What do relationships between extractable metals and soil organic carbon concentrations mean?

Abstract Aluminum (Al)‐bearing and iron (Fe)‐bearing minerals, especially short‐range‐ordered (SRO) phases, are thought to protect soil organic C (SOC). However, it remains methodologically challenging to assess the influence of Al vs. Fe minerals or metal complexes. Whereas SRO Al and Fe phases share some properties, Al dissolved by oxalate (Al ox ) often correlates stronger with SOC than Fe dissolved by oxalate (Fe ox ) or citrate–dithionite (Fe cd ). To further evaluate these relationships, we analyzed a large North American soil dataset from the National Ecological Observatory Network. A strong relationship between Al ox and SOC (and weaker Fe ox ‐SOC relationship) persisted even after excluding soils rich in SRO minerals (Andisols and Spodosols). Al dissolved by oxalate was strongly correlated with citrate–dithionite‐extractable Al (Al cd ; slope = 0.92, R 2 = .69), and discrepancies could be explained ( R 2 = .87) by greater dissolution of Al‐substituted Fe phases by citrate–dithionite and greater dissolution of aluminosilicates by oxalate. Aluminum dissolved by oxalate and Al cd were both strong SOC predictors despite their differing relationships with silicon (Si). Al dissolved by oxalate and Si ox strongly covaried ( R 2 = .79), but Al cd was inconsistently related to Si cd ( R 2 = .18). Similar relationships of Al ox and Al cd with SOC, despite differences in minerals extracted by oxalate and citrate–dithionite, suggest that Al‐OC complexes (as opposed to aluminosilicate or iron‐bearing minerals) were the best SOC predictor. This raises important questions: do Al‐OC complexes indicate protection from decomposition or simply reflect greater intensity of mineral weathering by organic acids; and, if the latter, then perhaps SOC input is driving Al ox and SOC correlations rather than Al phase composition or abundance.