Elemental stoichiometry and Rock-Eval® thermal stability of organic matter in French topsoils

Abstract

Abstract. The quality and quantity of soil organic matter (SOM) are key elements that impact soil health and climate regulation by soils. The Rock-Eval® thermal analysis technique is becoming more commonly used, as it represents a powerful method for SOM characterization by providing insights into bulk SOM chemistry and thermal stability. In this study, we applied this technique on a large soil sample set from the first campaign (2000–2009) of the French Soil Quality Monitoring Network (RMQS – Réseau de mesures de la qualité des sols). Based on our analyses of ca. 2000 composite surface (0–30 cm) samples collected across mainland France, we observed a significant impact of land cover on both the SOM thermal stability and elemental stoichiometry. Cropland soils had a lower mean hydrogen index value (a proxy for the SOM H/C ratio) and a higher thermal stability than grasslands and forests. Regarding the oxygen index (a proxy for the SOM O/C ratio), we observed significant differences among the values for croplands, grasslands, and forests. Positive correlations of the temperature parameters with the clay content and pH highlight the protective effect of clay on organic matter as well as the impact of pH on microorganisms' mineralization activity. Surprisingly, we found weak effects of climatic parameters on the thermal stability and stoichiometry of SOM. Our data suggest that topsoil SOM is on average more oxidized and biogeochemically stable in croplands. More generally, the high number and even distribution of data across the whole French territory allow one to build a national interpretative reference for these indicators in surface soils.