The Carbon Footprint of Western Agriculture - Carbon Cycles and

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The Carbon Footprint of Western Agriculture - Carbon Cycles and Powered By Docstoc
					Soil Association review of soil
 carbon and organic farming

           Gundula Azeez,
    Presentation at SA conference,
       Bristol, November 2008
                   Introduction
Soil carbon is a very important aspect of climate
  change:
• historically accounts for c.²/3 as much CO2 as
  fossil fuels
• unlike fossil fuels, the carbon loss from soil is
  reversible
• major gains are possible: theoretically, a 5%
  soil C increase can mean a 10% decrease in
  atmospheric CO2
• higher soil C may reduce the impacts of
Soil is losing carbon
   Current soil carbon losses
Myth: arable soil carbon levels are ‘low but
 stable’.
• Europe’s soil C store is reducing by c.
  300million t C/yr
• there have been four national soil surveys in
  Europe:
  – Belgium:   - 480kgC/ha/yr (0-30cm)
  – Austria:   - 240kgC/ha/yr (0-20cm)
  – Denmark: small loss 0-25cm BUT +170kgC/ha/yr
    25-50cm
     Reasons for falling soil C
             levels
Myth: ‘It’s all land use change’, ploughing of
 grassland
• inorganic N fertiliser - many indirect effects,
  eg. roots
• abandonment of mixed farming with grass
  (1950s +)
• less deposition and spreading of manure,
  more slurry
• higher yield, but reduction in non-yield crop
  biomass
 Organic farming - what
evidence is there on soil
     carbon levels?
      Evidence for comparative
               levels
Controlled long-term trials   Duration, years      Reference

Rodale Institute FST, US           21           Hepperly et al, 2006

FiBL DOK trial, Switzerland        28           Fließbach et al, 2007
IBR Darmstadt, Germany             18           Raupp and
   Oltmanns, 2006
Michigan University, US             9           Robertson et al,
   2000

Comparisons of organic & non-organic farms

30 pairs org & non-org farms, England           Armstrong Brown et
   al, 2000
Org & non-org tomato production, US             Drinkwater et al,
                Organic Farming
                evidence
• of 34 studies, OF has higher soil carbon in 31
  studies
• OF produces around 30%higher soil C
  (cultivated soil)
• in 8 of 10 long-term studies, organic farming
  increased soil C over time (sequesters
  100kg-1tC/ha/yr)
• carbon offset may reduce GHGs of European
  OF 5-30%
But:
 OF - reasons for higher levels
• dedicated fertility-building stages in the
  rotation
• additions of organic matter: FYM, green
  manure crops
• higher amount of straw
• greater root biomass
• higher proportion of soil organisms
• composting - organic horticultural &
  biodynamic farms
• less use arable land for feeding livestock
       Assumptions about C
          sequestration
Many negative assumptions are being used as
  reasons for not recognising the soil C benefits
  of organic farming:
• the use of ploughing is a weakness of organic
  farming
• higher yielding systems can build up more
  soil carbon
• the higher microbial life of OF may be a
  disadvantage
• applying high levels of organic matter has
 Assumptions - are they really
           true?
No, the evidence and analysis shows they are
  incorrect:
• higher crop yields means more organic
  matter is being removed from the farm. In
  FiBL trial, the non-organic system yielded
  more but did not build more soil carbon.
• Below-ground biomass & soil micro-
  organisms are also factors. In Rodale,
  despite similar above-ground C input, organic
  systems produced more SOM.
               Conclusion
• grassland and arable soil carbon levels falling
  rapidly
• good scientific evidence organic farming
  produces c. 30% higher soil carbon levels
  than non-organic farming
• gives positive potential for reducing GHGs (5-
  30%)
• good practices: mixed farming, composting,
  and FYM.
• soil carbon should be included in all GHG

				
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