Increasing temperatures in arctic-subarctic regions are warming soils and driving greenhouse gas releases (CO<sub>2</sub> and CH<sub>4</sub>) that further enhance global warming. Though increasing plant growth has been identified as viable solution to partially offset these rising temperatures, growth is limited by availability of nutrients including phosphorous (P). Phosphate effectively binds onto Fe-(oxyhyr)oxides, potentially limiting its bioavailability where Fe-(oxyhydr)oxides accumulate in northern high-latitude tundra. However, Fe speciation and phosphate sorption are sensitive to pH and redox changes that occur in these organic-rich soils. Although interactions between Fe and phosphate are well-studied in mineral soils, interactions in organic soils and with organic P are less understood. The objective of this study was to investigate the effects of pH on Fe speciation and its interactions with P in peat soils using laboratory incubation experiments. Batch experiments using aqueous Fe(II) and/or P were conducted with autoclaved peat soils buffered to different pH conditions (4, 6, or 7.5) for 24 hours under oxic conditions. Specifically, soluble P (as phosphate or phytic acid) was reacted with peat in the presence or absence of aqueous Fe(II), or with peat pre-reacted with aqueous Fe(II). Aqueous Fe(II) concentrations decreased over time, consistent with the formation of solid-phase Fe, and Fe(II) removal occurred more quickly at higher pH and in the presence of phosphorus. Phosphorus was completely removed from solution in the presence of Fe(II), and phytate removal was ~5x faster than phosphate. We hypothesize that Fe-(oxyhydr)oxide formation is driving rapid P sorption at high pH, but that slower oxidation kinetics and increased formation of organic-Fe phases slow P removal at lower pH. Spectroscopy on the reacted solids will be used to identify solid-phase Fe species and mechanisms for P removal. These results will improve understanding of how geochemical controls regulate nutrient bioavailability in arctic soils.

Interactions between Fe and P in peat soil under oxidizing conditions

ACS Fall 2021

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