Is the additional organic carbon stored thanks to alternative cropping systems and organic waste products application predominantly stable at a decadal timescale?

The implementation of agroecological practices can lead to an additional soil organic carbon (SOC) storage. The carbon sink effect will be more effective, even in the short and medium term, if the additional storage is realized in the form of persistent organic carbon (OC) and not in labile OC. The objective of this study was to evaluate the biogeochemical stability of additionally C stored by agroecological practices. Biogeochemical stability was assessed using particles size and density fractionation and Rock-Eval (RE) thermal analyses with PARTYsoc machine learning model. Samples were collected from the QualiAgro experiment, where organic wastes products (OWPs) including biowaste compost (BIOW), residual municipal solid waste compost (MSW) and farmyard manure (FYM) were applied, and from the La Cage experiment, where conservation (CA) and organic (ORG) agriculture had been established for 20 years. The plots that received the OWPs showed that 60-66% of the additional C was stored in mineral-associated organic matter (MAOM-C) and 29 - 39% in particulate organic matter (POM-C), whereas in CA and ORG, 77 - 84% of the additional C was stored in MAOM-C versus 15 - 23% in POM-C. While leading to additional C stocks of similar sizes, MSW and FYM exhibited higher proportions of the additionally stored C as POM-C (39 and 29% respectively) compared to CA (15%). This suggests a recalcitrance of POM under OWPs management compared to CA. The PARTY_SOC model using RE thermal analysis parameters allows to predict the active (30 - 40 years) and stable (>100 years) carbon pools as defined in the AMG model. The results revealed that most, if not all, of the additional C belonged to the active C pool. These findings suggest that although additional SOC is mainly associated with MAOM-C, it is probably not stored in a form with a mean residence time exceeding ~30 years. The agroecological practices implemented in both long-term field experiments have resulted in substantial short-term additional C storage, but this storage will only be maintained at a high level if these storing practices are continued.