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

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.

Automated summary

Aluminum and iron minerals, especially short-range-ordered phases, are believed to protect soil organic C. However, it is challenging to assess the influence of these minerals or metal complexes. Through analyzing a large soil dataset, it was found that aluminum dissolved by oxalate showed a strong relationship with soil organic C, while iron dissolved by oxalate or citrate-dithionite showed weaker correlation. The results suggest that aluminum-organic carbon complexes may be the best predictor of soil organic C.