It’s time for arable farming to join the Green Revolution.
Annual cropping systems make up a large part of the farming industry, so the chance to use them to improve carbon sequestration should be taken to help reduce greenhouse gas (GHG) emissions.
Among the negative emissions technologies (NETs — active atmospheric CO2 removal strategies) available to achieve CO2 reductions, soil carbon sequestration is the cheapest and easiest to use at scale in the near future.
Currently, cropping systems in the US favour genetics that improve yield rather than minimising GHG emissions or maximising CO2 sequestration, though genetic variation for these traits remains in many crop varieties.
Carbon levels in agricultural soils have greatly reduced over the last century, but they still have the capacity to store much of the CO2 currently in the atmosphere.
Fertilisers impact GHG emissions
Nitrogen, applied to annual cropping systems in the form of synthetic fertiliser, is the biggest problem for sustainability and GHG emissions. It inevitably leaches into local groundwater and rivers, as well as into the atmosphere as N2O, which has a warming effect 300 times that of CO2.
In parts of the world where the Green Revolution was successful, crop genotypes that require high levels of nitrogen input to thrive have been favoured almost exclusively.
To fix this, nitrogen use efficiency must be improved, and the use of fertilisers must be reduced. This can be partly achieved by removing a small number of N-dependent mutations favoured following the Green Revolution.
The general reduction of fertiliser use in the arable farming industry is a greater challenge.
For example, biennial overfertilisation of soy-maize rotation systems is a common practice across 73 million hectares of US farmland. Soybean is a nitrogen-fixing plant but still requires high levels of fertilisation to achieve the desired yields of modern systems.
Increases in fertiliser prices have been matched by the price of crops, so farmers remain incentivised to use the highest quantity of fertiliser necessary to ensure high yields.
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Previous sequestration efforts
Most efforts to improve carbon sequestration in annual cropping systems are practices originally implemented to improve soil health. These include reduced tillage and the planting of cover crops to increase the amount of above-ground plant biomass left in the field between crop rotations.
These efforts seem to input carbon mainly into the top 30 cm of soil, which is the least durable and much of the carbon is respired back into the atmosphere within a few years.
To achieve larger, more sustainable levels of sequestration in agricultural systems, carbon needs to be injected deeper into the soil. This will require genetic changes to the crops used.
Genetic changes
Genetic changes to annual crops should be made to reduce their nitrogen requirements and achieve multiple tonnes of carbon sequestration per hectare per year.
This can be achieved in part by selecting against mutations that grew in frequency as a result of the Green Revolution. Prior to this, large amounts of fertiliser were not used, as it promoted stem growth more than seed growth, resulting in these taller crops bending and breaking. Dwarf species were favoured to avoid this.
Ideal traits of annual crops used for food, animal feed and fuel must be considered when aiming for a carbon-negative supply chain. As well as a lower nitrogen requirement, they should include a deeper, larger root system to facilitate soil carbon storage.
Increasing plant density in fields will also increase root density and, therefore, carbon uptake.
Species not used in the Green Revolution will be the most suitable candidates for this. For example, industrial hemp has never been bred to require high nitrogen inputs, has a more substantial root biomass below 50 cm than any other major crop, and can be grown at 500,000 plants per hectare.
Root carbon composition should also be a genetic target; some forms of carbon are more resistant to degradation and so will remain longer in the soil. It is possible that roots can be engineered through genetics to create more long-lasting forms of carbon, such as lignin.
Root exudates, carbon compounds released by roots into the soil, contribute to soil organic carbon (SOC), so they must also be considered. However, little is known about whether root exudates are controlled by genetics or the environment, hindering understanding of their abundance and composition.
This is because it’s hard to measure which phenotypes are responsible for producing these exudates.
Recent models have been successful in separating the carbon inputs made from exudates versus biomass and have suggested that carbon inputs from exudates remain in the soil longer than carbon compounds from plant biomass.
Finally, the soil itself and the microbiome present need to be investigated in these cropping systems since there is a lack of data on how they influence carbon sequestration and storage.
In woodland systems, mycorrhizal fungi have been shown to drive SOC accumulation, so they could perhaps do the same in agricultural systems.
Changing the soil and root microbiome in large-scale crop systems is tougher than getting new crop seeds, but it could help improve carbon storage in yearly crop systems.