Soil Organic Carbon Dynamics

M. Francesca Cotrufo

Colorado State University

cotrufo@nrel.colostate.edu

Summer Soil Institute, July 12th -23rd 2010, Fort Collins Colorado

OUTLINES:

Soils in the context of the Global C cycle Soil C pools and fluxes Soils, are they sink or source of C? Approaches and challenges to quantify changes in soil C stocks Effects of global changes on soil organic matter dynamics and potential feedbacks

The Global Carbon Cycle

Soils store twice as much C as the atmosphere and for much longer timesThe C fluxes from the soil to the atmosphere is an order of magnitude higher than anthropogenic emissions of C to the atmosphere

IPCC, 2007

World Soil C stocks

0 1000 2000 3000

0-200 cm

0-100 cm

0-30 cm

AtmospherePg C

Nitrogen

Carbonate-C

Organic Carbon

Total soil C

Data derived from Batjes, 1996

http://soils.usda.gov/use/worldsoils/mapindex/soc.html

The map refers to 1 meter depth

TAKE HOME MESSAGE

Soils contain twice as much C as the atmosphere, both in an organic and inorganic form.

The C is distributed along the soil profile. Despite it is relatively more abundant in the first 20 cm of the soil, lower soil depths contain about half of the C in the soil

Soils around the world vary significantly in their capacity to store C. The richest C soils are at high latitude of the Northern Hemisphere

Soil Organic Carbon

Stabilized soil organic carbon

turnover

organic C input

turnover

Soil respiration: CO2 flux to the atmosphereLitter fall

Wood & Litter decomposition

Root respiration & decomposition

How does the C flow between the atmosphere and soil?

CO2

Photosynthesis

CO2

Pathways of C input, transformation and loss from soil

From Trumbore et al., 2009

Soil organic Carbon stabilization mechanisms

Intrinsic recalcitrance (i.e. chemical stabilization): some compounds are difficult to degrade because of their chemical structure. Examples are pyrolized carbon formed after fire, or some lipid compounds

Physical stabilization: organic matter may be stabilized by occlusion within aggregates

Chemical stabilization: organic matter may be stabilized by interactions with minerals

Inhibition of microbial activity. Microbial activity may be constrained by climatic conditions: freezing temperature, high moisture, low O2

can prevent microbial activity, resulting in organic matter accumulation. Also organic matter may become inaccessible to microbes or their enzymes, because of physical and/or physiological constrains (i.e. nutrient limitation, substrate limitation) .

Soil organic Carbon stabilization mechanisms

Jastrow & Miller, 1997

TAKE HOME MESSAGE

Soil C stores result from the net balance between production (i.e. C input) and decomposition. Changes in the rate of these two processes will likely result in changes of soil C stock.

Soil organic C is not a unique pool.

Organic matter decomposition patterns cannot be predicted by the substrate properties alone.

Any factor that modify any of the mechanisms contributing to soil C stabilization, will likely alter soil C stock and dynamics.

How do you define and/or measure SOM pools?

These are the kind of soil samples you can collect from the field.

How do you partition them in different SOM pools ?

What define a SOC pool, i.e. “different” for what?

SOC pools: kinetic definition

Time

CO

2m

iner

aliz

ed %

of

tota

l SO

C

Labile C pool

Stable C pool

Passive C pool

Method: Biological fractionation. SOM mineralization rates are measured, as CO2 efflux generally by IRGA, in the lab under optimal conditions and for a long time period

100

0

Assumption: Microbes mineralize the labile C first and leave recalcitrant compounds behind. Refractory compounds accumulate in carbon-depleted soils.

SOM pools: Chemical definition

Von Lützow et al., 2007

Chemical fractionation separate OM which is extractable by different solvents or “resistant” to oxidative /acid/basic treatments

Plant and fungal debris

Clay microstructures

Fungal or microbial metabolites

Biochemically recalcitrant organic matter

Silt-sized aggregates with microbially

derived organomineral associations

Microaggregates ~ 50-250 m

Particulate organic matter

colonized by saprophytic fungi

Decomposing roots and detritus become encrusted with mineral particles forming microaggregates

Decomposition continues at a slow rate in stable aggregates, due to formation of organo-mineral associations

Eventually, organic binding agents decompose sufficiently for aggregate to be destabilized, accelerating decomposition until new aggregate is formed

SOM pools: physical definition

The SOM aggregation concept

Microaggreagates , free POM and silt and clay may be bond together within macroaggreates.

SOM pools: physical definition

Bulk soil (intact)

Primary organo-

mineral complexes Uncomplexed OMSecondary organo-

mineral complexes

• 20-2000 m: sand-

sized

• 2-20 m: silt-sized

• < 2 m: clay-sized

• free POM

• occluded POM (in

aggregates)

• > 250 m:

macroaggregates

• < 250 m:

microaggregates

1. DensityDensity

Density separation

1. Complete dispersion

2. Sieving

3. Sedimentation

2. Dispersion,

sieving and

density

Sieving (dry or wet)

Microaggregate isolator

Bulk soil (intact)

Primary organo-

mineral complexes Uncomplexed OMSecondary organo-

mineral complexes

• 20-2000 m: sand-

sized

• 2-20 m: silt-sized

• < 2 m: clay-sized

• free POM

• occluded POM (in

aggregates)

• > 250 m:

macroaggregates

• < 250 m:

microaggregates

1. DensityDensity

Density separation

1. Complete dispersion

2. Sieving

3. Sedimentation

2. Dispersion,

sieving and

density

Sieving (dry or wet)

Microaggregate isolator

An example of fractionation by size and density

scheme

Density flotation

Light fraction(< 1.85 g cm-3)

Intra-microaggregatePOM (iPOM)

Density flotation

Light fraction(< 1.85 g cm-3)

Intra-microaggregatePOM (iPOM)

>250 m fraction (Macro aggregate)<53 m (SC)

53-250 m fraction (microaggregate)

Wet sieving

Micro-aggregateisolator

Silt + clayCoarse POM

Micro’s (mM)

8 mm sieved soil

Deal Galdo et al., 2003

Are the fractions homogeneous and unique pools?

How they relate to other SOM fractionations technique?

Can they be used in current SOM models, what changes are needed?

In search of the fractionation scheme most useful …. for modeling purposes

Can we trace C fluxes along the scheme?

δ13C (per mill)

LAND USE SITE FRACTION

TOTAL Macroaggregates Microaggegates Silt & Clay

Grassland Easter Bush -29,9 -29,6 -29,6 -29,2

0,3 0,4 0,6 0,2

Bugac -25,4 -26,2 -26,2 -25,6

0,7 0,4 0,5 1,3

Deciduous Forest Hesse -28,2 -28,3 -28,3 -29,2

0,2 0,3 0,2 0,9

Hainich -26,4 -27,0 -26,8 -26,8

0,5 0,4 0,5 0,5

Soro -27,2 -27,0 -26,4 -26,4

0,4 0,2 0,4 0,3

Coniferous Forest Wetzstein -26,7 -26,9 -26,8 -26,9

0,4 0,3 0,2 0,5

Norunda -26,5 -26,4 -26,3 -26,8

0,2 0,5 0,3 1,2

Agricultural Carlow -28,0 -28,0 -27,9 -27,7

0,2 0,3 0,2 0,2

Gebesee -26,6 -26,7 -26,5 -26,6

0,0 0,1 0,1 0,1

Grignon -28,3 -27,9 -28,8 -28,9

0,2 0,2 0,2 0,4

Stable C isotopes, have little to no use unless a C3-C4 vegetation change occurred at the site, are not powerful tool for SOM fractions characterization

14C in SOM aggregates

0

1

2

3

90 95 100 105 110pMC

Old Young

SC m M

SC_m LF_miPOM_mSC_MCoarse POMmM

SC_mM iPOM_mM

14C ages of SOM fractions

0

1

2

3

70 75 80 85 90 95 100 105

SC m M

Recalcintrant* Heavy F. Light F.

pMC

Fractionation schemes that do not separate further the Silt and Clay fraction fail to isolate the very recalcitrant fractions.

* Sensu Trumbore et al., 1996Marzaioli et al., 2009

13C of soil fractions in the 0-2 cm layer, before (●) and after 1 year incubation (●) of strongly enriched 13C leaf litter, on the top of the soil

-30 -28 -26 -24 -22 -20 -18 -16

13C

SCSC

Total soilTotal soil

m mFree OM Free OM

Litter derived C is sequestered in the mineral associated SC fraction of the top soil already within a year from litterfall

2 mm sieved soil Litter derived OM

Density flotation (< 1.85 g cm-3)

Heavy POM(> 1.85 g cm-3)

Glass beads dispersion/sieve

Total POM > 53 µm

Mineral-associated OM

Silt-associated

OM

Clay-associated OM

Hydrolizablesilt-associated OM

Recalcitrantsilt-associated OM

Hydrolizableclay-associated OM

Recalcitrantsilt-associated OM

centrifugationAcid Hydrolysis

Example of an alternative fractionation scheme to separate the recalcitrant fraction

14C in SOM fractions

80 85 90 95 100 105 110

pMC

LFPOMSilt, Clay

Clayrec Siltrec Clayhyd Silthyd

pMC = 14Rsample/14Roxll * 100WE, 0-10 cm

So… Which scheme should you use?

• There are a lot of different options and it does not exist the best one.

• Define your question first and then chose the scheme that would allow you to separate the OM pools of interest to you.

• The basic procedure are quite well established (i.e. dispersion, size or density separations etc.), however you can decide to combine them in different orders depending on if you want to isolate aggregates or not, if you are interested more in the short or long term dynamics, etc.

Modeling SOC using the kinetic pool approach

Century Model, Parton et al. 1996

Modeling SOC using physically defined pools

Six et al., 2002

Factors controlling soil C stabilization

For fast and intermediate turnover- C pools: a. Intrinsic Chemical recalcitrance b. Aggregation/accessibility

For slow turnover- C pools: a. Interaction with mineral surfaces , especially with Fe oxide

Kögel-Knaber et al., 2008

TAKE HOME MESSAGE

Soil organic matter can be separated in different pools, by biological, physical or chemical fractionation methods.

Most of the current models used to simulate soil organic matter dynamics use “kinetically” defined soil C pools

Understanding organic matter stabilization mechanisms is key to predict long term responses of SOM to global changes. Mineral association, and in particular interaction with Fe oxide, appears to be the most important long-term stabilization mechanism.

The Global Carbon Cycle

The Global C cycle is currently unbalanced, due to anthropogenic emissions of CO2 to the atmosphere. Oceans and terrestrial ecosystems are acting as sink of C, sequestering about two thirds of the anthropogenic emissions of CO2 – the rest remains in the atmosphere where it is the main contributor to warming

IPCC, 2007

Estimates of global C sinksBaker, 2007; Stephen et al., Science. 2007

Sink and Source processes driving the Land-Atmosphere C unbalance

Processes Sink, Source

Process driven by atmospheric composition change

CO2 fertilization of photosynthesis Sink

Nitrogen deposition fertilization on net primary production Sink

Air pollution effects on net primary production Reduced sink

Processes driven by climate change

Temperature and precipitation effects on net primary production Sink / Reduced Sink

Temperature and precipitation effects on heterotrophic respiration Source

Permafrost thawing Source

Shifts in natural disturbance (wildfires, insects attacks) Source

Shifts in vegetation types Sink/Source

Process driven by land-use change and land management

Afforestation and reforestation Sink

Forest regrowth in abandoned cropland Sink

Vegetation thickening and encroachment Sink

Shifts in fire regimes and other disturbances Source / Reduced Sources

Soil erosion and C burial Source / Sink

Crop management Source / Sink

Deforestation Source

Peat and drainage Source

TAKE HOME MESSAGE

We currently have an array of methods to quantify NET CO2 EXCHANGES between the atmosphere and the biosphere, with a high level of accuracy.

All methods concord in showing a significant, yet variable, terrestrial C sink. Latest evidence suggest that the sink is stronger in the tropical region than earlier thought.

Where is this “extra” C going?

How much of the NEP is stored in soil organic matter, and for how long?

The “long term” survey approach

Bellamy et al., 2005

This controversial study, for the claim that climate change is responsible for large losses of C from C rich soils in the UK, is based on the re-sampling of soils from a large number of sites surveyed 12 to 25 years before.

The “long term” survey approach

Problems:

- Availability of historical SOC data or soil samples to analyze- Comparability of past and recent methods to determine soil C- Availability of accurate bulk density data

Yang et al., 2009

Several studies report data on SOC changes through time using a survey approach – and vary from no changes to negative or positive – Results appear to be site specific and highly driven by land use history

The “chronosequence” approach: using space for time

Thuille & Schulze, 2006

0 yr

20 yr

40 yr

60 yr

80 yr

100 yr

Soil monitoring network

• Networks may comprise a large number of sites, across climatic, edaphic and management gradients • Allow for re-sampling in the same location through time.•Different sampling strategy can be designed: the scope is to maximize the detection power of changes over time. Thus , differences due to spatial variability need to be reduced as much as possible

Modeling soil C

Meersman et al., 2009

A good process based model, supported by solid initial data, can be extremely helpful at predicting soil C dynamics, in particular in response to changing conditions (i.e. climate, land-use, management etc.). It would be impossible to make a field experiment for every model run!

Lateral movements are often ignored in studies of soil C dynamics.However, the variation in soil C stores may strongly depend on topography and on a range of processes interrelated with slope (i.e. productivity, soil moisture, etc.)

Trumbore et al., 2009

Soil C sequestration: the challenges

Quantify a small change in a large stock

Determine the permanence time of the additional C stored, i.e. which pool is affected?

Verify that by sequestering C, other GHG (i.e. N2O) emissions are not promoted.

What is the cost of sequestering C in the soil for other resources, i.e. soil biodiversity?

The chronosequence approach can tell if the total C has changed. How do we determine which pools are

affected by the change?

0

1000

2000

3000

4000

5000

6000

Grassland Maize crop Afforested

So

il C

(g

m-2

)

0-10 cm

10-30 cm

100 years20 years

A case study in Northeastern ItalyG

A

C

Assessing changes at the level of SOM fraction, rather than bulk soil, increases the detection power. It also provides insights in the process occurring and the permanence time of the sequestered C.

If the system under study underwent a vegetation change (i.e. C3 to C4, or viceversa),

13C of the SOC can allow partitioning of the newly added C versus the native SOC (to be discussed tomorrow!)

Bulk soil was fractionated by size and density Individual fractions were analyzed for C%

Del Galdo et al., 2003

TAKE HOME MESSAGE

Several methods exist to quantify changes in SOC stocks. Many rely on the existence of historical data or specific site conditions (i.e. chronosequence, shifts in natural isotopic abundance).

The establishment of Soil C monitoring networks will ensure future accurate data of SOC changes

The analyses of SOM fractions rather then bulk soil allows to obtain more accurate estimates of SOC changes and information on SOC dynamics

Soil-Atmosphere Greenhouse Gas Exchanges

IPCC, 2007

Estimates of soil contribution to trace gases emissions to the atmosphere

70%

30%

Mosier, 1998

SPM 3

Component of the radiative forcing

SPM 1a

A measured fact: The Earth surface temperature increased by on average 0,6°C in the past 100 years

Will global change result in net losses or increases in soil organic C?

Climate controls on soil C mineralization

Carbon mineralization increases with temperatures, unless water becomes limiting (Rey et al., 2005)

However, the temperature sensitivity of SOC decomposition may depend on the specific SOC pool kinetic properties and stabilization mechanism (Davidson &

Janssen, 2006)

Additionally, warming may induce changes in microbial physiological properties, leading to reduced carbon-use efficiency (i.e. fraction of assimilated carbon that is allocated to growth). As a result, the warming stimulation of C mineralization would not be sustained over time (Allison et al., 2010)

Depleting Soil Organic Matter

Courtesy of K. Paustian

Positive feedback on warming

Rebuilding Soil Organic MatterNegative feedback on warming

Courtesy of K. Paustian

TAKE HOME MESSAGE

Because of soil microbial metabolism, soil is the largest contributor to atmospheric emissions of CO2, N2O and CH4

These are all greenhouse gases whose concentration has grown out of the natural range, and which are responsible for the occurring and projected global warming.

Land use and soil management, as well as the effects of global changes (e.g. climate, N depositions etc.) on soil C and N dynamics will determine the extent to which soils will mitigate or further accelerate climate change.

References

Allison et al., 2010. Nature Geosciences. 3: 336-340.Batjes, 1996. European Journal of Soil Sciences. 47: 151-163Bellamy et al., 2005. Nature. 437: 245-248Davidson & Janssen, 2006. Nature. 440:165-173.Del Galdo et al., 2003. Global Change Biology. 9: 1204-1213Jastrow and Miller, 1998. In: Soil Processes and the C cycle. Eds. Lal et al., CRC PressKögel-Knaber et al., 2008. J. Plant Nutr. Soil Sci. 171:61-82 (special Issue)Meersmans et al., 2009. Geoderma 152:43-52Mosier, 1998. Biology and Fertility of Soils. 27: 221-229. Parton et al., 1996. In: Evaluation of soil organic matter models using existing long term dataset. Eds. Powlson et al. SpringerSix et al., 2002. Plant and Soil 241:155-176Stephens et al., 2007. Science 316: 1732-1735Thuille & Schulze, 2006. Global Change Biology 12:325-342Trumbore, 2009. Annual Reviews of Earth and Planetary Sciences. 37:47-66.Von Lützow et al., 2007. Soil Biology and Biochemistry 39: 2183-2207.Yang et al., 2009. Global Change Biology. 15: 2723-2729