Phosphorus Fate and Transport in Biosolids Amended Soils: III. Phosphorus Forms in Soils and Losses in Runoff from Fescue Pastures

Land application programs for biosolids are designed to provide adequate nutrients for plant growth using biosolids that meet regulatory criteria for trace elements, organic toxics, pathogens, and vector attraction. The most commonly used agronomic rate for biosolids is based on application rates that meet the nitrogen (N) requirements of the crop to be grown (USEPA, 1994). Nitrogenbased biosolids applications, as for animal manures and other organic by-products, add more P to soils than is removed in the harvested portion of most agronomic crops (Kelling et al., 1977; Pierzynski, 1994; Petersen et al., 1994; Maguire et al., 2000a; Maguire et al., 2000b). As a result, with time, soil P concentrations increase to values above those needed for crop production, which may increase the risk of nonpoint P pollution of surface waters by erosion and runoff and of ground waters by leaching. In response to these environmental concerns, nutrient management laws and regulations that attempt to reduce P inputs to surface waters have recently been passed in Delaware (1999), Maryland (1998), and Virginia (1999) (Sims, 2000). These laws will employ soil test P concentrations to identify sites where P applications must be restricted or discontinued. For example, Maryland's Water Quality Improvement Act (1998) dictates that application of P in fertilizers, manures, and biosolids be based upon soil test P levels, site conditions (topography, drainage, proximity to surface waters), and P source management factors (application timing and method, relative availability of P in the organic P source) (Coale, et al., 2002). The Delaware Nutrient Management Act of 1999 states that total P application rates, from all P sources, to soils testing “high” in P must not exceed the “three year crop P removal rate” (Sims, 1999); thus, “high” P soils in DE will usually only receive P applications once every three years. The actions in Maryland are of considerable significance to municipalities in the region that rely upon land application for the disposal or beneficial use of biosolids because a high percentage of soils in these states currently test high in P (Sims et al., 2000).Many factors can affect the potential for P losses to water when biosolids are applied to crop land. Examples include the effects of wastewater treatment plant (WWTP) process on P solubility in biosolids, soil type, the method and rate of biosolids application (e.g., incorporation vs. surface), and the nature of the cropping system. Most research on the fate and transport of biosolids P has been conducted in laboratory studies or in controlled, small-scale runoff and leaching column studies where biosolids were incorporated into soils. Consequently, there is little field research available on P losses in runoff where biosolids are applied in accordance with current N-based management recommendations. There is also little data available on the loss of P in runoff from settings where biosolids are not incorporated into soils (e.g., no-tillage, pastures). Our objective was to determine, using a rainfall simulation study conducted under field conditions, the effects of biosolids applications to pastures on the forms of P in biosolids-amended soils and P losses in runoff.