Soil C in agriculture: the big uncertainty

Franco Miglietta

IBIMET-CNR, Firenze, Italy

Climate variability, extreme events increasing atmospheric greenhouse gas concentrations

Forest soils

Peatland

Permafrost

Agricultural soilsLand Use & Management

Terrestrial carbon storage, exchange flows and soil carbon dynamics

The climate feedback

+

Uncertainty onAnthropogenic Carbon

Emissions

Up to250 ppm

IPCC SRES 2000; Friedlingstein et al. 2006

Vulnerability of the Carbon Cycle in the 21st century

Up to 200 ppm

Uncertainty of theBiospheric-Carbon-Climate

Feedback

Slide courtesy of Pep Canadell, GCP

EU25:

Utilised agricultural area = 164 051 000 ha; Land under permanent crops = 11 594 000 ha; Land under cereals (excluding rice) = 51 610 000 ha;Permanent grassland = 57 124 000 ha; Arable land = 97 065 000 ha

Forest area ~ 140 000 000 ha

0

10

20

30

40

50

60

70

80

1955 1960 1965 1970 1975 1980 1985 1990

Year

t C

/ha

(0-2

3cm

)

Ploughed out grassland. Highfield, Rothamsted, UK

C-content of agricultural soils is sensitive to management

100 years 20 years

Micro's aggregates type

0 mM M

0

20

40

60

80

100

120

140

160

180

200

B

B

A

B

B

A

g C

kg

–1 s

and

free

mic

ro’s

100 agricultural use

20 years afforestation

A F N A F N

Source: II° Università di Napoli, Caserta – Università di UdineDel Galdo et al. Global Change Biology (2003)Assolari et al., Soil Biology and Biochemistry (2003)

+3% per year -1.3% per year

Davidson & Jansseens, Nature Vol.440, March 2006

Giardina, C.P. & Ryan, M.G. (2000) Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature. Nature, 404, 858-861.

Fang, C., Smith, P., Moncrieff, J.B. & Smith, J.U. (2005) Similar response of labile and resistant soil organic matter pools to changes in temperature. Nature, 433, 57-59.

Knorr, W., Prentice, I.C., House, J.I. & Holland, E.A. (2005) Long-term sensitivity of soil carbon turnover to warming. Nature, 433, 298-301.

C-content of agricultural soils sensitive to climate ?

Reductionist’s approach

Temperature sensitivity of decomposition

Irrigation Ploughing Fertilization

Relating fractions to the pools used in models

Experimentalist’s approach

Manipulation experiments:

Temperature +

Precipitation/Irrigation+

Elevated CO2 +

Crop management =

Model validation datasets

Short-term: Flux changes

Long-term: stock changes

OTC

Pro: Relatively easy to make

Con: Unavoidable decoupling between crop/soil-atmosphere

Free Air CO2 Enrichment (FACE)

Pro: Realistic

Con: Very-high CO2 demand, Difficult to sustain in the long-term

Manipulation strategies I: (CO2)

Automated Rain Shelters

Pro: Excellent control / manipulation

Con: Expensive, major infrastructure

Throughfall displacement

Pro: Simple installation, low cost

Con: Soil shadowing

Manipulation strategies II: (Water)

Soil warming

Pro: Relatively easy to make

Con: Unrealistic, due to the decoupling crop/soil processess

Free Air Thermal Enhancement (FATE)

Pro: Very Realistic

Con: High energy demand, Temp & Vpd, difficult to sustain in the long-term

Monolith transplanting

Pro: Very realistic, easy to make

Con: Requires proper and extended networking, long-term complications, potential confounding effects

Manipulation strategies III: (Temperature)

Passive heating

Pro: Relatively easy to make

Con: Not very realistic, sometimes small effects

CONCLUSIONS OF DOE(USA) ON FATE

• A plot 10 m wide with vegetation 0.5 m tall and an average wind speed of 1 m s-1.

• A volume of 18,000 m3 hr-1 would have to be heated.

• If the heat treatment was set at 4°C above ambient this would require 1,448 kWh.

• For a 24-hr operation, and at US$ 0.1/kWh, this would cost US$ 108,000

Z

Y X

Contours of Static Temperature (c)FLUENT 5.4 (3d, segregated, spe3, ke)

Dec 08, 2001

29.8

29.5

29.2

28.9

28.5

28.2

27.9

27.6

27.3

26.9

26.6

26.3

26.0

Lateral-view

Heaters/Blowers

°C

+2°C

Z

Y

X

Contours of Static Temperature (c)FLUENT 5.4 (3d, segregated, spe3, ke)

Dec 08, 2001

29.8

29.5

29.2

28.9

28.5

28.2

27.9

27.6

27.3

26.9

26.6

26.3

26.0

Top-view: surface 30cm above ground

°C

+2°C

Courtesy of Wayne Polley USDA-ARS, TX

European Soil Monolith Exchange Network

Western EuropeSweden - 2

Denmark - 2UK - 11

Belgium - 1Germany - 16

Switzerland - 2France - 1

Italy - 3Spain - 1

Central & Eastern EuropePoland - 2Romania - 1Hungary -10Czech Republic - 3 Slovenia - 2

Former Soviet UnionEstonia - 1Russian Federation - 25Belarus - 2Ukraine - 12Moldova - 5Georgia - 7

EuroSOMNET + Monolith Exchange

+2°C

Model validation

Fluxes Fluxes

Stock change

Stock change

Further manipulation (Water / CO2 / Management)

CONCLUSION

- We can’t predict climate change effects on soil C in croplands. The big uncertainty. New knowledge on thermal response of decompositoin of SOM fractions is required.

- Experimental manipulations are necessary to learn more about such response. Networking should be a priority, linking observations and modelling

-Detailed measurements (sensu Zimmermann et al. 2007) are also required to constrain our models. Towards standard fractions/pools protocols?

Very large C-sequestration in agricultural soils