GEOCHEMICAL ANALYSIS OF IRON AND PHOSPHOROUS IN ARCTIC TUNDRA SOILS

Arctic temperatures are increasing at double the global average rate, making permafrost prone to thawing that will release previously-frozen soil organic carbon (C) into the atmosphere as the greenhouse gases carbon dioxide and methane. Although increases in net primary productivity may partially mitigate C losses from tundra soils, current Earth systems models may overestimate future C storage in plant biomass because they do not consider nutrient limitations. In this study, we evaluated the potential for iron (Fe) oxyhydroxides to impact phosphorus (P) bioavailability in tundra soils obtained from the Barrow Environmental Observatory on the Arctic Coastal Plain in Alaska. Sequential extractions and synchrotron-source microprobe analyses were used to examine Fe and P associations in organic and mineral soil horizons collected from different topographic features of low-centered and high-centered polygons. We hypothesized that P adsorbs to poorly-crystalline iron (Fe) oxyhydroxides that precipitate during seasonal drying of topographic high features.

Iron and P geochemistry differed between organic and mineral horizons and varied as a function of soil saturation. Iron was primarily present as poorly-crystalline oxides in all soils (average 56 ± 6%), consistent with observations that organic and mineral constituents in the soil were coated with Fe(III)-phases. Organic horizons were enriched in poorly-crystalline and crystalline Fe-oxides relative to mineral horizons, while mineral horizons contained a higher proportion of organic-bound Fe and magnetite/ilmenite. Phosphorus was contained primarily in organic matter (91 ± 2%) with an additional 8 ± 2% bound to iron oxides. Consistent with our hypothesis, poorly-crystalline Fe oxides increased as soil saturation decreased; however, P was correlated with crystalline (p < 0.05) rather than poorly-crystalline Fe oxides. From these results, we infer that P availability to plants may be limited by incorporation into crystalline mineral structures, either iron oxides (e.g., hematite, goethite) or aluminum oxides dissolved in the crystalline extraction. In order to accurately predict global C budgets under changing climate, it is essential to evaluate geochemically-driven nutrient availability in tundra ecosystems.