Effects of grazing and fire on soil carbon in dry african savanna
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Transcript of Effects of grazing and fire on soil carbon in dry african savanna
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Effects of grazing and fire on soil carbon in dry African Savanna
Ermias BetemariamJan de LeeuwKeith Shepherd
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• Land productivity is key to feed the world - grasslands as “protein pools”
• Land degradation remains a global challenge and reducing/reversing land degradation is a development/research priority
• Soil comes to the global agenda: sustainable intensification
• Global Environmental Benefits - land degradation and soils are among the priority global benefits (GEF/UNCCD)
Context
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Context (cont.)
Erb et al. 2000
• Extensive grassland are mostly in arid and semi -arid zones.
• 75 percent of eastern Africa is dominated by grasslands
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• Moisture is a major limiting factor in the drylands
• Low and variable rainfall
• Sensitive to disturbances- rangeland degradation/bush encroachment
• Grass lands have hight SOC stocks (~12% of terrestrial SOM)
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Context (cont.)
• Topsoils (1m) contain 1400-1500 Pg (or Gt, billions of tonnes) of SOC
• Three times the amount that is present in the atmosphere (ca. 800 Pg) or vegetation (ca. 550)- small change could cause large effects on climate system• Climate change mitigation potential through C sequestration is less known (FAO, 2010).
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SOC balance• Inputs
• Litter, roots, branches
• Outputs
• Autotrophic respiration: roots
• Heterotrophic respiration: CO2
respiration of soil organisms that use dead plant matter as a food source
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SOC balance (cont.)What determines the amount of SOC in a soil?- the kitchen sink analogy (Govers, 2012)
SOC sequestration = Increasing SOC stocks through the increase of inputs and/or decreasing C decomposition (M. Stocking, 2012)
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SOC balance (cont.)Disturbance• Climate change:
• increase warming increase respiration rates = SOC increases
• Warming & CO2 fertilization stimulate plant growth = SOC increases
• Tillage: increases respiration: SOC decrease
• Harvesting: reduces inputs = SOC decrease
• Land use change = SOC increase or decrease
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Two decades of controlled burning did not affect soil C in Sudanian savanna-woodlands of West Africa
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• Fire is widely used as a management tool in tropical savanna ecosystems
• Long-term effect on soil carbon (SOC) is poorly documented
• Fire has the potential to alter soil carbon storage by influencing rates of net primary productivity, carbon allocation patterns, and rates of organic matter decomposition (Ojima et al., 1994)
• Carbon allocation - SOC gain
• Decrease input - SOC loss
Fire-a necessary evil
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The study site
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Methods (cont.)
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Experimental design (1992-2011)
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ResultsNo Sig difference in SOC between burned and unburned plots
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Results (cont.)No Sig difference in SOC between burned and unburned plots
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Results (cont.)No Sig difference in SOC between burned and unburned plots
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Effects of termite on SOC• Termites contribute for an increase
in SOC in savannah ecosystems
• Introduction of earth worms increased soil carbon in grasslands (Conant et al. 2001)
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Three decades of livestock exclosures did not affect soil organic carbon in dry savanna in southern Ethiopian
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Rangeland degradation• Bush encroachment following banning of
burning in the 1975s
• Several efforts to reverse bush encroachment
• Controlled grazing as a strategy to enhance soil carbon sequestration
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The study site
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Methods• 14 exclosures were mapped
• Pariwise sampling
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Results No sig. difference in SOC content between closed and open sites for all age categories
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Results (cont.) No sig. difference in SOC between closed and open plots for all age categories
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Exclosures for soil and water conservation
Exclosure have high sediment trapping capacity – Effective soil and water conservation measure (Descheemaeker et al. 2006)
Exclosures significantly reduces soil erosion and gives more grass production
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Rangeland degradation in Borena rangelands: soils matter
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Results (cont.)
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Results (cont.)
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Baringo - SOC difference between 25 years closed and open grazing lands (Vagen et al. 2007)
Some findings from Kenya
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Some findings from Kenya
http://www.coderedd.org/redd-project/wildlife-works-carbon-rukinga-redd-project/
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Final remarks • Effects of fire on soil processes are highly variable and no
generalized tendencies (Gonza Nlez-Pe Nrez, 2004)
• Exclosures are viable options to restore degraded lands
• Carbon loss by erosion
• Target multiple benefits: practices that sequester carbon in grasslands often enhance productivity, biodiversity, etc.
• SOC sequestration is not a commercially viable operation at present - response time decades (set realistic goals)
Site-specific solutionsChivenge et al., 2007:
•increase input in course textured soils
•reduce decomposition/disturbance in light textured soils
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• The role of water as the primary limiting factor on productivity may be more important (Semmartin et al., 2004).
• Where to invest? (BIODEV)
• mineralization rates are low (cold, moist climates) or
• input rates are high (warm, wet climates)
• It is easier (and probably cheaper to preserve than to restore - drylands of Africa
• Modeling SOC is difficult - may be better described with more complex models
Final remarks (cont.)
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