Cover Crops and Carbon Cycling

Cover crops have a long history of use in annual cropping systems worldwide thanks to benefits they can provide (such as reducing soil erosion, improving soil function, suppressing weed growth, breaking pest and disease cycles, and providing livestock fodder). Despite these benefits, cover crops fell out of favor amongst non-organic grain/oilseed/pulse commodity growers in North America following the Second World War. During the past decade a small but growing movement of commodity growers in Canada and the U.S. have found ways integrate cover crops back into their commodity production systems and cover crops are again becoming an important tool in profitable commodity production systems.

The resurgence in cover crop use has been facilitated by improvements in no-till (or direct) seeding and planting equipment, the development of scientifically-based cover crop extension knowledge, and a wealth of farmer-developed knowledge shared via social media. Recently, cover crops have become central in debates around commodity agriculture’s potential to sequester excess atmospheric carbon.
Commodity grain, oilseed, and pulse production is based on annual plant species. In North America, the land clearing and heavy annual tillage that make commodity production possible have made these farming systems significant net atmospheric carbon sources since colonization. Cover crops (combined with no-till, heavy cash crop residue, plant diversity, and ruminant livestock) have the potential to turn North American commodity grain/oilseed/pulse production systems into net atmospheric carbon sinks for the first time. It is due to these changes in soil management practices and their potential to play a partial role in climate change mitigation that has garnered the attention of lawmakers, government agencies, and corporations.  

It is important to point out that the biomass of cash crops and cover crops breaks down too quickly to sequester atmospheric carbon for any significant amounts of time. Instead, root exudates are the primary means by which excess atmospheric carbon can be fixed in the soil for long periods. Root exudates can take a number of different forms, but long chain starches are the most important regarding climate change mitigation. It is estimated that these long chain starches can be stable for up to 500 years. Therefore, if cover crops are fixing carbon in the long periods outside of the main growing season and cash crops are fixing carbon during the main growing season, the potential of annual agriculture systems to sequester excess atmospheric carbon is significant.

In conclusion, cover crops can be useful tool in commodity production systems thanks to the agronomic and soil health benefits they can provide. They also can play a key role in fixing excess atmospheric carbon. However the buzz around cover crops as a climate change mitigation tool should come with the following warnings:

1) The ability of commodity production systems to fix excess atmospheric carbon is highly dependent on local climatic conditions and farming practices
2) Methodologies for monitoring carbon cycling is still in its infancy and scaling these methodologies up to measure carbon cycling across commodity production landscapes is not possible at this time
3) Cover crops are inappropriate for large tracts of dryland commodity production areas in North America thanks to cold and dry climates.

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