Time and climate roles in driving soil carbon distribution and stability in particulate and mineral-associated organic matter pools.

Understanding the accumulation and stability of soil organic matter (SOM) pools as a function of time (i.e., soil age) and climate (i.e., precipitation and temperature) represents a crucial challenge. This study aims at investigating the effect of both climate and time on SOM distribution into particulate and mineral-associated organic matter (POM and MAOM, respectively), using two chronosequences located along a climate gradient. The contribution of POM and MAOM to soil organic carbon (SOC) storage differs between the climo-chronosequences and with depth. The ratio between MAOM and POM pools (MAOM/POM) ranges from 0.9 to 2.0 and from 1.4 to 3.5 in the wetter and cooler and in the drier and warmer chronosequence, respectively. Regardless of the chronosequence, the MAOM/POM ratio increases with depth, highlighting a more important role of the mineral-associated fraction in carbon storage in deeper soils. The concentration of organic carbon in mineral-associated (MAOC) and particulate (POC) pools along the soil profile in the wetter and cooler chronosequence is 2× and 3× higher, respectively, than in soils from the drier and warmer one. In particular, in the wetter and cooler chronosequence, MAOC and POC concentrations decrease with soil age. In the drier and warmer chronosequence, only POC concentration decreases with soil age, whereas MAOC concentration generally increases. The thermal stability of the MAOM fraction increases with soil age and depth only in the drier and warmer climatic conditions, whereas no differences with depth occur in the wetter and cooler chronosequence. Furthermore, the MAOM energy density decreases with soil depth and age in both chronosequences. Independently of the chronosequence, POM represents the most labile pool with higher energy density. In conclusion, time and climate play a different role in SOC distribution between the pools and on their relative stability. Soil age drives MAOM stability in drier and warmer conditions, whereas a wetter and cooler climate determines a higher SOC accumulation in both pools, although these greater carbon stocks are negatively correlated with their stability.