IRON REDOX CYCLING AND IMPACTS ON PHOSPHORUS SOLUBILITY IN TUNDRA AND BOREAL ECOSYSTEMS

Rapidly changing climate in high-latitude regions is altering biogeochemical cycles and potentially shifting Arctic and sub-Arctic ecosystems from sinks to sources of atmospheric carbon. Phosphorus (P) is an important nutrient whose availability may limit biological productivity, plant growth, and carbon storage in northern ecosystems. Microorganisms control the release of inorganic phosphate from decomposing organic matter and make it available for plant uptake. However, phosphate sorbs strongly to soil minerals such as iron (Fe) oxides, and bioavailability may also be regulated by microbially mediated iron oxidation/reduction transformations. Here, we investigate the potential for phosphate adsorption to iron oxide minerals that precipitate during drainage of anoxic, Fe-rich peat soils to limit P bioavailability in high-latitude peatlands. We compare Fe and P geochemistry in peat soils collected from relatively depressed and saturated microtopographic positions to elevated and dry positions along a latitudinal gradient in northern North America, including tundra (Barrow Environmental Observatory, AK; Toolik Lake Field Station, AK) and boreal (Bonanza Creek Environmental Forest, AK; Marcell Experimental Forest, MN) ecosystems. Differences in soil saturation generated by microtopography serve as a proxy for hydrologic changes driven by altered climate and permafrost degradation. To assess P sorption to Fe-oxides, we used a phosphate sorption index to evaluate the capacity for soils to bind phosphate and sequential extractions to quantify Fe phases including poorly-crystalline and crystalline iron oxides. Our results indicate that phosphate sorption capacity differs across microtopographic gradients that may reflect the position of Fe-oxide accumulation at oxic-anoxic interfaces. Consequently, projected temperature increases in Arctic and sub-Arctic regions may influence P availability due to increased association with iron oxides that precipitate as water tables lower in drying peatlands.