, Volume 125, Issue 2, pp 149–165 | Cite as

Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest

  • Steven J. Hall
  • Whendee L. Silver


Humid tropical forests support large stocks of surface soil carbon (C) that exhibit high spatial variability over scales of meters to landscapes (km). Reactive minerals and organo-metal complexes are known to contribute to C accumulation in these ecosystems, although potential interactions with environmental factors such as oxygen (O2) availability have received much less attention. Reducing conditions can potentially contribute to C accumulation, yet anaerobic metabolic processes such as iron (Fe) reduction can also drive substantial C losses. We tested whether these factors could explain variation in soil C (0–10 and 10–20 cm depths) over multiple spatial scales in the Luquillo Experimental Forest, Puerto Rico, using reduced iron (Fe(II)) concentrations as an index of reducing conditions across sites differing in vegetation, topographic position, and/or climate. Fine root biomass and Fe(II) were the best overall correlates of site (n = 6) mean C concentrations and stocks from 0 to 20 cm depth (r = 0.99 and 0.98, respectively). Litterfall decreased as reducing conditions, total and dead fine root biomass, and soil C increased among sites, suggesting that decomposition rates rather than C inputs regulated soil C content at the landscape scale. Strong relationships between Fe(II) and dead fine root biomass suggest that reducing conditions suppressed particulate organic matter decomposition. The optimal mixed-effects regression model for individual soil samples (n = 149) showed that aluminum (Al) and Fe in citrate/ascorbate and oxalate extractions, Fe(II), fine root biomass, and interactions between Fe(II) and Al explained most of the variation in C concentrations (pseudo R2 = 0.82). The optimal model of C stocks was similar but did not include fine root biomass (pseudo R2 = 0.62). In these models, soil C concentrations and stocks increased with citrate/ascorbate-extractable Al and oxalate-extractable Fe. However, soil C decreased with citrate/ascorbate-extractable Fe, an index of Fe susceptible to anaerobic microbial reduction. At the site scale (n = 6), ratios of citrate/ascorbate to oxalate-extractable Fe consistently decreased across a landscape O2 gradient as C increased. We suggest that the impact of reducing conditions on organic matter decomposition and the presence of organo-metal complexes and C sorption by short-range order Fe and Al contribute to C accumulation, whereas the availability of an Fe pool to sustain anaerobic respiration in soil microsites partially attenuates soil C accumulation in these ecosystems.


Iron reduction Poorly-crystalline minerals Redox Root biomass Soil carbon Soil oxygen 



All data reported here will be publicly available through the NSF Critical Zone Observatory web data repository ( We thank T. Baisden, M. Kramer, and anonymous reviewers for critical commentary. G. Sposito, A. Thompson, M. Firestone, and R. Rhew also provided valuable insights, and H. Dang, J. Treffkorn, T. Natake, J. Cosgrove, R. Ryals, A. McDowell, and C. Torrens helped in the field and lab. SJH was funded by the DOE Office of Science Graduate Fellowship Program supported by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100. WLS received support from CA-B-ECO-7673-MS from the A.E.S. Funding was also provided by DOE grant DE-FOA-0000749 and NSF grant EAR-08199072 to WLS, the NSF Luquillo Critical Zone Observatory (EAR-0722476) with additional support provided by the USGS Luquillo WEBB program, and grant DEB 0620910 from NSF to the Institute for Tropical Ecosystem Studies, University of Puerto Rico, and to the International Institute of Tropical Forestry USDA Forest Service, as part of the LTER Program.

Supplementary material

10533_2015_120_MOESM1_ESM.docx (1.3 mb)
Supplementary material 1 (DOCX 1375 kb)


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Copyright information

© Springer International Publishing Switzerland 2015

Authors and Affiliations

  1. 1.Department of Environmental Science, Policy, and ManagementUniversity of California-BerkeleyBerkeleyUSA
  2. 2.Department of Ecology, Evolution, and Organismal BiologyIowa State UniversityAmesUSA

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