Cover Cropping to Enhance Carbon Sequestration in Agriculture
Benefits of Cover crops
As global attitudes shift towards adopting environmentally sustainable production practices to mitigate climate change, cover cropping can play a meaningful role in improving soil health by returning CO2 from the atmosphere to the soil. Cover crops (or catch crops) provide multiple benefits that can improve soil productivity. They provide organic matter full of carbon (C) and nitrogen (N) which can increase soil water holding capacity, porosity, aggregate stability, nutrient cycling, and microbial activity12,19,28, as well as reduce erosion, disease pressure, pest pressure and nitrogen leaching, and fix atmospheric nitrogen and sequester carbon1,2,4,6,9,16,20,25,31,38,41,44. Increasing the quantity and quality of plant biomass returns with favorable soil conditions for incorporation can increase soil organic carbon.
What are Cover Crops?
Cover crops are typically planted in the fallow period between cash crops. The cover crop provides a vegetative cover over the soil which protects it from erosion and reduces nitrate leaching48. They are typically produced in the fall and early spring when low temperature and light availability are likely to promote high ratios of root to shoot production30. Deeper cover crop roots can provide deeper delivery of carbon into the soil profile1. Cover cropping can utilize a single species or a mixture of species and can be annual, biennial or perennial vegetation1. They can be ploughed‐in during the winter or spring or grazed then incorporated in soils by tillage to prevent competition with the primary crop, and to promote mineralization of organic N1,11. Most cover crops do not reach maturity, therefor are not harvested. Their residues are left to decompose in the field adding C and N to the soil, especially when legumes are added into a rotation6,7,9,11,13,18,20,22,24,31,39,45,47.
Quantifying the Correlation Between Cover Cropping and Enhanced Carbon Sequestration
The following table displays a summary of some of the carbon sequestration potentials observed by other studies using various cover cropping strategies. These numbers show an increase in total soil carbon from the use of cover crops compared to cropping systems without cover crops (expressed in percentage increase or added tonnes per hectare per year).
|Reference||Description of Crops||Carbon Sequestration Potential|
|Abdalla et al. (2019)||Legume/non-legume mixes||0.54 tonnes C/ha/yr|
|Poplau and Don (2015)||Legumes and non-legumes||0.32 tonnes C/ha/yr|
|Follet et al. (2001)||Legumes and grasses||0.6–0.9 tonnes C/ha/yr|
|Lal et al. (1998)||Legumes and grasses||0.3–0.7 tonnes C/ha/yr|
|Bruce et al. (1999)||Grassland||0.8 tonnes C/ha/yr|
|Sperow et al. (2003)||Winter cover crops||0.2 tonnes C/ha/yr|
|Jian et al. (2020)||Legumes and grasses||0.56 tonnes C/ha/yr (+15.5 %)|
|Mcdaniel et al. (2014)||Legume/non-legume mixes||+8.5% increase of total soil carbon|
This C sequestration potential varies with different soil, climate, and management conditions. Studies suggest that overall cover crop biomass is the main driver of soil C accumulation whereas others suggest the importance of using cover crop combinations that use both legumes and nonlegumes14,26,42,43,50. Mcdaniel et al. (2014) found legumes to be an important cover crop for soil C accumulation across a wide variety of rotations, soil types, and climates.
NRCS/SWCS photo by Lynn Betts. https://www.flickr.com/photos/[email protected]/34983997550
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Crop Types and Functions
It is important to consider the cover crop best suited for a crop rotation as well as timely seeding to maximize vegetative growth of the cover crop while minimizing competition with the subsequent cash crop. Different cover crops can be used for different purposes in a field and can be divided into four categories: legumes (e.g. alfalfa, vetches, and clover), non‐legumes (spinach, and flax), grasses (e.g. ryegrass and barley) and brassicas (e.g. canola, radishes, turnips)1.
In soils where nitrogen (N) is limiting, legume crops can improve soil nutrition by not only adding organic matter, but by fixing atmospheric N which is added to the soil for use by subsequent cash crops29.
Conversely, non-legume cover crops that are higher in carbon can be used to absorb excess nitrate from the soil which reduces nitrate leaching as well as increases green manure biomass3,15,24,40. Brassica species with deep taproots like radish or rapeseed can be used to alleviate soil compaction as they break through compacted soil25. Maize yields have been reported to have increased following radish or rapeseed cover crops due to reduced compaction and increased access to water at lower depths, overall increasing drought resistance9. Furthermore, C sequestration can be increased by combining cover crop rotations with minimal or zero-till practices due to reduced soil disturbance21,32-37,49.
Current Opportunities for Growers
Producers implementing a cover crop strategy for reducing their carbon footprint can generate carbon offsets through scope 1 voluntary carbon offset protocols. Alfalfa is a great example of a cover crop that can be seeded to increase carbon sequestration potential and create on farm carbon offsets. Legumes are very effective nitrogen fixers; when added to the rotation as a cover crop, can provide additional N to the cash crop, reducing high nitrogen fertilizer requirements, enhancing nitrogen use efficiencies, and mitigating runoff and leaching losses. Nitrous oxide emissions reductions protocols (NERP) are available for producers managing their nitrogen according to sustainable 4R nutrient management practices to generate carbon offset credits that can then be traded in the carbon market for revenue.
Contact Fertoz to stay up to date on current and upcoming carbon opportunities.
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