Soil management: Longer crop rotations reduce carbon emissions

Changing the way farmland is used could help increase the amount of organic carbon retained in soils. According to recent research, studies of different crop rotation patterns will help decision makers design policies which help reduce carbon emissions.

Farming and forestry practices aimed at maximising the amount of carbon at land level are broadly referred to as 'LULUCF' - land use, land use change and forestry. Although carbon sequestered through LULUCF does not count towards Kyoto Protocol targets, these practices are nonetheless considered an important way of decreasing atmospheric carbon concentrations in national greenhouse gas inventories1.

The researchers studied the effects of different land management practices on soil concentrations of carbon in order to help decision makers design appropriate policies to increase concentrations. Computer modelling was used to reveal how the various practices may affect soil quality and carbon content in the long term. They conclude that encouraging land owners to increase soil carbon content through longer crop rotations could contribute towards carbon emissions targets, while also increase agricultural productivity.

The study focuses on the effects of different management practices on farmland soils in the La Higueruela region of Central Spain. Most farmland here is used for winter cereals. Traditionally, cereals are cultivated here in three year cycles - wheat is grown the first year, then the land is planted with legumes and turned into pasture. It is then left to rest (fallow) in the third year. Soil samples from different field locations revealed that carbon levels were lower in soils used for growing cereal crops compared to levels in soils from pasture land.

Results from previous long term soil studies in the same region were used to validate a computer model. The effects of different management were then projected over 25- and 100-year periods. According to the model, management strategies play an important role in determining carbon content. Land which has been used to cultivate cereal and then turned into continuous pasture would contain the most carbon. After 25 years, it is projected to contain 1.7-1.8 times more carbon than land which continues to undergo traditional three-year crop rotations.

However, there is an alternative strategy, which allows the farmer to continue growing crops while increasing soil carbon levels, although to a lesser degree than the continuous pasture scenario. Five-year crop rotations - where the land is turned into pasture for five-year periods between five-year wheat-fallow rotations - increases carbon levels by 1.3 times after 25 years.

By replacing three-year rotations with five-year rotations, farmers would only lose three rotations over 100 years (30 versus 33), meaning that economic impact is likely to be small. Nonetheless, policies addressing land use change would need to consider how changes to crop rotations affect income and government subsidies. However, the study points out that humus (stable organic material) levels could also increase by 2-3 times under five-year rotations. Humus is important to soil health and can increase crop productivity.

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