What is the stability of additional organic carbon stored thanks to alternative cropping systems and organic wastes products application? A multi-methods evaluation

Abstract. The implementation of agroecological practices often leads to an additional soil organic carbon storage in these soils, of which we aimed to assess the biogeochemical stability. To achieve this, we implemented a multi-method approach using particles size and density fractionation, Rock-Eval® thermal analyses and long-term incubation (484 days), that we applied to topsoil samples (0–30 cm) from temperate luvisols that had been subjected, in > 20 years long-term experiments in France, to conservation agriculture (CA), organic agriculture (ORG) and conventional agriculture (CON-LC) in La Cage experiment, and to organic wastes products (OWPs) applications in QualiAgro experiment, including biowaste composts (BIOW), residual municipal solid waste composts (MSW), farmyard manure (FYM) and conventional agriculture without organic inputs (CON-QA). The incubations provided information on the additional carbon stability in the short term (i.e., MRT <2 years) and showed that the additional soil organic C mineralized faster than the baseline C at La Cage but slower at QualiAgro. In OWPs-treated plots at QualiAgro, 60–66 % of the additional carbon was stored as mineral-associated organic matter (MAOM-C), and 34–40 % as particulate organic matter (POM-C). In CA and ORG systems at La Cage, 77–84 % of the additional carbon was stored in MAOM-C, versus 16–23 % as POM-C. Management practices hence influenced the distribution of additional carbon in physical fractions. Utilizing the PARTYSOC model with Rock-Eval® thermal analysis parameters, we found that most, if not all, of the additional carbon belonged to the active carbon pool (MRT ~ 30–40 years). In summary, our comprehensive multi-methods evaluation indicates that the additional soil organic carbon is less stable over decadal and pluri-decadal time-scales compared to soil carbon under baseline practices. Our results show that particle size and density fractions can be heterogenous in their biogeochemical stability. On the one hand, while additional carbon was mainly associated with MAOM, we suggest that it has a mean residence time exceeding ~30 years, rather than of ≈ 50 years. On the other hand agroecological practices with equivalent additional carbon stocks (MSW, FYM vs CA) exhibited a higher proportion of additional carbon in POM-C under MSW (40 %) and FYM (34 %) compared to CA (16 %), which suggests a high chemical recalcitrance of POM-C under OWPs management relative to CA. Additional soil organic carbon deriving from organic wastes, i.e., biomass that has been partially decomposed and transformed through its processing prior to its incorporation in soil, would be more biogeochemically stable in soil than that deriving directly from plant biomass. The apparent contradictions observed between method can be explained by the fact that they address different kinetic pools of organic C. Care must be taken to specify which range of residence times is considered when using any method intending to evaluate the biogeochemical stability of soil organic matter, as well as when using the terms stable or labile. In conclusion, the contrasting biogeochemical stabilities observed in the different management options highlight the need to maintain agroecological practices to keep these carbon stocks at a high level over time, given that the additional carbon is stable on a pluri-decadal scale.