Phosphorus bioavailability as a function of increasing permafrost thaw and surface ponding in Abisko, Sweden

Abstract:

Phosphorus (P) is an important limiting nutrient in many terrestrial ecosystems including those in the arctic and subarctic. Phosphate, the major ion used by plants and microbes, binds strongly to iron (oxyhydr)oxide minerals (henceforth called Fe oxides). Consequently, Fe oxides play an important role in controlling the bioavailability of phosphate in soils. Increasing temperatures in the arctic can cause permafrost thaw and radically change hydrological and redox conditions in soils. Changes in redox conditions can either cause the precipitation or dissolution of redox sensitive minerals like Fe oxides and influence the phosphate nutrient cycle by sorption onto Fe oxides.

To explore P bioavailability under these complex interactions between permafrost thaw, Fe oxides, and redox conditions, we conducted an in situ incubation experiment along a permafrost gradient in a subarctic peatland in Abisko, Sweden. The permafrost gradient included an elevated palsa with permafrost located close to the soil surface, a collapsed bog with permafrost located deep beneath the soil surface, and a fen with no underlying permafrost. Soil redox conditions were measured continuously over 34 days in the palsa, bog, and fen. Mesh bags, filled with local Fe-rich sediments comprised of ferrihydrite, organic-bound Fe, and goethite and with or without adsorbed phosphate, were buried under the topsoil at each site and incubated for one or eight weeks. Incubated materials were subsequently analyzed with sequential extractions to quantify changes in total Fe and P and with x-ray absorption fine structure spectroscopy to evaluate changes in Fe speciation.

Redox conditions transitioned from oxic to anoxic as permafrost thaw progressed and caused surface ponding, although redox was variable across the thaw season. The proportion of total Fe present as ferrihydrite increased over time in sediments incubated in the bog and fen, particularly in phosphate-added treatments, while remaining unchanged in the palsa. We conclude that fluctuating redox conditions in the bog and fen topsoils and P addition facilitated precipitation of new ferrihydrite in the incubated material. Increases in ferrihydrite during permafrost collapse could decrease bioavailable phosphate in the topsoils and potentially limit plant growth and microbial activity.