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, along with nitrogen, may limit biological productivity and carbon storage in northern ecosystems. Here, we investigate the potential for phosphate adsorption to poorly-crystalline iron (Fe) oxide minerals that precipitate during drainage and drying of anoxic peat soils to limit P bioavailability in high-latitude wetlands. We compare Fe and P geochemistry in organic-rich soils collected from either relatively depressed and saturated or elevated and dry microtopographic positions in sites spanning a latitudinal gradient in 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. We consider differences in soil saturation between microtopographic positions to be proxies for hydrologic changes driven by altered climate. To assess P sorption to Fe-oxides, we use phosphate sorption assays to evaluate the capacity for soils to bind phosphate and sequential extractions to quantify Fe phases including poorly-crystalline iron oxides. Our results indicate that phosphate sorption capacity differs across microtopographic gradients, and zones of high phosphate sorption capacity may coincide with Fe-oxide accumulation at oxic-anoxic interfaces. Consequently, projected temperature increases in arctic and boreal regions may influence P availability due to increased association with poorly-crystalline Fe-oxides that precipitate as water tables lower in drying peatlands, wetlands, and polygonal landscapes.