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Understanding the dynamics of soil organic matter

Delphine Derrien (INRA) takes the floor

Understanding the dynamics of soil organic matter, a key component of forest ecosystem equilibrium


The international initiative "4/1000" hopes to increase by 0.4% each year the global amount of carbon stored in the first 30 centimetres of the soil. Theoretically, this would slow down, or even stop, the current rise in CO2 levels in the atmosphere. What is more, increasing carbon (C) stocks in the soil would improve soil quality and reduce erosion. Of course, soils would be richer in organic material, and in nutrients, yields would improve, and our ability to feed the planet would be strengthened. For the moment, 82 countries are willing to commit to applying this initiative through different international programmes.

Adding plant material would appear to be the most obvious way to increase soil carbon. However, adding organic material to the soil could in fact increase carbon release... That is why it is important to understand the fine-tuned mechanisms that control soil carbon storage: Where does soil carbon come from? What forms does it take? How long will it remain stored in the soil?

In this talk, we will take a look at the studies and reflections going on within the CarboSMS group (Soil Carbon, Stabilisation Mechanisms - https://carbosms.wordpress.com/). The group has 120 members from the French-speaking scientific community who are working on soils and their capacity to store carbon.

We will first look at recent advances in our knowledge of carbon stabilisation mechanisms in the soil. Two major types of mechanisms influence stabilisation / destabilisation of organic C in soils: those related to living organisms and biodiversity (plants, fauna, micro-organisms), and abiotic mechanisms (location in the physical soil structure and interactions among the different mineral particles).

Next, we will talk about how silvicultural practices affect carbon stocks in the soil. By acting on both the biotic and abiotic mechanisms, the choice of species and planting density, harvesting and extraction intensity, soil amendments, fertilisation or working the soil determine not only the amount of organic matter brought to the soil over time and in space, but also influence the sensitivity of the soil organic matter to mineralization. We will illustrate the complexity of the interactions between these mechanisms and their effect over time on carbon stocks through meta-analyses and long-term field studies.

Finally, we will show how taking these mechanisms into account in global models of carbon dynamics or when determining indicators for carbon storage stability can improve our predictions of trends in soil organic carbon storage. However, these new models or indicators, which include the fine mechanisms of carbon storage, must first go through a validation process before they can be applied at a larger territorial scale. Here, networks of monitoring sites like RENECOFOR are especially precious. Indeed, detecting changes in carbon stocks necessitates repeated analyses over extended periods of time. Furthermore, determining whether a model or indicator is robust enough to be generalised requires comparing predictions and estimates to actual recorded data from a variety of pedological and climatic contexts.

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