Decomposition of Biosolids in a Disposal Site Soil

The objectives of this study were to determine if residual biosolids in a disposal site soil altered fresh biosolid decomposition and characterize the decomposition behavior of the residual biosolids in that soil. This information is requisite to predicting the impact of residual biosolids on release of N via net N mineralization from fresh and residual biosolids at the disposal site. Aerobically digested municipal sewage sludge from Laredo, TX, was incubated in LaGloria silly clay loam soil (coarse-silly, mixed, hyperthermic fluventic Ustochrepts) from three locations within the disposal area. Sites I and 2 had been amended with biosolids at disposal rates of 647 and 418 Mg ha -1 , respectively, over a 7-yr period while Site 3 was the control. Carbon dioxide evolution was monitored for 62 d and decomposition of fresh biosolid was evaluated using first-order kinetics for simultaneous and sequential decomposition models. Decomposition of the active and total residual biomass pools was also assessed. Decomposition of the fresh biosolid was characterized as a two phase (rapid and slow fractions) process. The rate constants for these phases and percentages of biosolid in the rapid fraction were independent of previous additions for this biosolid/soil combination. The active pool or the residual biosolid that was most labile was from 6 to 10% of the total residual biosolid C pool. First-order rate constants for the total pool of residual biosolids approached those that have been used for more labile portions of soil organic matter.