Geologic Control of Soil Carbon Geologic Control of Soil Carbon Sequestration Sequestration ––

Examples from Western Conifer ForestsExamples from Western Conifer Forests

Craig RasmussenCraig RasmussenDept. of Soil, Water and Environmental ScienceDept. of Soil, Water and Environmental Science

The University of ArizonaThe University of Arizona

Global Carbon Cycle – why do we care?

Image from the Climate and Vegetation Research Group, Boston University

Global Carbon Cycle

Pool size and fluxAtmospheric CO2 sinks:

land-atmosphere; ocean-atmosphere

Soil carbon – largest terrestrial pool

Source/sink dynamic

Terrestrial Carbon Stocks

Ecosystem Type Soil Carbon Litter Carbon Total Carbon % of Total

Tropical Forest 255 3.6 259 17Temperate Forest 142 14.5 157 10Boreal Forest 179 24.0 203 13Woodland and Shrubland 59 2.4 61 4Tropical Savanna 56 1.5 58 4Temperate Grassland 173 1.8 175 12Tundra and Alpine 173 4.0 177 12Desert Scrub 101 0.2 101 7Extreme 3 0.0 3 0Cultivated 178 0.7 179 12Swamp and Marsh 137 2.5 140 9Total 1456 55 1511From Schlesinger (1992)

mt C x 109

Forests 40% of Global SOC

Forests ~1/3 of US carbon

North America Carbon Stocks

Forests – east and west

West predominantly conifer forests

Source: Climate Change Science Program

Forests ~1/2 of US terrestrial C sink

Western Conifer Forests

• Complex physiography– Geology– Climate– Ecosystems

• California and Arizona

Bedrock Geology

MAT (Cx100)Value

High : 2549

Low : -420

MAP (mmx100)Value

High : 710807

Low : 4759

Winter PPT (% of MAP)Value

High : 96

Low : 15

WinterWinter

XericXeric

SummerSummer

UsticUstic

DesertForest

Agriculture

Savanna

Chaparral

Grassland

Other

Rel

ativ

e P

erce

nt

0

10

20

30

40

50

% Land Area % of Total SOC

Forest

Upper Desert

Lower Desert

Shrub-grass

Grassland

Rel

ativ

e Pe

rcen

t

0

10

20

30

40

% Land Area% Total SOC

Data derived from GAP and STATSGO – 4-km pixel resolution

California

ArizonaForests – disproportionate SOC relative to land area

Clay content not a good predictor of SOC in these systems

California Conifer EcosystemsCalifornia Conifer Ecosystems

Significant difference in SOC content among geologic parent materials

Parent Parent MaterialMaterial

SOC SOC (kg m(kg m--22))

AndesiteAndesite 8.7 A8.7 A

BasaltBasalt 8.1 B8.1 B

GraniteGranite 5.9 C5.9 C

How does geology control SOC sequestration?

• Metal and mineral interactions– Organo-metal complexation– Mineral adsorption

• Microbial community composition– Diversity and function

• Aggregation– Mineral-mineral and organo-mineral– Occlusion and physical protection

• All are highly dependent on soil mineral assemblage and should vary predictably with geologic parent material

Geology and Soil Carbon Studies• Case studies

– Sierra Nevada Conifer Forests• Parent material and elevation gradients

– Arizona Ponderosa Pine Forests • Soil pH and Al gradient

• Empirical and Mechanistic data– Soil C mineralization– Microbial community composition– Aggregate stability and occlusion

Red Fir, MAST 0-8ºC, Entisols

White Fir, MAST 8-10ºC, Inceptisols/Andisols

Ponderosa Pine, MAST 10-15ºC, Alfisols/Ultisols

2

4

6

8

10

12

14

Grassland

Savanna

Ponderosa Pine

White Fir

Red Fir

Alpine

Elevation (1000ft)

Alp (g kg-1)

0 2 4 6 8 10 12

Min

eral

ized

C (m

g C

g-1

soi

l C)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Feo (g kg-1)

0 1 2 3 4 5 6 7 8

Min

eral

ized

C (m

g C

g-1

soi

l C)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Soil C Mineralization

Pooled by Parent Material

0

2

4

6

8

10

12

14

16

GR BS AN

[mg

C (g

soi

l C)-

1]

a

bc

Pooled by Parent Material

0

2

4

6

8

10

12

14

16

GR BS AN

[mg

C (g

soi

l C)-

1]

a

bc

Study 2: Arizona Forests• Arizona Ponderosa Pine Forests

– Al effects on microbial community structure and soil C sequestration

– Lithosequence of four parent materials• pH gradient and variable soil mineral assemblage

Heckman et al. (2009) Chemical Geology.

Limestone Basalt Granite Rhyolite

High pH (7.0) Low pH (5.0)

Low available Al High available Al

Significant correlation between soil C and metal-organic complexes

Significant variation in soil C mineralization by parent material

Alp (kg m-2)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

soil

orga

nic

carb

on (k

g m

-2)

0

2

4

6

8

10

12

14

LimestoneRhyoliteGraniteBasaltR2 = 0.72, P<0.0001

Incubation Day

0 10 20 30 40

mg

C re

spire

d (g

soi

l C)-1

0

20

40

60

80

100

120

LimestoneBasaltGraniteRhyolite

Microbial Community Composition

Significant variation by parent material

Correlated with:• pH• exchangeable Al• metal-organic complexes• SRO Fe-oxides

• Soil pH, diversity and Al-tolerant microbes

Microbial Community Composition

Study 3: Physical Mechanisms

Free Light Fraction (FLF) Occluded (OCC)

Mineral (MIN)

Physical Mechanisms - Aggregation

FLF OCC MIN

Δ14C

-400

-300

-200

-100

0

100

200

RhyoliteGraniteBasaltLimestone

0%

20%

40%

60%

80%

100%

Rhyolite Granite Basalt Limestone

MINOCLFLF

“Young”

“Old”

Overall Summary:• Geology and soil mineralogy matter

– Non-crystalline mineral species and metal- organic complexation

• Mineral control of soil C recalcitrance– Microbial community activity and composition– Chemical and physical protection mechanisms

• Within a given ecosystem soil carbon sequestration varies predictably with geology

FLF OCC MIN

Δ14

C

-400

-300

-200

-100

0

100

200

Rhyolite

Granite

Basalt

Limestone

Alp (g kg-1)

0 2 4 6 8 10 12

Min

eral

ized

C (m

g C

g-1

soi

l C)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Acknowledgements:• Students

– Katherine Heckman, Amy Welty-Benard, Jessica Hagerlin-Jones, Angelica Vazquez-Ortega, Mateo Cagnasso

• NSF DEB#0543130