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

Abstract

Aluminum (Al)- and iron (Fe)-bearing minerals, especially short-range-ordered (SRO) phases, are thought to protect soil organic carbon (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 (Alox) often correlates stronger with SOC than Fe dissolved by oxalate (Feox) or citratedithionite (Fecd). To further evaluate these relationships, we analyzed a large North American soil dataset from the National Ecological Observatory Network (NEON). A strong relationship between Alox and SOC (and weaker Feox-SOC relationship) persisted even after excluding soils rich in SRO minerals (Andisols and Spodosols). Alox was strongly correlated with citrate-dithionite extractable Al (Alcd; slope=0.92, R2=0.69), and discrepancies could be explained (R2=0.87) by greater dissolution of Al-substituted Fe phases by citrate-dithionite, and greater dissolution of aluminosilicates by oxalate. Alox and Alcd were both strong SOC predictors despite their differing relationships with silicon (Si). Alox and Siox strongly covaried (R2=0.79), but Alcd was inconsistently related to Sicd (R2=0.18). Similar relationships of Alox and Alcd with SOC despite differences in minerals extracted by oxalate and citrate-dithionite suggest that Al-OC complexes—as opposed to aluminosilicate or ironbearing 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 Alox and SOC correlations rather than Al phase composition or abundance.