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New insights into phosphorus mobilisation from sulphur-rich sediments: time-dependent effects of salinisation.

van Diggelen JM, Lamers LP, van Dijk G, Schaafsma MJ, Roelofs JG, Smolders AJ - PLoS ONE (2014)

Bottom Line: Although several studies have addressed the effects of salinisation, interactions between salinity changes and nutrient cycling in freshwater systems are not fully understood.Although salinisation was shown to lower short-term P mobilisation as a result of increased calcium concentrations, it may increase long-term P mobilisation by the interactions between sulphate reduction and oxygen availability.Our study showed time-dependent responses of sediment P mobilisation in relation to salinity, suggesting that sulphur plays an important role in the release of P from FeSx-rich sediments, its biogeochemical effect depending on the availability of Fe(2+) and O2.

View Article: PubMed Central - PubMed

Affiliation: B-WARE Research Centre, Radboud University Nijmegen, Mercator 3, Nijmegen, The Netherlands; Institute for Water and Wetland Research, Department of Aquatic Ecology and Environmental Biology, Radboud University Nijmegen, Nijmegen, The Netherlands.

ABSTRACT
Internal phosphorus (P) mobilisation from aquatic sediments is an important process adding to eutrophication problems in wetlands. Salinisation, a fast growing global problem, is thought to affect P behaviour. Although several studies have addressed the effects of salinisation, interactions between salinity changes and nutrient cycling in freshwater systems are not fully understood. To tackle eutrophication, a clear understanding of the interacting effects of sediment characteristics and surface water quality is vital. In the present study, P release from two eutrophic sediments, both characterized by high pore water P and very low pore water iron (Fe(2+)) concentrations, was studied in a long-term aquarium experiment, using three salinity levels. Sediment P release was expected to be mainly driven by diffusion, due to the eutrophic conditions and low iron availability. Unexpectedly, this only seemed to be the driving mechanism in the short term (0-10 weeks). In the long term (>80 weeks), P mobilisation was absent in most treatments. This can most likely be explained by the oxidation of the sediment-water interface where Fe(2+) immobilises P, even though it is commonly assumed that free Fe(2+) concentrations need to be higher for this. Therefore, a controlling mechanism is suggested in which the partial oxidation of iron-sulphides in the sediment plays a key role, releasing extra Fe(2+) at the sediment-water interface. Although salinisation was shown to lower short-term P mobilisation as a result of increased calcium concentrations, it may increase long-term P mobilisation by the interactions between sulphate reduction and oxygen availability. Our study showed time-dependent responses of sediment P mobilisation in relation to salinity, suggesting that sulphur plays an important role in the release of P from FeSx-rich sediments, its biogeochemical effect depending on the availability of Fe(2+) and O2.

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Schematic overview of the proposed mechanism, showing key processes in the upper millimetres of the S-rich, peat sediments involved in P mobilisation.Salinisation leads to an increased SO42- influx, affecting Fe diffusion to the sediment surface, enabling increased P mobilisation in the longer term.
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pone-0111106-g006: Schematic overview of the proposed mechanism, showing key processes in the upper millimetres of the S-rich, peat sediments involved in P mobilisation.Salinisation leads to an increased SO42- influx, affecting Fe diffusion to the sediment surface, enabling increased P mobilisation in the longer term.

Mentions: Most likely Fe redox cycling played a dominant role in the absence of P mobilisation, as was also indicated by the strongly increased P release under anaerobic conditions compared to aerobic conditions (Fig. 5). It has been demonstrated that diffusive P release should be prevented under aerobic conditions if pore water Fe:P ratios are relatively high (at least >1) [19], [38], [39]. In our sediments, however, pore water Fe:P ratios were very unfavourable. Nevertheless, oxidation processes might be able to mobilise Fe2+ from FeSx at a spatial micro-scale in the sediment surface at relatively low O2 levels [40], catalysed by S oxidising microbes [41]. Our O2 profiles showed that O2 was available in the surface water and in the top millimetres of the sediment. The observed high S mobilisation rates in the low salinity treatment, where no S was added, indeed showed that SO42- is being mobilised from the sediment by the oxidation of FeSx. Simultaneously, Fe2+ thus becomes available to be oxidised [40], and is able to sequester dissolved P. So the intrusion of O2 in reduced sediments may mobilise S bound Fe at a millimetre spatial scale, providing dissolved Fe2+ for the formation of ferric Fe(OH)x at the sediment surface (Fig. 6). This mechanism may very well explain the unexpected lack of P release from the sediments in the long term under aerobic conditions.


New insights into phosphorus mobilisation from sulphur-rich sediments: time-dependent effects of salinisation.

van Diggelen JM, Lamers LP, van Dijk G, Schaafsma MJ, Roelofs JG, Smolders AJ - PLoS ONE (2014)

Schematic overview of the proposed mechanism, showing key processes in the upper millimetres of the S-rich, peat sediments involved in P mobilisation.Salinisation leads to an increased SO42- influx, affecting Fe diffusion to the sediment surface, enabling increased P mobilisation in the longer term.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4219700&req=5

pone-0111106-g006: Schematic overview of the proposed mechanism, showing key processes in the upper millimetres of the S-rich, peat sediments involved in P mobilisation.Salinisation leads to an increased SO42- influx, affecting Fe diffusion to the sediment surface, enabling increased P mobilisation in the longer term.
Mentions: Most likely Fe redox cycling played a dominant role in the absence of P mobilisation, as was also indicated by the strongly increased P release under anaerobic conditions compared to aerobic conditions (Fig. 5). It has been demonstrated that diffusive P release should be prevented under aerobic conditions if pore water Fe:P ratios are relatively high (at least >1) [19], [38], [39]. In our sediments, however, pore water Fe:P ratios were very unfavourable. Nevertheless, oxidation processes might be able to mobilise Fe2+ from FeSx at a spatial micro-scale in the sediment surface at relatively low O2 levels [40], catalysed by S oxidising microbes [41]. Our O2 profiles showed that O2 was available in the surface water and in the top millimetres of the sediment. The observed high S mobilisation rates in the low salinity treatment, where no S was added, indeed showed that SO42- is being mobilised from the sediment by the oxidation of FeSx. Simultaneously, Fe2+ thus becomes available to be oxidised [40], and is able to sequester dissolved P. So the intrusion of O2 in reduced sediments may mobilise S bound Fe at a millimetre spatial scale, providing dissolved Fe2+ for the formation of ferric Fe(OH)x at the sediment surface (Fig. 6). This mechanism may very well explain the unexpected lack of P release from the sediments in the long term under aerobic conditions.

Bottom Line: Although several studies have addressed the effects of salinisation, interactions between salinity changes and nutrient cycling in freshwater systems are not fully understood.Although salinisation was shown to lower short-term P mobilisation as a result of increased calcium concentrations, it may increase long-term P mobilisation by the interactions between sulphate reduction and oxygen availability.Our study showed time-dependent responses of sediment P mobilisation in relation to salinity, suggesting that sulphur plays an important role in the release of P from FeSx-rich sediments, its biogeochemical effect depending on the availability of Fe(2+) and O2.

View Article: PubMed Central - PubMed

Affiliation: B-WARE Research Centre, Radboud University Nijmegen, Mercator 3, Nijmegen, The Netherlands; Institute for Water and Wetland Research, Department of Aquatic Ecology and Environmental Biology, Radboud University Nijmegen, Nijmegen, The Netherlands.

ABSTRACT
Internal phosphorus (P) mobilisation from aquatic sediments is an important process adding to eutrophication problems in wetlands. Salinisation, a fast growing global problem, is thought to affect P behaviour. Although several studies have addressed the effects of salinisation, interactions between salinity changes and nutrient cycling in freshwater systems are not fully understood. To tackle eutrophication, a clear understanding of the interacting effects of sediment characteristics and surface water quality is vital. In the present study, P release from two eutrophic sediments, both characterized by high pore water P and very low pore water iron (Fe(2+)) concentrations, was studied in a long-term aquarium experiment, using three salinity levels. Sediment P release was expected to be mainly driven by diffusion, due to the eutrophic conditions and low iron availability. Unexpectedly, this only seemed to be the driving mechanism in the short term (0-10 weeks). In the long term (>80 weeks), P mobilisation was absent in most treatments. This can most likely be explained by the oxidation of the sediment-water interface where Fe(2+) immobilises P, even though it is commonly assumed that free Fe(2+) concentrations need to be higher for this. Therefore, a controlling mechanism is suggested in which the partial oxidation of iron-sulphides in the sediment plays a key role, releasing extra Fe(2+) at the sediment-water interface. Although salinisation was shown to lower short-term P mobilisation as a result of increased calcium concentrations, it may increase long-term P mobilisation by the interactions between sulphate reduction and oxygen availability. Our study showed time-dependent responses of sediment P mobilisation in relation to salinity, suggesting that sulphur plays an important role in the release of P from FeSx-rich sediments, its biogeochemical effect depending on the availability of Fe(2+) and O2.

Show MeSH
Related in: MedlinePlus