Are carbon-storing soils more sensitive to climate change? A laboratory evaluation for agricultural temperate soils

A range of agroecological practices allow to increase soil organic carbon (SOC) stocks, which makes a positive impact on climate change mitigation and soil health, but the permanence of this additional SOC storage can be questioned, in particular in a climate change context. Increased temperatures, accentuated evaporation of terrestrial water and increased atmosphere moisture content are anticipated, resulting in more frequent droughts and heavy precipitation events. Understanding the SOC dynamics and assessing the sensitivity of carbon mineralization to these climatic events is necessary to anticipate future carbon losses in terrestrial ecosystems. To this respect, it seems relevant to investigate carbon-storing soils as increased carbon mineralization induced by climate change may limit the carbon storing potential in agricultural soils. Thus, we evaluated the sensitivity of SOC mineralization to increased temperature, decreased soil moisture and drying-rewetting cycles using soils from long-term field experiments. We performed an incubation experiment on topsoil (0–30 cm) samples from temperate luvisols that had been under 20 years under conservation agriculture (CA), organic agriculture (ORG) and conventional agriculture (CON-LC) at the La Cage experiment, and under organic waste 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). Soil samples were incubated in the lab for 3 months under different temperature conditions (20, 28 and 35 °C) or under different moisture conditions (matric potential: pF1.5; pF 2.5 and pF 4.2) or under several dry (pF 4.2)-wet (pF 1.5) cycles (DWC). The results shown that, whatever the agricultural practices, soil moisture regime and temperature significantly affect the SOC mineralization. Overall, the DWC did not stimulate soil carbon mineralization relative to wet controls (pF1.5 and pF2.5). Whatever the soil moisture regime and temperature, specific carbon mineralization was similar between agricultural practices at La Cage, while at QualiAgro, specific carbon mineralization was lower in soils receiving organic waste products (OWPs) compared to the baseline soil. These results suggest a strong carbon stabilization by OWPs in soils as assessed by laboratory incubation experiments. Within each long-term experiment, we observed no significant difference between the carbon-storing soils (CA, ORG, MSW, FYM and BIOW) and their respective baseline soils (CON-LC and CON-QA) in the delta SOC mineralized whatever the soil moisture regime. The Q10 also indicated no significant difference between carbon-storing soils and their respective baseline soils. These results indicate that the SOC mineralization in carbon-storing soils had a similar sensitivity to the soil moisture regime and temperature as the baseline ones. Hence, the implementation of these agroecological practices appears beneficial for climate change mitigation, even in the context of extreme climatic events.