Soil organic matter and long-term agricultural experiments

Johan Six

University of California - Davis

Questions

• Quantity versus quality?

• Is stable SOM predominantly microbial in origin?

• SOM dynamics at steady state?

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Maize Calliandra Tithonia

So

il o

rga

nic

C (

g C

kg

-1 s

oil)

Silt + Clay

Microaggregates

Macroaggregates

Residue-Derived Soil C - 0.25 yr

B

AA

Gentile et al 2008 SBB

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Maize Calliandra Tithonia

So

il o

rga

nic

C (

g C

kg

-1 s

oil)

Silt + Clay

Microaggregates

Macroaggregates

Residue-Derived Soil C - 1.5 yr

AAA

Gentile et al 2008 SBB

0

5

10

15

20

25

30

35

40

45

Control Maize Calliandra Tithonia

Soil

org

an

ic C

(g C

kg

-1 s

oil)

Silt + Clay

Microaggregates

Macroaggregates

Soil Organic Matter Stabilization

B

A A A

Gentile et al 2011 Ecol Appl

0

10

20

30

40

50

60

70

80

90

100

C In

pu

t (M

g C

ha-1

)

RWC IWC RWF IWF RWL IWL CWT LCT CCT OCT

Russell Farm Cropping Systems

Kong et al. 2005 SSSAJ

C input

C inputKong et al. 2005 SSSAJ

Conceptual Diagram

Denbow et al 2008

Carbon Saturation Model

m

t

C

Ik

IC

mC

C-Input (I)

So

il C

-co

nte

nt

(Ct)

k

SOC at equilibrium time

Inputs = Outputs

mC

k

= maximum SOC content

= rate of C saturationCSAT Model

Six et al 2002 Plant Soil

Carbon Saturation Model

m

t

C

Ik

IC

mC

C-Input (I)

So

il C

-co

nte

nt

(Ct)

k

SOC at equilibrium time

Inputs = Outputs

mC

k

= maximum SOC content

= rate of C saturation

C-Input (I)

Linear Model

k

ICt

CSAT Model

Six et al 2002 Plant Soil

Saturation vs steady state

Soil C equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

C content versus time, C inputs are constantSoil C equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

C content versus time, C inputs are constantSoil C equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

Carbon

content

Equilibrium level

input > respiration

{input = respiration

Time

increased

inputs result in

new equilibrium

C content versus time, C inputs are constant

West & Six 2007

Soil C Saturation: need to test it in long-term experiments at steady state

West & Six 2007

Carbon Saturation Lethbridge, Alberta

Gulde et al. 2008 SSSAJ

Dark Brown Chernozemic (Typic Haplustoll) Clay Loam Annual manure additions since 1973

mC

Distinct Saturation behavior of SOC poolsSite

soil USDA sand silt clay MAT MAP

texture classification (% composition) (° C) (mm)

Lethbridge, Alberta LT clay loam Typic Haplustoll 31 39 30 5.9 389

Lexington, KY KY silt loam Typic Paleudalf 5 68 27 12.7 547

Malhi, Alberta MH silty clay loam Typic Cryoboroll 0-19 45-64 36 1.7 452

Breton, Alberta AB loam Typic Cryoboralf 31 39 30 3.9 240

Scott, Saskatchewan. SC clay loam Typic Boroll 28 44 28 2.4 483

1

Distinct effect of microaggregation

Carbon SaturationLignin (VSC) Response

VSC = lignin SFA = cutin + suberin

Carrington et al 2012 SBB

Carbon SaturationCutin + Suberin (SFA) Response

VSC = lignin SFA = cutin + suberin

Carrington et al 2012 SBB

Management changes (regional)

De Gryze et al 2011 AGEE

Answers

• Is stable SOM predominantly microbial in origin?

it looks like

• SOM dynamics at steady state?

we need to consider C saturation

• Quantity versus quality?

Quantity!

Answers like this can only be answered through long-term experiments linked to biogeochemical models

Thanks!

… the saturation level of a soil, in combination with the initial soil C or steady state level,

determines the rate and duration of potential C sequestration. (West and Six, 2007)

Innate property of a soil Independent of climate, redox, and land management. Impacts how land use change can increase soil organic carbon stocks.

Soil Carbon Saturation

(Hassink, 1996)

Carbon SaturationConclusions

• Preferential Stabilization or Preservation:– Lignin, cutin, and suberin are not preserved by inherent

recalcitrance.

– Lignin, cutin, and suberin stabilization does not change

with C saturation, despite decreasing C stabilization

potential.

Carrington et al 2012 SBB

Carbon SaturationConclusions

• Preferential Stabilization or Preservation:– Lignin, cutin, and suberin are not preserved by inherent

recalcitrance.

– Lignin, cutin, and suberin stabilization does not change

with C saturation, despite decreasing C stabilization

potential.

• Microbial v. Plant-Derived C – There is no preferential stabilization of microbial C in the

chemically protected fraction with C saturation.

– Microbial C may preferentially accumulate in the non-

protected and aggregate fractions with C saturation.

Carrington et al 2012 SBB

Carbon SaturationConclusions

• Preferential Stabilization or Preservation:– Lignin, cutin, and suberin are not preserved by inherent

recalcitrance.

– Lignin, cutin, and suberin stabilization does not change

with C saturation, despite decreasing C stabilization

potential.

• Microbial v. Plant-Derived C – There is no preferential stabilization of microbial C in the

chemically protected fraction with C saturation.

– Microbial C may preferentially accumulate in the non-

protected and aggregate fractions with C saturation.

• Quantity, not Quality– Only quantity affects the chemical saturation response.

Carrington et al 2012 SBB