Iron (Fe) is identified to play a crucial role in driving phosphorus (P) mobilization in sediment. However, the role of manganese (Mn) on P mobilization is poorly understood. Herein, P mobilization mechanism influenced by Mn was investigated both in filed investigation and incubation experiment. In field investigation, significant positive correlations (r²>0.63, p<0.01) and synchronize distributions between DGT-P(Ⅴ) and DGT-Mn(Ⅱ) in sediment profile suggest that P cycle is couple with Mn cycle. This finding is further confirmed by similar results of DGT-P(Ⅴ) and DGT-Mn(Ⅱ) in sediment profile from incubation experiments. Concentration of DGT-Fe(Ⅱ) is low (mean<0.18 mg L^-1) in top 6 cm sediment and high (mean>0.36 mg L^-1) in the depth of -6 to -9.4 cm sediment while DGT-S(-Ⅱ) have opposite concentration pattern, indicating that reduced Fe(Ⅱ) in top 6 cm sediment were co-deposited with S(-Ⅱ). The possible mechanism of Mn affected P mobilization in sediments, evidenced from X-ray photoelectron spectroscopy (XPS) and Mn XANES spectra results, is P was mainly trapped by Mn(Ⅲ) to form P-bearing Mn oxides then coated on sediment nodule surface under aerobic conditions, and those P-bearing Mn oxides were preferentially reduced under hypoxic environment led P cycle couple with Mn cycle. Fe XANES spectra and ⁵⁷Fe Mössbauer spectra demonstrated that goethite and pyrite are the dominate Fe phases (range from 60 to 95%) in sediments, this leads weak influence of Fe on P cycle. Overall, this work indicate that Mn(Ⅲ) oxides strongly influence fate and cycle of P, and provides new insights into the coupled cycles of Mn and P in reducing sediments.

Phosphorus mobilization, Manganese, Iron, Eutrophication, Freshwater sediments.