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
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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.
Baribault TW, Kobe RK, Finley AO (2011) Tropical tree growth is correlated with soil phosphorus, potassium, and calcium, though not for legumes. Ecol Monogr 82:189–203. doi:10.1890/11-1013.1CrossRefGoogle Scholar
Chacon N, Silver WL, Dubinsky EA, Cusack DF (2006) Iron reduction and soil phosphorus solubilization in humid tropical forest soils: the roles of labile carbon pools and an electron shuttle compound. Biogeochemistry 78:67–84. doi:10.1007/s10533-005-2343-3CrossRefGoogle Scholar
Chadwick OA, Derry LA, Vitousek PM et al (1999) Changing sources of nutrients during four million years of ecosystem development. Nature 397:491–497. doi:10.1038/17276CrossRefGoogle Scholar
Gee G, Bauder J (1986) Particle size analysis. In: Klute A (ed) Methods of soil analysis, Part 1, physical and mineralogical methods, 2nd edn. American Society of Agronomy, Madison, pp 383–411Google Scholar
Ginn B, Meile C, Wilmoth J et al (2017) Rapid iron reduction rates are stimulated by high-amplitude redox fluctuations in a tropical forest soil. Environ Sci Technol 51:3250–3259. doi:10.1021/acs.est.6b05709CrossRefGoogle Scholar
Grybos M, Davranche M, Gruau G, Petitjean P (2007) Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxides reduction? J Colloid Interface Sci 314:490–501. doi:10.1016/j.jcis.2007.04.062CrossRefGoogle Scholar
Hall SJ, Silver WL (2015) Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest. Biogeochemistry 125:149–165. doi:10.1007/s10533-015-0120-5CrossRefGoogle Scholar
Hall SJ, McDowell WH, Silver WL (2013) When wet gets wetter: decoupling of moisture, redox biogeochemistry, and greenhouse gas fluxes in a humid tropical forest soil. Ecosystems 16:576–589. doi:10.1007/s10021-012-9631-2CrossRefGoogle Scholar
Hall SJ, Treffkorn J, Silver WL (2014) Breaking the enzymatic latch: impacts of reducing conditions on hydrolytic enzyme activity in tropical forest soils. Ecology 95:2964–2973. doi:10.1890/13-2151.1CrossRefGoogle Scholar
Hall SJ, Liptzin D, Buss HL et al (2016) Drivers and patterns of iron redox cycling from surface to bedrock in a deep tropical forest soil: a new conceptual model. Biogeochemistry 130:177–190. doi:10.1007/s10533-016-0251-3CrossRefGoogle Scholar
Henderson R, Kabengi N, Mantripragada N et al (2012) Anoxia-induced release of colloid- and nanoparticle-bound phosphorus in grassland soils. Environ Sci Technol 46:11727–11734. doi:10.1021/es302395rCrossRefGoogle Scholar
Lloyd J, Domingues TF, Schrodt F et al (2015) Edaphic, structural and physiological contrasts across Amazon Basin forest–savanna ecotones suggest a role for potassium as a key modulator of tropical woody vegetation structure and function. Biogeosciences 12:6529–6571. doi:10.5194/bg-12-6529-2015CrossRefGoogle Scholar
Lovley DR, Phillips EJP (1987) Rapid assay for microbially reducible ferric iron in aquatic sediments. Appl Environ Microbiol 53:1536–1540Google Scholar
Markewitz D, Davidson EA, de O Figueiredo R et al (2001) Control of cation concentrations in stream waters by surface soil processes in an Amazonian watershed. Nature 410:802–805. doi:10.1038/35071052CrossRefGoogle Scholar
Phillips IR, Greenway M (1998) Changes in water-soluble and exchangeable ions, cation exchange capacity, and phosphorus max in soils under alternating waterlogged and drying conditions. Commun Soil Sci Plant Anal 29:51–65. doi:10.1080/00103629809369928CrossRefGoogle Scholar
Pinheiro J, Bates D, DebRoy S et al (2014) nlme: linear and nonlinear mixed effects modelsGoogle Scholar
Soil Survey Staff (2002) Soil survey of Caribbean National Forest and Luquillo Experimental Forest, Commonwealth of Puerto Rico. United States Department of Agriculture, Natural Resources Conservation ServiceGoogle Scholar
Unger M, Leuschner C, Homeier J (2010) Variability of indices of macronutrient availability in soils at different spatial scales along an elevation transect in tropical moist forests (NE Ecuador). Plant Soil 336:443–458. doi:10.1007/s11104-010-0494-zCrossRefGoogle Scholar
Vitousek P, Sanford R (1986) Nutrient cycling in moist tropical forest. Annu Rev Ecol Syst 17:137–167CrossRefGoogle Scholar
Wright SJ, Yavitt JB, Wurzburger N et al (2011) Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest. Ecology 92:1616–1625. doi:10.1890/10-1558.1CrossRefGoogle Scholar