Soil Carbon and Agricultural Land Management in Semi-Arid

Central Spain

Darcy BoellstorffAssistant Professor, Bridgewater State College,

Massachusetts

• Economic and societal benefits– Food source

stability– Biodiversity

• Soil carbon and soil restoration determined by site-specific processes– Land use – Climate

Overview: soil restoration and soil carbon

• Soil organic carbon (SOC) – Physical – controls aggregate stability,

aeration, water storage– Chemical – allows soil to hold nutrients

for plant growth – Biological – energy source for microbes

• SOC level determined by balance between interrelated factors– Vegetation decomposition rates– Temperature and moisture regimes– Land management

• Where SOC is low, land management that increases SOC is linked to restoration of degraded soil (Parshotam and Hewitt 1995, Lal 2003, Kong et al. 2005)

Soil restoration and soil carbon

• Rates of carbon cycling, SOC pools–Fast turnover

• Plant matter• Biomass

–Slow turnover • Humus

• Positive correlation between humus and aggregate stability (Drogovoz 1994, Hernanz et al. 2002, Sevink et al. 2005)

SOC

aggregate stability

Modified from Hernanz et al. 2002

AS = 1.67SOC - 8.6r2 = 0.62

Soil restoration and soil carbon

Soil type - Alfisols

• Arkose pedisediment

• Well-drained• Topsoil

–SOC stored in upper horizon

–Varying texture, clay content

image source: CSIC-CCMA

• Chemical– mining of soil

fertility and loss of nutrients

• Biological– decline of soil

microbes • Physical

– lowered aggregate stability from tillage

– crusting and redistribution of topsoil

Soil degradation

Lower image source: CSIC-CCMA

T C ha-1 percent

pasture 12.8 1.0

rotated 6.4 0.5

T C ha-1 percent

pasture 13.5 1.2

rotated 7.8 0.7

Soil carbon cycling models

• Models– SOCRATES: southern Australia (P. Grace)– RothC: northeast United Kingdom (D.

Jenkinson, K. Coleman)

• Applied in range of ecosystems– Semi-arid systems in U.S., Europe and

Australia – Range of SOC values

• Benefits– Calculate changes for different SOC

pools over long time scales– Can be used to predict outcome of land

use and climate change– Crop yields (SOCRATES)

SOCRATES

r2 = 0.90

1:1 line

8.00

9.00

10.00

11.00

12.00

8.00 9.00 10.00 11.00Modeled SOC (T ha-1)

Scenario comparison: SOCRATES

• CP: Continuous pasture • T: Traditional cereal rotation• P5: 5 year pasture, 5 year

cereal rotations• P10: 10 year pasture, 5

year cereal rotations

Scenario comparison: SOC

8

13

18

23

28

33

38

43

year

T CP P5 P10

Scenario comparison: Humus

5

10

15

20

25

30

year

T CP P5 P10

Scenario comparison: Grain yields

T CP P5 P10Years pasture 33 100 50 65

Years cereal 34 -- 30 21

Years fallow 33 -- 20 14

Average annual yield (kg/ha) 2,638 -- 2,994 2,976

Total yield (kg/ha) 89,708 -- 89,825 62,513

• Longer pasture rotation positively correlated with:– Improved humus

conditions – Increased aggregate

stability – Crop yields

• Longer pasture could present policy-makers with win-win scenarios– Restoration of soil– Maintained crop

output

Conclusions and application

• Bridgewater State College Center and the Advancement of Teaching and Research.

• Senior research scientists Dr. Gerardo Benito and Dr. Gonzalo Almendros and Carlos LaCasta, manager of La Higuerela experimental farm

• The Centro de Ciencias Medioambientales, part of the Consejo Superior de Investigaciones Cientificas in Madrid, Spain for sharing their data sources.

Acknowledgements