Investigating Microbial Phosphorus Acquisition in the Presence of Iron across a Permafrost Thaw Gradient in Abisko Sweden.

Abstract:

Northern latitudes are rapidly warming at a faster rate than any other region on Earth. As permafrost thaws, soil carbon stocks are at risk of being released into the atmosphere, transitioning arctic systems from carbon sinks to sources. Soil microbial carbon metabolism is constrained by temperature, water, and nutrient availability. Nutrients such as phosphate (PO₄^3-), are regulated by iron geochemistry, which varies across redox conditions as a function of the hydrologic fluctuations in permafrost-affected soils. Where present, poorly crystalline iron (oxyhydr)oxides sorb PO₄^3-, limiting its bioavailability. As warming continues, increased PO₄^3- sorption may occur in drying soils, further constraining carbon mineralization. Changes in PO₄^3- bioavailability and microbial phosphorus (P) acquisition remain poorly understood as arctic and subarctic systems change in response to warming.

To assess microbial P acquisition across a permafrost thaw gradient, we measured microbial biomass P storage of the microbial community in subarctic soils in Abisko, Sweden. We conducted an in-situ incubation experiment in which mesh bags were filled with natural iron rich sediment that was either amended with PO₄^3- or left unamended. Bags were incubated for one or eight weeks along a permafrost thaw gradient representing different redox conditions. The thaw gradient included a raised palsa with permafrost located near the soil surface, a subsided bog with permafrost located deeper in the soil, and an inundated fen with no underlying permafrost. Incubated sediments were analyzed for microbial biomass P to determine the total amount of P accessed and stored in microbial biomass in the presence of iron rich material.

Preliminary results show P concentrations in microbial biomass are less than 0.01mmol P/kg^-1 compared to average temperate soil concentrations of 0.67 mmol P/kg^-1. This suggests that microbes are acclimated to low P conditions and/or may be limited by nutrients other than P in subarctic soils, resulting in limited storage of P. Based on these results, low P bioavailability will be an important factor that may affect how these systems respond to changes in climate. To further elucidate phosphorus dynamics in permafrost soils, continued analysis of soil iron and phosphorus interactions are necessary.