carbon fluctuation may stabilize soil carbon during winter.
• The underlying mechanisms (physiology, community shifts, substrate allocation) remain unclear.

Purpose of the Study:

• Investigate microbial physiological and carbon allocation responses to cooling.
• Determine if soil cooling affects microbial respiration, growth, and glucose uptake.
• Understand how soil microorganisms prepare for winter conditions.

Main Methods:

• Laboratory incubation of temperate forest and cropland soils.
• Cooling soils from 11°C to 1°C.
• Measuring microbial growth via ¹⁸O-incorporation into DNA.
• Tracing ¹³C-labeled glucose into biomass, CO₂, and phospholipid fatty acids (PLFAs).

Main Results:

• Soil cooling significantly reduced microbial growth and cell division.
• Total respiration decreased, but glucose uptake and glucose-derived respiration remained unchanged.
• Microbes increased investment in unsaturated PLFAs at lower temperatures, enhancing membrane fluidity.
• Maintained glucose uptake despite reduced cell division was observed.

Conclusions:

• Soil microorganisms exhibit an immediate physiological response to cooling, potentially preparing for freezing.
• Altered carbon allocation (increased unsaturated fatty acids) is a key adaptation to cold.
• The discrepancy between carbon uptake and cell division may explain winter accumulation of microbial biomass carbon.