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Phosphorus Chemistry and Release in Restored and Agricultural Floodplains Following Freezing and Thawing
Disturbance regimes like freezing and thawing (FT) can have potentially significant impacts on nutrient release from soil and are predicted to increase with climate change. This is particularly important in biogeochemical hotspots like floodplains that can both remove and release nutrients to surface waters during flooding. Connection between the river and floodplain can improve water quality by reducing nutrient loads through microbial processes and sedimentation. However, conditions during flooding can also lead to phosphorus (P) release from pools that are not normally bioavailable. Disturbance events like FT can also lead to changes in bioavailable P due to microbial cell lysis. This study investigates differences in P chemistry and flux during flooding from intact soil cores that have undergone a FT cycle compared to soils that have not undergone freezing. Floodplain soils were collected from four sites along the Wabash and Tippecanoe Rivers in Indiana. We hypothesized that (i) the primary pools of P within the soil would change with freezing (ii) and flooding; (iii) frozen treatment cores would release more P during flood incubations than unfrozen control cores; and (iv) processes controlling P release during flood incubations would change after FT due to changes in the primary pools of P in the soil cores.
On average, soil cores that underwent FT released greater amounts of P than unfrozen cores over the course of the 3-week experimental flood incubation. Phosphorus release in both unfrozen control and FT treatment cores during flooding was explained in part by soil extractable Al and Fe and redox status; however, P release was influenced by soil Ca-P in the FT cores to a greater extent than unfrozen cores. Phosphorus release in FT cores occurred faster than in control cores with overlying water concentrations peaking 2 weeks after onset of flooding, followed by lower concentrations at 3 weeks. Whereas control cores had some release and uptake early on but then released P throughout the 3-week incubation—supporting the hypothesis that drivers of P release were different after FT. Interactive effects of FT and flooding suggest that concentration gradients between soil pore water and overlying surface water could have enhanced dissolution of the Ca-P pool, highlighting the importance of floodwater chemistry to P dynamics following FT. This study provides an important link between observed winter floodplain P loss and potential drivers of release and retention, which is critical to informing floodplain restoration design and management through all seasons.