The Relation of Biosolids Source and Process Type to Phosphorus Availability in Biosolids-Amended Soils

Phosphorus (P) is an essential nutrient for plant growth, and is often added to croplands as a fertilizer. Unfortunately, over the years, there has been an excess of phosphorus applied to agricultural lands in the Chesapeake Bay watershed. This is due to applications of biosolids and manure (primarily poultry litter) at rates needed to maintain the required levels of plant available nitrogen (PAN), which leads to excessive amounts of P. Significant amounts of biosolids from wastewater treatment plants in the Chesapeake Bay region are land applied. The quantity of P in the biosolids applied is dependent upon the processes at the individual wastewater treatment plant as can be seen below.Because this excess phosphorus is often applied in excess of nutrient requirements, it has the potential to run off into nearby waterways, and P can cause eutrophication of surface waters of lakes, streams, and reservoirs at concentrations as low as 0.02 mg P/L. The quantity of nutrients entering the watershed in this nonpoint source form, and the transformations of P species when soils are amended with biosolids, should be well understood. This will allow the amount of bioavailable P entering waterways to be more carefully controlled.Previous studies (reviewed in Dentel et al., 2000) have shown that the means used for P removal in wastewater treatment can have a significant effect on P mobility in the resulting biosolids. Wastewater treatment plants that utilize chemical precipitation for P removal can produce biosolids with relatively immobile P species. However, treatment facilities that use biological P removal (BPR) produce biosolids from which the P is readily released.When the biosolids are combined with soil, however, the question of P mobility becomes more complex. If the soil has a high affinity for P, for example, mobile P from the biosolids can still be retained. Interactions between the soil and the biosolids can also alter P mobility, e.g. when the pH of limed biosolids is decreased when incorporated into soil. Previous research has not examined this question in a sufficiently quantitative manner, such as by attempting mass balances on the significant P species when incubating biosolids in the presence of soils.In many areas it is becoming necessary to implement improved practices for land application of biosolids in order to avoid excessive loading of P. In developing these practices–such as the acceptable P application rate–it is important to consider the biosolids source and process type in assessing the likely P release level. However, interactions with the soil matrix should also be understood and quantified if a reasonable and predictive approach is to be possible. It was the intent of this research to incorporate all of these considerations in its experiments.