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Arctic and boreal ecosystems are experiencing pronounced warming that is accelerating decomposition of soil organic matter and releasing greenhouse gases to the atmosphere. Future carbon storage in these ecosystems depends on the balance between microbial decomposition and primary production, both of which can be regulated by nutrients such as phosphorus. Phosphorus cycling in tundra and boreal regions is often assumed to occur through biological pathways with little interaction with soil minerals; that is, phosphate released from organic molecules is rapidly assimilated by plants or microorganisms. In contrast to this prevailing conceptual model, we use sequential extractions and spectroscopic techniques to demonstrate that iron (oxyhydr)oxides sequester approximately half of soil phosphate in organic soils from four arctic and boreal sites. Iron (III) (oxyhydr)oxides accumulated in shallow soils of low-lying, saturated areas where circumneutral pH and the presence of a redox interface promoted iron oxidation and hydrolysis. Soils enriched in short-range ordered iron oxyhydroxides, which are susceptible to dissolution under anoxic conditions, had high phosphate sorption capacities and maintained low concentrations of soluble phosphate relative to soils containing mostly organic-bound iron or crystalline iron oxides. Thus, substantial quantities of phosphorus in these organic soils were associated with minerals that could reduce bioavailability but potentially also serve as phosphorus sources under anoxic conditions. The implication of this finding is that mineral surfaces effectively compete with biological processes for phosphate and must be considered as a nutrient regulator in these sensitive ecosystems.

Plain Language Summary The ability of plants to derive nutrients from the soil influences their capacity to photosynthesize and draw carbon out of the atmosphere. Plants compete for nutrients such as phosphate with soil microorganisms and with soil minerals. Iron oxides, in particular, effectively bind phosphate and keep it sequestered from plants. We demonstrate that iron oxides bind high quantities of phosphate in arctic and boreal systems where minerals are often assumed to have negligible influence on biological processes. Although plant biomass is likely to increase in a climate that is warmer and enriched in carbon dioxide, iron oxides may increasingly limit phosphate availability to plants and constrain ecosystem productivity.