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

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2021-10-06
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Hall, Steven
Thompson, Aaron
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© 2021 The Authors. Soil Science Society of America Journal © 2021 Soil Science Society of America
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Ecology, Evolution and Organismal Biology
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.
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This is the peer reviewed version of the following article: Hall, Steven J., and Aaron Thompson. "What do relationships between extractable metals and soil organic carbon concentrations mean?." Soil Science Society of America Journal. (2021), which has been published in final form at DOI:10.1002/saj2.20343. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.
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