Forage Crops

HCS612 Forage Crops
Carbon Sequestration

The situation (pg 17)

World carbon balance

Atmospheric increase	=	Emissions from fossil fuels	+	Net emissions from changes in land use	-	Ocean uptake	-	Missing carbon
3.3 Pg		5.5 Pg		1.6 Pg		2.0 Pg		1.8 Pg
One Pg [petagram] = one billion metric tonnes = 10^15 g www.woodshole.org

Background

	OM is 45-58% carbon
	Grasslands contain 10-30% of the world’s soil organic carbon
	3 reservoirs of carbon in grasslands standing biomass (1%) soil organic carbon (90%) soil inorganic carbon (carbonates in dry environments & CO2) (9%)
	World wide reserves are 50 Pg as standing biomass, 300 Pg as organic carbon, 500 Pg as inorganic carbon)
	In USA cropland 20-50% of soil carbon has been lost
	In grassland - overgrazing, erosion and weak stands have resulted in some loss of soil carbon

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Typical levels of Organic Matter

	Prairie is typically 2-3% OM
	Ohio natural vegetation is forest (typically 3-5% OM) – cropping and to a lesser extent grasslands have lowered Ohio soil OM to ~2%
	Cropped soils are typically 1-2% OM

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Soil OM matter sources & fractions

<1% of OM is above-ground plant material (<2 year cycle time)

~11% is below-ground plant material (~5 years cycle time).

Roots are the main mechanism for transfer of carbon belowground in extensive pasture <400 kgC/ha/yr, in intensive pastures ~1200 kgC/ha/yr.

1% is bacterial OM with a rapid cycle time (<2 year)

12% is fungal and animal-derived OM with a slow cycle time (50 years)

75% is recalcitrant (200-1000 year cycle time)

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Soil Organic Carbon Forms

	Litter (dead vegetation) (8%)
	Light OM (small litter 0.25-2 mm) (50%)
	Plant roots (25% of OM)
	Particulate OM (10% of OM) 50-2000 mm (Fine and coarse OM)
	Microbial OM (5% of OM)
	Humic and fulvic acid – small amount but are bound to soil particles
	Soluble carbon (passes a 0.45 mm pore) (10-500 kg/ha/yr)

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Management factors affecting sequestration

	Reduced soil erosion
	Greater vegetative cover
	Pasture species
	Grazing – promotes root ‘turnover’ – cycles of growth and death adding carbon to soil. Grasses are adapted to this treatment. Grazing responses vary – some studies report decreased sequestration with grazing. Grazing has more sequestration than mowing.
	Fertilizer use

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Conclusion

The potential for sequestering carbon on grasslands is smaller than in crop land or if land is converted to forestry. However the extensive scope of grasslands suggests they have potential for significant carbon sequestration

Roots can build soil organic matter at around 1,200 kgC/ha/yr, 2400 kgOM/ha/yr. This will increase soil OM by 0.1% per year up to a limit around 2-3% OM.

If a carbon tax were introduced it would give opportunity to farmers for diversified income

The potential for income would be greater on conversion from cropping land, and conversion to forestry – little benefit to grasslands

Management companies would be likely to purchase the carbon rights for lump-sum payments – with little on-going benefits to grazing farms. Subsequent land management options would likely be restricted.

If a carbon tax were sufficiently valuable – industrial processes could extract CO2 from air, react it with Ca and bury the products in abandoned mines. Agriculture might be excluded from any benefits.

	<1% of OM is above-ground plant material (<2 year cycle time)
	~11% is below-ground plant material (~5 years cycle time).
	Roots are the main mechanism for transfer of carbon belowground in extensive pasture <400 kgC/ha/yr, in intensive pastures ~1200 kgC/ha/yr.
	1% is bacterial OM with a rapid cycle time (<2 year)
	12% is fungal and animal-derived OM with a slow cycle time (50 years)
	75% is recalcitrant (200-1000 year cycle time)