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The Fate of Phosphorus from Long Term Biosolids Application

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Phosphorus is the limiting nutrient in the eutrophication of many lakes and rivers. In wastewater treatment facilities, phosphorus is generally concentrated in the biosolids, which are often land applied. Therefore, the fate of phosphorus from long-term biosolids application to agricultural land is of interest. The primary goal of this study is to determine the fate of phosphorus after many years of biosolids application to nearby farmland by the City of Sioux Falls' Wastewater Treatment Plant. Two sites were selected that have had 10 years of surface application of biosolids. One site was a grass pasture land, the other site was an alfalfa field. Soil samples were collected starting at the summit of the site and continuing down the slope to the toe, with samples taken at various landscape positions. This pattern of sampling allowed for the tracking of phosphorus down the slope. At each landscape position, samples were collected in 6-inch increments to a depth of 48 inches, which allowed for the tracking of phosphorus down through the soil depth.

The soil samples were analyzed for the following parameters; total phosphorus, organic phosphorus, inorganic phosphorus, plant available phosphorus (Olsen Phosphorus), phosphorus buffering, free Fe and Mn, inorganic carbon, bulk density and pH. Results of the soil analysis show little movement of total phosphorus down the slope at either site; however, the results indicate that Olsen phosphorus was accumulating down slope. Results also show an increase in Olsen phosphorus into the surface at a depth of 12 inches at both sites. A mass balance of the total phosphorus on the alfalfa site was able to account for 99% of the applied phosphorus, which would indicate that little phosphorus has left the site. Based on the results, it was concluded that current application practices preformed by the City of Sioux Falls are sufficient to prevent the movement of total phosphorus off site.

Document Type: Research Article


Publication date: 2004-01-01

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