Moisture and soil depth govern relationships between soil organic carbon and oxalate-extractable metals at the global scale

An important control on long-term soil organic carbon (SOC) storage is the adsorption of SOC by short-range-ordered (SRO) minerals. SRO are commonly quantified by measuring oxalate-extractable metals (M_ox = Al_ox + ½ Fe_ox), which many studies have shown to be positively correlated with SOC. It remains uncertain if this organo-mineral relationship is robust at the global scale, or if capturing regional differences is needed to maximize model accuracy. We used a global synthesis of Al_ox and Fe_ox data to test their role in controlling SOC abundance across regions. We compiled 37,344 individual soil horizon measurements, with soil depth ranging between 0 and 200 cm, from 11,122 profiles. We used the Holdridge Life Zones, which are characterized by biotemperature, precipitation, and potential evapotranspiration, to group the soil profiles by their climatic conditions that also correlate with other important soil-forming factors. Based on linear mixed-effects models, we found a positive relationship between M_ox and SOC across regions and depths, accounting for 49% of the SOC variation. This relationship is strongest in wetter regions and at depths between 20 and 100 cm. Across all environmental conditions, Al_ox is a stronger predictor of SOC than Fe_ox. Our analysis suggests oxalate-extractable metals are good proxies for mineral-induced SOC protection at the global scale. However, our findings also indicate that the importance of organo-mineral interactions at the global scale varies with climatic conditions and depth. The underlying mechanisms need to be considered when incorporating these relationships as proxies for mineral sorption capacity into soil C models.