The influence of minerals and cations contents on carbon sequestration in temperate French forests’ soils, and their effects on carbon dynamics

Association of organic matter (OM) with mineral phases have been identified as one important process to explain organic carbon sequestration in soil. Associations include cationic bridging between OM and mineral surfaces, OM adsorption on mineral surface, and OM co-precipitation with minerals. Some recent studies underlined the crucial role of poorly crystalline aluminium (Al) and iron (Fe) oxides to sequestrate organic carbon. These findings have been mostly observed so far at a global or continental scale. Moreover, the links between soil mineralogy and geochemistry and carbon are still unclear, given the time span needed to unveil them.

The aim of the study is to verify if relationships between geochemical parameters and soil carbon content are still noticeable at the scale of the French European territory, in temperate forests covering a wide range of soil types. Taking advantage of a double inventory of soil organic carbon at a 15-year interval, we further aim to assess if a relationship exists between soil geochemical parameters and C dynamics over a decade. In this forest monitoring network of 102 sites across mainland France (RENECOFOR) three soil depths were analysed at each campaign: 0–10, 10–20, 20–40 cm. The particulate organic carbon (POC) fraction proportion of the 0–10 cm soil depth was known for 53 sites for the second campaign. The mineral-associated organic carbon (MAOC) content could be therefore inferred.

The soil carbon content was linked to the oxalate-extractable Al and Fe (Al_ox and Fe_ox) at each of the three depths. The relation with the MAOC content of the 53 sites subset was also highly positively correlated. The cation content (exchangeable calcium and magnesium, Ca_ex and Mg_ex) showed a positive effect on carbon content for a subset of the sites. These had a higher pH and were mainly located on alkaline parent rocks. However, the stock change was mainly unaffected by neither minerals nor cation content, or by other soil characteristics.