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 climate change: flooding, effects of drought and need for irrigation. But soil carbon is omitted from GHG assessments!
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 • UK: loss at 92% of sites, maybe 1% of total each year • in the UK, most land is losing carbon, arable and grass
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 • excessive grazing livestock stocking rate (1976 - 2005) • deeper ploughing … negative or positive? • avermectin livestock wormers reduces dung decay ... ?
Evidence for comparative levels Controlled long-term trialsDuration, yearsReference 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, 1995 Org & non-org wheat field, US Reganold et al, 1987
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: • in the UK, little evidence of soil carbon sequestration • impact of organic farming on grassland soil carbon?
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 • agro-forestry in the tropics - up to 2.5tC/ha/yr
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 little result • sequestration is not continual but mainly in early years • soil carbon sequestration is non-permanent, reversible
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. • ability to build soil C within 20 years is a real strength!
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 assessments • OF should be more promoted by climate policy • research on OF and grassland, and the deeper soil layer