How reactive metals and base cations influence soil organic carbon content and dynamics in temperate French forests

Abstract. Soil organic carbon (SOC) immobilisation and sequestration have been found to be influenced by a wide range of environmental factors, notably the climate, stand characteristics, and soil properties. Recent research has suggested that some soil properties traditionally regarded as major predictors of SOC stocks (e.g. clay content) may be less influential than those pH–dependent, reactive constituents, including crystalline aluminium (Al) and iron (Fe) oxides, and basic cations. However, these soil properties have rarely been considered for explaining large-scale variation in SOC storage. Here, by taking the advantage of the RENECOFOR network double forest soil inventory at a 15–year interval over the French European territory, we investigate how both SOC storage and its temporal dynamics vary as a function of climate, stand characteristics, and soil properties. Furthermore, we used Rock-Eval® (RE) thermal fractionation to identify descriptors correlated with proxies of SOC biogeochemical stability, in particular the hydrogen index (HI – hydrocarbon proportion of SOC) and T50 CO2 PYR (thermal stability indicator positively correlated to SOC biogeochemical stability). While the effect of the cations only occurred when pH ≥ 5, the metal oxides increased SOC for both pH ≥ 5 and pH < 5 groups. These effects were also identified on the thermal stability of C (T50 CO2 PYR) for the 0–10 and 10–20 cm soil layers. No unequivocal relationship was found between HI and soil properties. When considering reactive soil constituents, clay content played a limited role in explaining variations of SOC and RE metrics. While HI was lower in coniferous forests, there was no other clear effect of climate nor stand characteristics. Regarding the C dynamics, only the 0–10 cm layer showed sufficiently high variations to be confidently analysed. For this layer, the expected variables (e.g. temperature or litter input) did not explain any of the variations of SOC or RE metrics. However, the composition of SOC, through tree species or HI, had an influence on the SOC dynamics, with lower SOC variation the more the carbon was labile. Additionally, the variation of SOC was highly influenced by the dynamics of holorganic layers, with a decrease of SOC content variation with an increase of the change of forest floor stock. Our results highlight the generally stronger role of soil factors than climatic or stand ones in explaining large-scale variation in forest SOC storage, largely due to the inclusion of refined soil geochemical properties beyond clay content.