Why store more carbon in our soils? In a context of climate urgency the goal of 4 ‰ that was fixed to neutralise annual increases in atmospheric carbon is supplemental to the principal objective which remains a reduction in our greenhouse gas emissions. Soils that contain more carbon – or in other words, more organic matter – are more fertile. This initiative is therefore favourable to the environment, as well as to agriculture and global food security.
Increasing the storage in France
How can we increase this storage in France? The INRA study first of all identified the farming and forestry practices that favour soil carbon storage and are compatible with agroecology.
Using agronomic and economic modelling, it was possible to simulate the effects of these practices on the evolution of storage over a 30-year period. An original methodology was deployed, using km² by km² estimates to evaluate the additional storage contributed by each new practice compared to changes in carbon storage if no appropriate measures were implemented.
These new findings are intended to inform public policies and were reported and discussed during a symposium on 13 June 2019. They demonstrate how important it is for these policies to favour the maintenance of permanent grasslands, wetlands and forests, where the soils generally contain high carbon stocks, and to halt land take. They supplement policies designed to increase carbon stocks where they are low, or in other words in major arable areas. By implementing these two complementary objectives throughout the country, it would be possible to achieve an increase in French soil carbon storage of close to 4‰ per year. This calculation was conditional upon reducing the uncertainties that affect current storage trends. As well as redirecting public policies and their associated funding towards sustainable systems that favour the preservation of soils and carbon storage, considerable changes will be necessary to the strategies adopted by actors in agriculture and forestry and by regional government bodies.
The initial value of soil carbon stocks was determined from data generated by the Soils Scientific Interest Group (GIS Sol) using a 1 km² grid covering mainland France. The resulting map demonstrated contrasting initial findings due to land use and also different soil types and climate.
At a national scale, forested land accounts for 38% of all stocks; this is closely linked to the history of land use and the trend is towards a rise in levels. Permanent grasslands account for high (22%) and stable stocks, but the trend is towards slight depletion. Because they cover so much land, temporary grasslands and arable crops contribute to 26.5% of total storage and the trend is downwards.
Preserving and maintaining high stocks under forests and grasslands
Stocks fluctuate as a function of carbon inputs (litter, organic residues, etc.), biotransformation and the duration of stabilisation in the soil, as well as outputs that are principally due to the respiration of decomposer organisms. Estimates of these variations are highly sensitive to calculation hypotheses, but it is hoped that by 2020 the French Soil Quality Monitoring Network (RMQS) will be producing new measurements to reduce these uncertainties.
Without changes to land use, and without modifying agricultural and forestry practices, the evolution of soil carbon stocks, all types of land use taken together, is currently 2.3‰ per year, but with marked uncertainty (-0.2‰ to +3.2‰ per year). However, this increase will partly be offset by changes to land use that deplete carbon stocks: land take and the ploughing of grasslands. For this reason, public policies in favour of maintaining permanent grasslands and forests and halting land take will be necessary to attain the 4‰ goal.
It was not possible to identify practices that could ensure more storage than at present in forests: the challenge is therefore to maintain existing stocks and the practices that sustain them. For permanent grasslands, and at a moderate cost, two practices could potentially achieve additional storage reaching around 12% in France. This would require moderate intensification through the input of fertiliser or the extension of grazing rather than cutting, which favours a return to the soil of residues and animal waste.
Potential for +1.9‰ additional storage at a national scale
It is arable land – where current stocks are lowest – which has the greatest potential for additional storage (86% of the total) by implementing five practices:
- The use of cover crops and catch crops. Applied throughout the country, this practice could account for 35% of the potential total, at a moderate cost;
- The introduction and extension of temporary grasslands in crop rotations: 13% of total potential, at a high cost;
- The development of agroforestry: 19% of the total potential, at a high cost;
- Input of composts or organic residues, with a negative cost (slight gain for the farmer);
- Planting of hedges, at a high cost.
A sixth practice (a switch to direct sowing) was also studied. This could increase storage in the surface horizon, but the effect was no longer perceptible when the entire soil profile was considered. Its cost was therefore not evaluated.
In vineyards, grasses as a permanent or winter cover crop between rows displayed significant potential for a low or negative cost. But because this only concerns a small land area, its contribution to total potential would remain low.
Deploying the right practices in the right places
The study also demonstrated that the additional storage potentials, implementation bases and costs all varied not only depending on practices but also from one region to another. The most effective solution was thus a combination of “the right practices in the right places”. Indeed, it remains essential to maintain and protect existing stocks through appropriate practices and not permit any depletion by putting a halt to the tillage of grassland and land take.
In total, additional storage could reach a maximum of +1.9 ‰ for all agricultural and forestry land (3.3‰ for agricultural land alone and 5.2‰ if restricted to arable land), or 41% of agricultural carbon emissions. Any further additions would require new research in order to overcome other obstacles and clarify the estimates. It is necessary to model at a detailed spatial scale and use massive data on climates, soils and farming systems, to take account in the economic calculations of the additional benefits procured by certain practices such as cover crops (which also improve water quality) or agroforestry, and to explore in depth the value and limitations of exploiting composts and organic residues. It would also be necessary to broaden the study to all greenhouse gases, including the N2O linked to the use of synthetic fertilisers and the methane resulting from livestock farming, in order to take account of different climate change scenarios in the calculations and produce scenarios for alternative agricultural production systems.
This study was carried out by INRA’s Unit for Collective Scientific Expertise, Foresight and Advanced Studies (DEPE). It involved 40 experts: agronomists, economists, modelling specialists and study engineers. It was based on the methodological framework of collective scientific expertise and on simulation studies.
The simulations were made using INRA’s STICS model for arable crops and the PaSim model for permanent grasslands. These models include an explicit representation of the carbon cycle in the soil-plant-animal system, and reflect the effects of the numerous pedoclimatic factors and practices that drive the evolution of carbon stocks and other interesting output variables (yield, nitrogen leaching, N2O emissions, etc.). Several French databases on soils, climate and farming practices were mobilized to provide inputs for these models.
The simulations were performed on the 0-30 cm soil horizon and on the final aggregated results, although storage calculations throughout the depth of the soil were also made as this method is the most appropriate in terms of of climate change attenuation. The BANCO model combining agronomic simulations and cost calculations was used to optimise the storage efforts that need to be implemented.