Biosolids and inorganic fertilizers have both been a boon for the agricultural industry because they are able to bump up crop output and increase food production. But nutrient runoff from farmlands treated with fertilizers can have a devastating effect on surface water in nearby areas because of the risk of algal blooms.
Researchers from the University of Florida attempted to compare which type of fertilizer have a greater propensity for nutrient runoff and thereby have a more negative effect on bodies of water. They have discovered that higher levels of nutrients from inorganic fertilizers caused higher levels of pollutants compared to organic biosolid fertilizers.
The study was led by Maria Silveira, a professor in the UF/IFAS department of soil and water sciences. They determined the nitrogen and phosphorus content from collected runoff coming from a common pasture grassland. Both biosolids and chemical fertilizers were applied to the grass which the researchers then proceeded to analyze for nutrient runoff content.
“Although we have seen that public perception remains uncertain about using biosolids to amend soils, this study reveals a positive attribute: biosolids-amended soils generally released the same amount of nitrogen and phosphorus as untreated soils,” Silveira said.
To provide a granular focus on their study, they tested 7 types of biosolids and two kinds of inorganic fertilizers that were commercially available. They extracted the biosolids from different treatment facilities throughout Florida. Samples were placed in a runoff box developed by the UF/IFAS Range Cattle Research and Education Center (REC) in Ona.
To provide better control of the environmental conditions, the study was conducted indoors. The boxes were each packed with around 20 pounds of soil before being covered with transplanted bahiagrass, a type of grass which is typically used for grazing livestock in the state. The REC was responsible for providing the needed soil and bahiagrass for the study. Only a few studies have ever evaluated the amount of phosphorus loss when fertilizers are applied on an established vegetative cover like bahiagrass.
“Most biosolids are spread directly onto pastureland in Florida. Establishing a grass cover for the study was intended to better represent the soil cover conditions where biosolids application typically occurs,” Silveira said.
A uniform application of both the biosolids and inorganic fertilizer was required for the study. A simulated rainfall event was introduced 48 hours after the biosolids or fertilizers were applied to a runoff box. The rainfall was set by the researchers for one hour which was consistent with the 25-year average in central Florida. Three simulations were performed on alternating days from late March to early April and each rainfall were stopped after 30 minutes of runoff was generated for each box.
The results showed that approximately 38% of phosphorus and 46% of nitrogen were found in the runoff that came from the inorganic fertilizer application. The runoff from the biosolids-amended soils accounted for only 3% of phosphorus and 6% of nitrogen.
She believes that further research in the field with biosolids and commercial fertilizer is necessary to deepen our understanding of how nitrogen and phosphorus behave in sandy soils.
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