, Volume 136, Issue 1, pp 91–102 | Cite as

Iron reduction: a mechanism for dynamic cycling of occluded cations in tropical forest soils?

  • Steven J. HallEmail author
  • Wenjuan Huang


Nutrient cations can limit plant productivity in highly weathered soils, but have received much less attention than phosphorus and nitrogen. The reduction of iron (Fe) in anaerobic microsites of surface soils can solubilize organic matter and P sorbed or occluded with short-range-ordered (SRO) Fe phases. This mechanism might also release occluded cations. In the Luquillo Experimental Forest, Puerto Rico, we measured cation release during anaerobic laboratory incubations, and assessed patterns of cation availability in surface soils spanning ridge-slope-valley catenas. During anaerobic incubations, potassium (K), calcium (Ca) and magnesium (Mg) significantly increased with reduced Fe (Fe(II)) in both water and 0.5 M HCl extractions, but did not change during aerobic incubations. In the field, 0.5 M HCl-extractable Fe(II) and Fe(III) were the strongest predictors of K, Mg, and Ca on ridges (R2 0.57–0.75). Here, both Ca and Mg decreased with Fe(III), while K, Ca, and Mg increased with Fe(II), consistent with release of Fe-occluded cations following Fe reduction. Manganese in ridge soils was extremely low, consistent with leaching following reductive dissolution of Mn(IV). On slopes, soil C was the strongest cation predictor, consistent with the importance of organic matter for cation exchange in these highly weathered Oxisols. In riparian valleys, cation concentrations were up to 16-fold greater than in other topographic positions but were weakly or unrelated to measured predictors, potentially reflecting cation-rich groundwater. Predictors of cation availability varied with topography, but were consistent with the potential importance of microsite Fe reduction in liberating occluded cations, particularly in the highly productive ridges. This mechanism may explain discrepancies among indices of “available” soil cations and plant cation uptake observed in other tropical forests.


Cation Luquillo Experimental Forest Iron Occluded Redox Walker-Syers model 



This work was supported by NSF grant DEB-1457805, by the NSF Luquillo Critical Zone Observatory, and by Iowa State University. SJH gratefully acknowledges mentorship by W. Silver on related research at this site. We thank A. Russell for discussion about the conceptual model, S. Rathke and S. Bakshi for assistance with ICP analyses, the USFS International Institute of Tropical Forestry for logistical support, and O. Gutierrez del Arroyo for collecting soil for the incubation experiment.


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© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Department of Ecology, Evolution, and Organismal BiologyIowa State UniversityAmesUSA

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