Rock-Eval®-RMQS: Monitoring the Characteristics of SOM on the French Territory with Rock-Eval® 6 Thermal Analysis to Assess its Stability

Soil organic matter (SOM) is widely recognized as of critical importance for both soil quality and climatic mitigation. The quality and quantity of SOM are key to assess the characteristics of soils, and thus must be accurately monitored in order to protect the integrity of soils. In the last few years, a thermal analysis technique called Rock-Eval® that provides insights on bulk SOM chemistry and thermal stability has been recognized as a powerful method for SOM characterization. It can moreover be applied on large sets of samples.

The RMQS is the French monitoring network of soil quality. The first sampling campaign took place from 2000 to 2015 and resulted – among others – in about 2200 composite surface (0-30 cm) samples taken all over France. It represents an unprecedented collection of precise and complete data on French soils.

We observed significant effects of land cover on both SOM thermal stability and bulk chemistry. The mean values of hydrogen index (HI, which is a proxy for SOM H/C ratio) for arable lands (190.5 ± 43.4 mg HC per g of SOC, n=786) was lower than for grassland soils (228.4 ± 46.3 mg HC per g of SOC, n=486) and forest soils (240.4 ± 66.4 mg HC per g of SOC, n=528). Regarding the oxygen index (OIre6, which is a proxy of SOM O/C ratio), we observed significantly different values (P<0.001) for arable land soils (188.8 ± 30.4 mg O₂ per g of SOC), grassland soils (172.4 ± 26.8 mg O₂ per g of SOC) and forest soils (164.2 ± 29.6 mg O₂ per g of SOC). We also observed that thermal stability of SOM was significantly higher in cropland soils compared to grassland and forest soils. Our data suggest that topsoil SOM is on average more oxidized and biogeochemically stable in croplands. Further analyses will investigate the influence of pedo-climatic conditions on SOM characteristics.

The high number and even repartition of data on the French territory allow for the constitution of a national interpretative referential for these indicators. The Rock-Eval® parameters will also be used to calculate the centennially stable SOC fraction using the PARTYsocv2.0 model and map it at the scale of France.