Are decreasing soil sulphate concentrations really causing increased iron mobility?

Abstract

In the last couple of decades a strong increase in surface water iron (Fe) concentrations has been observed in the Northern hemisphere. Since Fe in surface waters derives from the soil in the catchment it is important to understand what biogeochemical processes are governing this increase. It has been proposed that reduced atmospheric sulphate deposition increases Fe concentrations in soil solution. Reduction of Fe(III) and sulphate (SO42-) has produces FeS which precipitate from solution, which would decrease Fe in solution. In this study this hypothesis was tested through a microcosm lab experiment where two different concentrations of SO42- were added to soil slurries and sampled over a period of 83 days. Fe, SO42-, dissolved organic matter (DOC) and water chemistry were measured during this period in both oxic and anoxic treatments. Opposite to expectations Fe concentrations were higher at high SO42- concentrations, which is in part because no SO42- reduction occurred. In the absence of FeS precipitation, difference in pH was likely the biggest factor between S treatments, with Fe being more soluble at low pH. DOC on the other hand shows a negative response to SO42- concentrations and increased significantly more in anoxic treatments. Interestingly fungal growth in oxic treatments caused a sharp increase in Fe concentrations as well. This might have been caused by partial anoxia in those treatments or potentially due to Fenton reactions induced by the fungi. All in all this study does not offer support for the reduced atmospheric sulphate deposition and rather suggest that low pH caused by added SO42- was the main factor governing Fe concentrations in soil water.

Popular Abstract

In the last couple of decades a strong increase in surface water colour has been observed in the Northern hemisphere. This is in part due to increasing organic material export from soils into surface waters and in part due to an increase of iron concentrations in surface waters. While it is understood that iron release from soils is governed by different biological, chemical and geological factors it is unclear what changes caused the increased release in recent decades. One of several theories is that a decline in sulphur release from industrial sources could have caused the increase in iron release from soils. Iron (Fe) and sulphur (S) in soil are both redox active, meaning they can act as either electron acceptors or donors, depending on redox conditions in the soil. At reducing conditions in soil, iron can act as an electron acceptor receiving electrons from other molecules and getting reduced from the natural ferric (Fe3+) to the more water soluble ferrous (Fe2+) form. Sulphate (SO42-), the sulphur form usually found in soil water, on the other hand, gets reduced to sulphide (S2-). When both Fe2+ and S2- are both present in soil they can form a solid precipitate called iron sulphide (FeS). Therefore we assume that under reducing conditions and at higher S concentrations, Fe would get removed from soil solution and therefore be less likely to get flushed from the soil.

To test this hypothesis, a lab experiment simulating natural flooding conditions was performed using glass bottles filled with soil and water. Two treatments using a high and a low concentration of sulphate were created. Each treatment was run at oxic and also at anoxic conditions, to create a more reducing environment where Fe and SO42- are likely to get reduced. Concentrations of iron in soil water were then measured on 5 occasions during 83 days, to see if there is any difference in Fe solubility between the two sulphate treatments.

Surprisingly Fe concentrations in soil solution were higher in the high sulphate treatment, which goes against our hypothesis that reduced sulphate release is responsible for increased iron concentrations in soil water. Since SO42- lowers pH it is likely that lower pH in the high sulphate treatments was responsible for the increased amount of Fe in solution, since Fe is more soluble at low pH. Growth of fungi in the oxic treatments also caused an increase in soil water iron concentrations, however it is unclear whether that is due to anoxic conditions in the soil of those treatments or due to an active reduction of iron by the fungi.

In conclusion this study does not confirm the decreased sulphur deposition theory and suggests that other more local factors might be behind the increase in iron release from soil.

Supervisor: Emma Kritzberg
Master´s Degree Project 45 credits 2016
Department of Biology, Lund University

• author: Škerlep, Martin
• supervisor: Emma Kritzberg
• organization: Degree Projects in Biology
• course: BION01 20161
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8902649
• date added to LUP: 2017-02-07 11:09:59
• date last changed: 2017-02-07 11:09:59