Significance and driving forces of dark CO2 fixation for organic

carbon inputs in temperate forest soils

1Aquatic Geomicrobiology group, Institute of Biodiversity, Jena2Department of Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Jena.

Rachael Akinyede1,2, Martin Taubert1, Marion Schrumpf2, Susan Trumbore2, Kirsten Küsel1

Soils of temperate forests are currently regarded as

organic carbon sinks(1). Although soils are also one of

the largest terrestrial sources of atmospheric CO2(2),

this can by modulated by dark (non-phototrophic) CO2

fixation by microbes(3), contributing to microbial

biomass (MB) and soil organic carbon (SOC)(4).

• Dark CO2 fixation by autotrophs increases with soil depth.

• Dark CO2 fixation is enhanced by increase in soil CO2 concentration.

• Dark CO2 fixation contributes to MB and SOC across the soil profile.

Dark CO2 fixation in soils

4. Dark CO2 fixation rates

Pine (SEW 17)

Mixed spp. (Hainich)

Beech (SEW 49)

* (p<0.05), *** (p<0.001)

cbbL IC (facultative)

Vegetation type: (R2 = 0.44, p < 0.01)

Depth: (R2 = 0.14, p < 0.05); Permanova.

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30

13C

up

tak

e ra

te /

µg

C g

(dw

) so

il-1

d-1

Time / days

0

100

200

300

400

500

600

0 5 10 15 20 25 30

13C

up

tak

e ra

te /

µg

C g

MB

-1 d

-1

Time / days

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20 25

13C

up

tak

e ra

te /

µg

C g

(dw

) so

il-1

d-1

A horizonB horizon

C horizon

+ve

correlation

-ve

correlation

0.00

0.01

0.02

0.03

0.04

0.05

7 14 28Time / days

CO

2d

eriv

ed c

arb

on

/ %

SO

C

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

7 14 28

Time / days

CO

2d

eriv

ed c

arb

on

/ %

MB

0.000

0.005

0.010

0.015

0.020

0.025

0.030

2 20

CO2 / % v:v

CO

2d

eriv

ed c

arb

on

/ %

SO

C

*

***

CO2 / % v:v

A horizon

C horizon

B horizon

Hypotheses:

Microbial biomass levelSoil level

Microbial biomass levelSoil level

Soil level

Microbial biomass levelSoil level

Water content explains 77.5% variability (R2 =

0.79; p = 0.02); Multiple step-wise regression.

cbbL IA (obligate)

MB determines dark CO2 fixation rates in soils from mixed spp. forests

Dark CO2 fixation contributes to SOC and MB

Increase in CO2 concentration enhance dark CO2 fixation rates

Dark CO2 fixation rates differ with Vegetation type

Autotrophs constitute less than 2% of the bacterial community

δ-

13C

ny

Summary

• MB, CO2 and water content are important factors driving dark CO2

fixation in temperate forest soils.1. Lorenz et al., 2011. AREPS. 29: 535-562.

2. Batjes, N.H. 2016. Geoderma. 269: 61-68.

3. Miltner et al., 2005. Plant and Soil. 269: 193-203.

4. Nowak et al., 2015. Biogeosciences. 12: 7169-7183.

References

• Autotrophs represents only a small fraction of the microbial community.

A horizon B horizon

C- horizon

A horizon

C- horizon

B horizon

μg C gMB-1d-1: expressed per g of microbial biomass

* (p<0.05), *** (p<0.001) denotes significant differences

across depth for the beech and pine plots, respectively.

Pine (SEW 17)

Mixed spp. (Hainich)

Beech (SEW 49)

Red arrows represent dark

CO2 fixation by microbes

0.0

1.0

2.0

3.0

A B C A B C A B CGen

e ab

un

dan

ce /

%

Bac

teri

al c

om

mu

nit

y

Depth/ Horizon

aclA (obligate)

13C uptake rate / µg C g(dw) soil-1 d-1 13C uptake rate / µg C gMB-1 d-1

5. Bacterial community structure

and autotroph abundance

6. Effect of soil properties on

dark CO2 fixation rates

Mixed spp.

(Hainich)

A horizon

B horizon

C horizon Min

eral

hori

zon

(0 –

100 c

m)

Schorfheide-chorin exploratory

(SEW 17 & SEW 49)

(3 cores per plot)

Hainich

National Park

Germany

Berlin

1. Soil sampling

7, 14, 28 days incubation;

2, 5, 10, 20% (v:v) 13CO2

2. 13C metabolic labelling

Chloroform Fumigation

Extraction (CFE)

3. 13C quantification in MB

Soil properties affect

dark CO2 fixation rates

Bacterial community shift with

vegetation type and depth

μg C gdw-1d-1: expressed per g dry weight of soil

*** (p<0.001) denotes significant difference across

depth for all three soil plots.

CRC 1076 AquaDiva www.geomicrobiology.de

μg C g(dw) soil-1 d-1 : expressed per g dry weight of soil

μg C gMB-1 d-1 : expressed per g of microbial biomass

• Estimated fixation of ~0.4Gt C/yr (1m deep) reduces global temperate forest

soil CO2 emissions by 3%.

Average SOC accumulation rate per unit area: 57.36 ± 16.75 g C m2 yr-1 (1m deep)

0.0 0.2 0.4 0.6 0.8 1.0

C-h

B-h

A-h

Dep

th /

Ho

rizo

n

0 150 300 450 600

C-h

B-h

A-h

Dep

th /

Ho

rizo

n

Temperate forests