Decomposition and Plant-Available Nitrogen in Biosolids: Laboratory Studies, Field Studies, and Computer Simulation

This research combines laboratory and field studies with computer simulation to characterize the amount of plant-available nitrogen (PAN) released when municipal biosolids are landapplied to agronomic crops. In the laboratory studies, biosolids were incubated in or on soil from the land application sites. Mean biosolids total C, organic N, and C to N ratio were 292 g kg-1, 41.7 g kg-1, and 7.5, respectively. Based on CO2 evolution at 25°C and optimum soil moisture, 27 of the 37 biosolids–soil combinations had two decomposition phases. The mean rapid and slow fraction rate constants were 0.021 and 0.0015 d-1, respectively, and the rapid fraction contained 23% of the total C assuming sequential decomposition. Where only one decomposition phase existed, the mean first order rate constant was 0.0046 d-1. The mean rate constant for biosolids stored in lagoons for an extended time was 0.00097 d-1. The only treatment process that was related to biosolids treatment was stabilization by storage in a lagoon. Biosolids addition rates (dry basis) ranged from 1.3 to 33.8 Mg ha-1 with a mean value of 10.6 Mg ha-1. A relationship between fertilizer N rate and crop response was used to estimate observed PAN at each site. Mean observed PAN during the growing season was 18.9 kg N Mg-1 or 37% of the biosolids total N. Observed PAN was linearly related to biosolids total N. Predicted PAN using the computer model Decomposition, actual growingseason weather, actual analytical data, and laboratory decomposition kinetics compared well with observed PAN. The mean computer model prediction of growing-season PAN was 19.2 kg N Mg-1 and the slope of the regression between predicted and observed PAN was not significantly different from unity. Predicted PAN obtained using mean decomposition kinetics was related to predicted PAN using actual decomposition kinetics suggesting that mean rate constants, actual weather, and actual analytical data could be used in estimation of PAN. There was a linear relationship between predicted N mineralization for the growing season and for the first year. For this study, the mean values for the growing season and year were 27 and 37% of the organic N, respectively.