Iron concentrations and speciation across a soil moisture and soil pH gradient in the arctic tundra and its impact on phosphate availability

Climate change in the Arctic is leading to warmer soil temperatures and an increase in the seasonal thaw depth. A warmer soil environment can stimulate microbial activity, causing deep soil carbon to become available for decomposition. Increases in atmospheric CO2 and warmer temperatures are predicted to heighten plant productivity, causing more carbon capture and mitigating further increases in atmospheric CO2. The idea of plant productivity increasing in tandem with atmospheric CO2 does not consider how nutrient availability constrains productivity. In the arctic tundra, low phosphorus availability causes a tight cycling of phosphate between plants and microbes; availability is further restricted by the ability of phosphate to adsorb to iron(oxyhr)oxides in oxic soil conditions. The concentration of phosphate adsorbed to iron can increase or decrease across the landscape with changes in water saturation and soil pH. With the micro topographic variation present across the arctic tundra, we ask 1) is the concentration of iron greater in more saturated, acidic soils? 2) do specific species of iron show greater ability to adsorb phosphorus? To answer these questions, we sampled soils near Toolik Field Station, AK across three time points during the growing season in 2021. We collected organic surface soils along a hillslope transect with dry, semi-saturated, and saturated soil moisture conditions in both acidic and non-acidic soils to observe how both pH and soil saturation would affect phosphorus-iron associations. We completed phosphorus and iron fractionations, along with a phosphate sorption index, a comparative measure of a soil's capacity to sorb phosphate. Concentrations of iron were highest in the acidic soils with saturated soil moisture conditions. The most abundant species of iron was found to be colloidal and organic-bound iron across all sites, with especially high concentrations in the sites with saturated soil conditions. The concentrations of organic-bound iron showed a strong relationship with the phosphate sorption index, indicating that organic-bound iron may play an important role in phosphate adsorption. With warming temperatures increasing the rate of microbial decomposition, the presence of organic-bound iron in saturated soils could limit the bioavailability of phosphate.