Predicting plant available nitrogen in land-applied biosolids

The rate at which biosolids (municipal sewage sludge) may be applied to land is dependent on factors including concentrations of metals, pathogens, toxic organic compounds, and nutrients. Where other properties are not limiting, land application rates are often based on matching crop TV needs with the plant available IV (PAN). The objectives of this study were to quantify biosolids PAN under field conditions and to propose methods including computer simulation to estimate biosolids PAN in a land application program. Six biosolids were evaluated over a 2-yr period. Laboratory incubations were used to obtain decomposition kinetics, Field studies provided a relationship between inorganic fertilizer N rate and sorghum sudangrass [Sorghum bicolor (L.) Moench] tissue N concentration, which was used to determine biosolids PAN in a Captina silt loam soil (fine-silty, siliceous, mesic Typic Fragiudult), Biosolids PAN released during the field experiment was linearly related to biosolids C/N ratio, organic N, or total N, Computer model predictions of PAN in the field were also linearly related to field estimates of biosolids PAN. Decay series (first and second year N mineralization percentages) obtained using the computer model, average biosolids decomposition kinetics, and average application site weather were very similar to decay series obtained using the computer model, actual weather, and kinetic data. Either decay series and routine analytical data for biosolids are proposed to estimate PAN for a given situation. Use of the computer model and weather data makes the approach site-specific, while analytical date for a specific biosolids makes the approach biosolids-specific.