How will conversion to organic cereal production affect carbon stocks in Swedish agricultural soils?

Soil carbon changes were modelled over 30 years with the focus on cereal crops, since leys are often managed similarly in organic and conventional agriculture. Other crops were not considered due to difficulties in large-scale cropping of oilseed rape and potatoes organically because of pest problems. Four scenarios were used: 0%, 8% (current), 20% and 100% organic cereal production. Conversion to organic cereal crop production was found to reduce the amount of carbon stored as organic matter in agricultural soils. Three factors contributed to decrease soil carbon levels in a given field: (i) a yield decrease, resulting in less C input through roots and above-ground crop residues; (ii) lower leaf area causing less water uptake, which resulted in higher water content in soil and an increased decomposition rate of soil organic matter; and (iii) more frequent and intensive mechanical cultivation for weed control, which resulted in increased mixing and exposure of soil organic matter to oxidative processes, speeding up decomposition. Due to lower yields in organic agriculture, more land must be used to produce the same amount. With 20% organic cereal production, land currently in fallow would have to be taken into production, while with 100% of cereals produced organically, all fallow land plus conversion of forest land to agriculture would be required. An 8% level of organic cereal production would lead to losses of 0.3 Tg C over a 30-year period, 20% would cause losses of 1.1 Tg C and 100% would cause losses of 12.8 Tg C. The annual CO₂ losses from 100% organic cereal production would be equivalent to the amount emitted by 675 000 average cars in Sweden annually. Losses of soil carbon under organic cultivation would continue for a much longer period than 30 years until a new equilibrium is reached.