The Effect of Drought and N Availability on Soil Microbial Production, Respiration, and Carbon-use Efficiency

The Effect of Drought and N Availability on Soil Microbial Production, Respiration,

and Carbon-use Efficiency

John M. StarkDept of Biology, & the Ecology Center

Utah State University

© zuki70 / Fotolia

CHO

CO2

ATP + NADH

CHO

CO2

ATP + NADH

BiomassCHO

CO2

ATP + NADH

BiomassCHO CUE

CO2

ATP + NADH

BiomassCHO CUE = 0

CO2

ATP + NADH

BiomassCHO CUE = 0.8

CO2

ATP + NADH

BiomassCHO CUE = 0.8

C sequestration

How do environmental factors regulate CUE?

• Temperature ?


• Temperature Maintenance E requirements


• Temperature • Substrate supply ?


• Temperature • Substrate supply

type of substrate C:N stoichiometry



type of substrate C:N stoichiometry Limiting resource




• Soil moisture ?




• Soil moisture soil water potential soil water content





Microbial physiology






Diffusion of substrates






Diffusion of substratesSoil texture

Objective

Determine how microbial CUE changes with: • Substrate supply (C & N)• Soil moisture (water potential & content)• Substrate x moisture

Approach

• Range of soil water potentials & contentsin soils of different textures

• Tracer and isotopic dilution assays with13C and 15N-labeled substrates

• Gas-phase additions of substrates(13C-acetic acid vapor & 15NH3)

Measure microbial respiration, C & N assimilation, and CUE using:

Approach

Two methods for calculating CUE:13C-acetate assimilation:

CUE =13C in microbial biomass

13C-biomass + 13CO2

15N immobilization (by 15NH4+-pool dilution):

CUE =Inorg N immob * C:Nmicrob

Inorg N immob * C:Nmicrob + CO2

Approach

Preliminary Experiment:

Approach

Preliminary Experiment: Can microbes assimilate C and N

supplied through the gas phase in acid, dry soils?

Approach



• Sitka spruce forest soil, pH 3.8• Moist (-0.03 MPa) and dry (-3.6 MPa) soils• Gas-phase additions of substrates

(13C-acetic acid vapor & 15NH3)

• 0 to 48-h incubations

Approach



Herron et al. 2009 SBB

Approach



Glass beads w/ 13C-acetic acid

Aquarium pump (90 mL/min)


Approach





Soil cores


Approach





15NH3


Can acetic acid vapor supply 13C substrate to soil microbes in acidic and/or dry soils?


Dry soil (-3.6 MPa)

Moist soil (-0.03 MPa)


Dry soil (-3.6 MPa)


• Yes. 13C-acetate was readily respired.• Respiration rates were higher

after injection of larger amounts of acetic acid vapor.


Dry soil (-3.6 MPa)


• Yes. 13C-acetate was readily respired.• Respiration rates were higher

after injection of larger amounts of acetic acid vapor.

• Acetic acid injections had no negativeimpact on respiration of native SOC.



• 13C assimilation rates were higher after injection of larger amounts of acetic acid vapor and in moister soil.

Does acetic acid vapor affect N cycling rates?


• Mineralization: No


• Mineralization: No

• Microbial N immobilization: Yes• Addition of larger quantities of

acetic acid vapor stimulatedmicrobial immobilization of Nin both moist and dry soil.

So, what about microbial CUE?


15NH3

13C-acetic acid


15NH3

13C-acetic acid

• 15N and 13C methods gave very similar results after 24 to 48 h


15NH3

13C-acetic acid

• 15N and 13C methods gave very similar results after 24 to 48 h

• CUE in different C-addition and soil moisture treatments converged to the same values (0.4 to 0.5) after 24 to 48 h

Conclusions

• Gas-phase additions of substrate can be used to provide isotopically labeled C & N to microbes, and to manipulate substrate supply

Conclusions


• 15N and 13C methods gave similar CUE estimates after 24 – 48h

Conclusions


• 15N and 13C methods gave similar CUE estimates after 24 – 48h

• Surprisingly, C-partitioning into biomass and respiration did not appear to be affected by C supply or soil moisture.

Acknowledgments

Jeanette NortonJennifer MacAdam

Katrina SlabaughShay Peterson

Funding from:

Collaborators:

Technicians:

The Effect of Drought and N Availability on Soil Microbial Production, Respiration, and Carbon-use Efficiency

Environment