Potential and durability of carbon storage in agricultural soils – StoreSoilC

The 4 per mil initiative promotes soil organic carbon (SOC) sequestration to improve soil fertility, adapt to climate change and reduce greenhouse gas emissions. It targets agricultural soils in priority. Rendering soils « climate smart » requires however an understanding and assessment of the SOC storage potential of different soils. SOC is very heterogeneous with residences times ranging from hours to millennia so that the durability of the stored C needs assesment. Hence, the sustained mitigation of climate change by SOC management requires reliable routine methodologies forecasting SOC stocks evolution. The SOC storage potential of a soil may be defined as the maximum gain in SOC stock attainable under a given climate, a given land use and a given timeline (e.g. time to attain a new equilibrium or IPCC time period: 20y). To date, the different available methods (e.g. Tier 1 to 3) have seldom been compared on a variety of soils, nor with the C saturation approach. Furthermore, SOC dynamics models are limited by the lack of calibration of the different SOC kinetic pools, for which indicators are needed.
The project aims to :
• Investigate the ability of different methods to determine the size of SOC kinetic pools and test how their routine use to initialize models of SOC dynamics may improve their predictions
• Develop and compare different methods to estimate the C storage potential of agricultural soils
• Determine the persistence of stored C, particularly in relation to its interaction with soil minerals.
The project associates 6 partners, with high and complementary expertise from process level understanding and biogeochemistry, pedology, chemistry, SOC modelling, agronomy, databases and monitoring networks, to spatial statistics and mapping (Ecosys, Grignon, Laboratoire de Géologie Paris, Infosol Orléans LSCE Gif sur Yvette, Institut de Géologie Paris and AgroImpact Laon.
The project combines measurements, experiments and modelling. It takes advantage of well-documented Long Term Experiments (LTE) with agroecological cropping practices (conservation agriculture, application of organic wastes, temporary grasslands) and considers the diversity of agricultural soils in a small territory (Versailles plain), in the Region Centre as well as in a series of sites from the French soils monitoring network (RMQS). To identify indicators of SOC kinetic pool sizes Rock Eval pyrolysis will be compared to physical fractionation methods, and these indicators tested against 13C and 14C measurements of the turnover rate of SOC in LTEs. SOC stocks will be modelled with RothC and Century models and with AMG, a simpler one extensively calibrated for temperate agricultural soils. Several approaches will be compared to estimate the SOC storage potential: (i) a statistical approach (identifying in a territory soils with highest SOC stocks per soil type); (ii) a Tier 2 approach (applying literature-based emission factors for practices changes); (iii) a modelling approach (modelling the effect of changes in practices) and (iv) a C saturation approach (quantifying SOC stabilized by its association with < 20 µm soil particles). The project will propose a methodological framework for estimating the SOC storage potential of agricultural soils, to be used also in other pedoclimatic contexts and will compare the ability of several agricultural practices to increase SOC stocks and stabilize organic matter.
A strong attention will be dedicated in to promoting international cooperation (e.g. by organizing dedicated workshops) and to dissemination to the scientific community, to stakeholders (i.e. agricultural extension services, small companies, managers in charge of implementing GHG policies at the territorial scale) and to students.