Nutrient Mobility from Biosolids Land Application Sites

Three types of biosolids (lime-stabilized, aerobically digested, and anaerobically digested biosolids) were applied on 0.13-ha test plots on disturbed rangelands in Western Utah at rates of up to twenty times (20X) the estimated N-based agronomic rate. Soil samples at depths up to 1.5 m were collected and analyzed for nitrogen, phosphorus, regulated metals, pH, and electrical conductivity for up to two years after biosolids application. NH4-N at the soil surface (0.2 m) was primarily lost through ammonia volatilization and nitrification. This observation was consistent with reported increases in nitrate (NO3-N) concentrations found within the soil surface on the biosolids-amended sites. A nitrogen mass balance on the surface soil control volume indicated that the nitrogen residual field measurements were significantly higher than the nitrogen level estimated by accounting for nitrogen inputs (biosolids) and outputs (vegetative yield, nitrogen volatilization and nitrate leaching). Biosolids land application led to increases in vegetative growth and dry matter yield when compared to vegetation grown on control plots. Based on the Root Zone Water Quality Model (RZWQM), the model predicted NH4 and NO3 storage values at biosolids-amended sites were significantly different from the field data, which suggests that the model default and limited measured values were inappropriate for a non-irrigated rangeland landscape. The majority of total P and plant available P accumulation was found to occur primarily within the soil surface (0.2 m). Phosphorus soil residual measurements were higher than phosphorus accumulation based on a phosphorus mass balance at soil surface. The phosphorus leachability to ground water at the biosolids-amended treatment sites was low based on the molar ratio of ([P]/([Al]+[Fe])) and the potential formation of calcium phosphate (Ca3(PO2)2). Aerobically digested biosolids appeared to be the optimal biosolids type with regard to minimizing the adverse environmental effects of phosphorus based on the Phosphorus Site Index (PSI). Regulated metal concentrations (As, Cd, Cu, Pb, Mo, Ni, Se, and Zn) were well below the cumulative pollutant loading limits for biosolids-amended soils. Finally, nutrients as well as regulated heavy metals associated with biosolids land application to disturbed rangelands do not pose any significant threat to the environment