Soil Organic Matter Stabilization By Fe-C Interactions in Temperate and Tropical Soils: A Cross-CZO Comparison.

Increasing evidence suggests that the long-term stability of soil organic matter (SOM) is dominated by organomineral interactions, particularly those involving Fe short-range-order (SRO) minerals. SRO minerals are highly reactive and capable of SOM stabilization through sorption or co-precipitation, and thus SRO-SOM associations may be a disproportionally important driver of SOM stabilization capacity in highly weathered soils lacking 2:1 phyllosilicate clays. The Critical Zone Observatory (CZO) network provides an ideal framework for comparative research into the role of SRO-SOM associations, particularly the Luquillo and Calhoun soils, which are formed from highly weathered, metal-rich parent materials, yet differ drastically in surface processes. Surface (0-20 cm) and subsurface (50-80 cm) samples were taken from 20 quantitative soil pits within the Luquillo site in northeast Puerto Rico, stratified across two parent materials. A comparative set of 26 samples from similar horizons were collected from six temperate Calhoun sites across North Carolina under three distinct land treatment types. We will present results from inorganic selective dissolution used to isolate three forms of Fe-C interactions: sodium pyrophosphate to isolate organomineral complexes, HCl-hydroxylamine and ammonium oxalate to isolate SRO Fe hydroxides, and inorganic dithionite to isolate crystalline Fe oxides. Extracts were analysed for Fe concentrations via ICP-OES, and for dissolved organic C (DOC) to estimate SOM concentrations associated with each mineral phase. Preliminary results indicate a dominance of crystalline and semi-crystalline Fe phases in surface horizons across both CZOs, while DOC is primarily pyrophosphate-extractable in Luquillo and dithionite-extractable in Calhoun. At depth, we observe a decline in SRO phases at both sites, in correlation with decreased weathering, accompanied by an increase in pyrophosphate-extractable Fe and DOC in Luquillo and dithionite-extractable Fe and DOC in Calhoun. These results demonstrate important differences in Fe-SOM associations between the sites, driven by pedogenic gradients in parent material, depth and climate.