Fertile speculation erodes faith in earthy wisdom

Posted on December 28, 2013

Rainfall events erode agricultural soils and transport them into local waterways, where the release of nutrients (nitrogen, phosphorus) from the soil can lead to eutrophic conditions: high levels of primary production and the development of algal blooms. To avoid eutrophication, it’s necessary to prevent soil from entering water bodies – e.g. through the use of reduced tillage, cover crops, buffer strips, riparian zones, settling ponds and wetlands. Although this is the conventional wisdom linking soil erosion to aquatic eutrophication, recent studies suggest that the relationship may be more complicated, and that in some circumstances, soil erosion may actually counteract eutrophication. The new findings concern the processes that take place after eroded soil has settled on the bed of the receiving water body, especially those involving the reduction of iron oxides. It turns out that the fate of soil-borne phosphorus depends crucially on whether iron reduction is carried out microbially or chemically. Iron that’s reduced by microbes in the sediment is highly soluble and diffuses up to the sediment surface, oxidizes, and forms a layer that captures phosphorus that is also diffusing up through the sediment. As a result, little phosphorus reaches the main water body. In contrast, in the chemical reduction process, sulphides reduce iron oxides and trap iron below the surface in the form of insoluble compounds. In this case no oxidized layer is produced at the sediment surface and phosphorus can escape into the water. Because eroded soil is rich in iron oxides, which inhibit sulphide production, erosion may increase the ability of the sediment to retain phosphorus. So the overall effect on eutrophication may depend on the balance between available phosphorus and iron oxides in the transported soil. More work is needed to test this idea.

Reference: Ekholm, P. & Lehtoranta, J. 2012. Does control of soil erosion inhibit aquatic eutrophication?
Journal of Environmental Management 93, 140-146. http://www.jlakes.org/web/1-s2.0-S0301479711003409-main.pdf