Soil conservation farming and climate change

In the context of the current discussion on climate change and the energy transition, most people can correctly identify energy production and transport as the sectors that produce most carbon dioxide (CO2). However, depending on how it is measured, climate scientists estimate that agriculture is the fourth largest contributor to global CO2 emissions. Mechanisation (the use of tractors and other vehicles) is a significant source of CO2 emissions through ploughing, sowing, spraying, and harvesting, as is — to a lesser extent — the burning of crop residues. It is also well established that livestock farming and the use of synthetic fertilizers generate other (even more potent) greenhouse gases like methane and nitrous oxide.

However, we rarely hear about innovations in farming techniques that could improve agriculture’s carbon balance. One such innovation is soil conservation farming, a suite of techniques developed by Brazilian farmers in the 1970s to offset the demineralisation of their land and leaching caused by heavy rain. Now being adapted by French grain farmers all over the country, soil conservation farming involves ensuring that the soil is never bare by using cover crops and minimising soil disturbance through the use of no-till seed drills.

So how does this help to reduce carbon emissions? Well, as you may remember from biology class, during photosynthesis the plant absorbs CO2 in the air and uses light energy to convert it into glucose. This glucose is used in part by the plant to synthesize starch, which is stored in the grain that is harvested, while the rest is used for the plant’s metabolism and goes into making the stem, leaves, etc.

A one-hectare field of wheat can absorb up to 3.8 tonnes of CO2 from the atmosphere, 44% of which enters the food system. The destiny of the crop residue (essentially straw) can have a significant impact on a farming system’s carbon emissions. The challenge is to ensure as much as possible of it is trapped underground, thereby also enriching the soil and improving future yields.

When the grain is harvested, most of the crop residue remains on the ground. This straw creates a “mulch”; an important source of biomass that will nourish the soil. However, in many farming systems, the soil is turned over to create a good seedbed for the next crop. This process releases carbon dioxide into the atmosphere. No-till farming techniques avoid this phenomenon by seeding directly through the mulch, minimising soil disturbance, and ultimately creating a “carbon sink”.

The use of cover crops — planted alongside farming crops for agronomic purposes but not harvested — is another crucial piece in the jigsaw. Legumes are excellent companion crops for some cereals, protecting them from pests, and they have the added benefit of capturing nitrogen in the air and returning it to the soil, thereby reducing the need for chemical fertilisers. When you consider that around one third of the energy used in U.S. agriculture is for the production and transportation of fertiliser, their environmental impact becomes clear.

According to some estimations, a one-hectare field of wheat in conventional farming currently sequesters between 100 and 300 kg of carbon, but agronomic trials in China have shown that switching from conventional to soil conservation farming methods have tripled sequestration levels). Studies by the IAD (French institute for sustainable farming) shows a five-fold improvement in farms’ carbon balance using these techniques.

Although a recent study by the INRA (French agricultural research institute) is more conservative about the potential for carbon storage in France, it does estimate that large-scale farming crops could increase their storage potential by 86% for little cost — even without no-till farming — thereby offsetting 41% of agricultural carbon emissions.

Moreover, as well as the aforementioned benefits, soil conservation farming also reduces erosion and the leaching of pesticides into the groundwater by protecting the soil’s structure. So, whereas ecologists often point the finger at agriculture for its environmental impact, soil conservation farming could have a massive role to play in reducing atmospheric greenhouse gas emissions and mitigating the overall ecological impact of farming.