Time and climate influence on the formation and stability of soil organic matter pools

This study aims at investigating the effect of climate and time on particulate organic matter (POM) and mineral-associated organic matter (MAOM) dynamics. Two chronosequences located along a climate gradient were investigated. The driest chronosequence (ADI) consisted of four fluvial terraces, whereas the wettest one (LED) included three fluvio-glacial terraces. The age of the ADI sites (Q2, Q3, Q4 and Q5) ranged from about 125,000 to 2,000 years BP, whereas that of the LED sites (Q1, Q2, and Q3) from about 16,000 to 10,000 years BP. All sites were grasslands. Soil samples (1 profile and 2 cores per site) were collected by horizon, and each horizon sub-sampled by depth (each 5 cm). From each sample, POM and MAOM were isolated and characterized by elemental and thermal analyses. The contribution of POM and MAOM to carbon (C) storage differs among the climo-chronosequences. In the topsoil (0-15 cm), POM represents the main pool, especially in the wettest chronosequence. The average MAOM/POM ratio is almost twice in the driest chronosequence (ca. 2.6). The concentration of organic C in MAOM and POM along the whole profile is about 2x and 3x, respectively, in LED soils compared to ADI. In both chronosequences, the MAOM/POM ratio increases with depth with an unsaturated level of MAOM. Thermal indices show that the stability of the MAOM pool increases with soil age and depth in the driest chronosequence (ADI), whereas no significant differences were observed in the wettest (LED). For the POM fraction, no significant differences in stability were observed in both chronosequences. In conclusion, our results show that soil age plays an essential role in forming stable MAOM, in particular in drier climatic conditions. In contrast, a wetter climate determines a higher accumulation of C in both pools, although such higher C contents are negatively correlated with their thermal stability.