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 (Alox) often correlates stronger with SOC than Fe dissolved by oxalate (Feox) or citrate–dithionite (Fecd). To further evaluate these relationships, we analyzed a large North American soil dataset from the National Ecological Observatory Network. A strong relationship between Alox and SOC (and weaker Feox‐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 (Alcd; slope = 0.92, R2 = .69), and discrepancies could be explained (R2 = .87) by greater dissolution of Al‐substituted Fe phases by citrate–dithionite and greater dissolution of aluminosilicates by oxalate. Aluminum dissolved by oxalate and Alcd were both strong SOC predictors despite their differing relationships with silicon (Si). Al dissolved by oxalate and Siox strongly covaried (R2 = .79), but Alcd was inconsistently related to Sicd (R2 = .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 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 Alox and SOC correlations rather than Al phase composition or abundance.

Association of Organic Carbon With Reactive Iron Oxides Driven by Soil pH at the Global Scale

“…It should be noted that Al is commonly substituted within Fe mineral phases and is also released following dithionite extraction, and Al accounted for 20 ± 15% of total reducible Fe and Al on a molar basis in a previous study of diverse soils (Wagai & Mayer, 2007). A similar estimate of Al substitution in Fe phases was obtained from statistical comparison of dithionite-and oxalate-extractable metals from a broad range of NEON soils (Hall & Thompson, 2021). Therefore, the direct association of organic C with Fe atoms as opposed to Al atoms substituted in the Fe phase cannot be ascertained with the dithionite extraction.…”

“…However, our data indicate that in many soils, secondary Fe phases may be sufficiently abundant that other factors such as pH (Figure 3a) or soil organic C input (Figure 4d) become more important than Fe abundance per se. This view is further supported by statistical analyses of relationships between bulk soil organic C concentrations and Fe o in large soil datasets, where soil organic C tends to increase with Fe o at very low Fe o concentrations, but the response plateaus as Fe o exceeds approximately 1 or 2 mg Fe o g −1 (Hall & Thompson, 2021;Yu et al, 2021;von Fromm et al, 2021). Together, these results demonstrated that soil pH exerts strong control over the concentrations of C associated with reactive Fe minerals, consistent with previous work highlighting a central role for soil pH in modulating organo-metal complexation (Kleber et al, 2015;Wagai & Mayer, 2007).…”

Contrasting geochemical and fungal controls on decomposition of lignin and soil carbon at continental scale

“…The metals extracted from these NEON soils likely represent ions (e.g. Ca cd ), metals dissolved from mineral phases of varying crystallinity (e.g., Fe HCl , Fe ox , Fe cd-ox ), or a mixture of ions and mineral phases (e.g., Mn cd and Al ox ) 32 . Synthesis studies and lab experiments demonstrate that soil mineral and metal cations as well as fine particles (silt+clay) are important predictors of SOC concentration due to protection by sorption, precipitation, and aggregation 16,20,33 .…”

Contrasting geochemical and fungal controls on decomposition of lignin and soil carbon at continental scale

“…The metals extracted from these NEON soils likely represent ions (e.g. Ca cd ), metals dissolved from speci c mineral phases of varying crystallinity (e.g., Fe HCl , Fe ox , Fe cd−ox ), or a mixture of ions and mineral phases (e.g., Mn cd and Al ox ) 32 . Soil mineral and metal cations as well as ne particles (silt + clay) are important predictors of SOC concentration due to protection by sorption, precipitation, and aggregation 13,19,33 .…”