Biosolid nitrogen (N) ammonification, followed by nitrification in soil, produces nitrate (NO3-), which is not only a plant nutrient, but also a contaminant for ground water. Determining the most relevant factors influencing mineralization will help to manage N in biosolid-treated soils. Biosolid application rate, biosolid carbon (C):N ratio, biosolid organic N content, biosolid type, soil organic N content, soil pH, temperature, and time were compared among 12 published studies. Biosolid application rate, biosolid C:N ratio, and temperature significantly affected the mineralization rate and accounted up to 87% of the final model variability (R2 = 79.1). Although the partial coefficients for soil pH and a dummy variable for biosolid were significant in the final model, their total contribution to the sum of partial coefficients was low (13%). There is insufficient evidence to conclude that soil organic N content and time contribute to biosolid N mineralization variability.