Abstract

Iron (Fe) is essential to plants, microbes, and animals, is an important element in weathered soils from tropical and subtropical regions due to its reactivity toward carbon (C) and nutrients and its ability to serve as an electron acceptor for anaerobic respiration. Humid (sub)tropical and iron-rich soils naturally experience fluctuations in soil moisture, oxygen content, and hence, redox potential due to elevated but intermittent rainfall and high inputs of labile carbon from decomposed litter. Soils from the Luquillo Critical Zone Observatory (LCZO), Puerto Rico, are well-suited for studying the impact of redox fluctuations on Fe and C biogeochemistry. I conducted two laboratory experiments, exploring coupled Fe-C mechanisms, and one field experiment, using LCZO soils. Both lab experiments were conducted using soil in a slurry, which minimizes spatial variability and involved shifting between anoxic and oxic conditions. In the first lab study, I found that iron reduction rates increased when redox oscillations occurred more frequently. In the second lab experiment, I varied the time under oxic conditions (τ_oxic) in both long and short oscillation periods. For the long treatments (τ_anoxic at 6 d), I observed that as τ_oxic decreased from 72 to 24 to 8 hours, Fe reduction rates increased, CO2 emissions remained unchanged, and CH4 emissions decreased; and for the short treatments (τ_anoxic at 2 d), Fe^II and trace gases emissions decreased throughout the experiment. For the field experiment, I monitored several biogeochemical variables involved in Fe-C redox processes in triplicate catenas at ridge, slope, and valley positions. I found that soil moisture was a predictor for changes in Fe^II, rapidly-reducible Fe oxides (Fe^III_RR), pH, Eh, and DOC. Valleys were more responsive to environmental changes than the other landscape positions. I also conducted three other lab studies (using LCZO soils) and one field experiment at the Calhoun CZO, in South Carolina (each are reported briefly in the Appendices). In conclusion, under natural and laboratory redox fluctuating systems, iron exerts a strong biogeochemical influence on the carbon dynamics of soils from humid (sub)tropical regions with important climate change and environmental implications.

Citation

Barcellos, Diego (2018): Biogeochemical cycling of iron and carbon in humid (sub)tropical forest soils under fluctuating redox conditions. PhD Dissertation, Department of Crop and Soil Sciences, University of Georgia.

This Paper/Book acknowledges NSF CZO grant support.