Carbon and Nitrogen Mineralization Kinetics in Soil Previously Amended with Sewage Sludge Michael Boyle* and E. A. Paul ABSTRACT Microbial mineralization rates of organic carbon (C) and nitrogen (N) were determined on the same sludge-amended and nonamended soil samples. The purpose of this integrated approach was to high- light the long-term dynamics of N release with C stabilization in sludge-affected soil. Three application rates of digested municipal sludge, check, 45 Mg ha~' and 180 Mg ha~', were incorporated into field plots annually for 8 years, with no addition during the subse- quent 3 years. Barley was grown on the site each spring of the 11 years. In an 87-week laboratory incubation experiment conducted on soil samples collected 3 years after the last sludge addition, N and C mineralization rates (*„, k c ) increased with sludge application rate. Soil nitrogen mineralization potentials (N 0 ) increased with sludge application, unlike carbon mineralization potentials (C 0 ) which did not correlate with sludge application. The C/N ratio of the miner- alized organic matter decreased with sludge application rate. Three years after field incorporation of sludge, decomposition of the or- ganic fraction can be described as a set of two first-order rate re- actions. One fraction is characterized by a large stable element (high /V 0 , C 0 and low &„, k e ); the second fraction consists of a smaller labile Michael Boyle, IPH, Harvard School of Public Health, Boston, MA 02115; and E.A. Paul, Dept. of Crop & Soil Sciences, Michigan State Univ., East Lansing, MI 48824. Contribution from the Dept. of Plant & Soil Biology, Univ. of California, Berkeley. Received 20 Jan. 1988. "Corresponding author. Published in Soil Sci. Soc. Am. J. 53:99-103 (1989). portion which is characterized by low TV,,, C 0 and high *„, k c values. The microbial biomass decreased to less than half of its original amount after 20 weeks of incubation in all soil treatments. T HE FORMATION of soil organic matter (SOM) is a reversible reaction. Additions of organic waste, such as municipal sludge, has been proposed as one method of maintaining levels of organic matter in ag- ricultural as well as forested and disturbed lands. Data on N mineralization in soils that had recent applica- tions of sludge (Epstein et al., 1978; Lindemann and Cardinas, 1984), as well as studies on land previously amended with sludge (Stark and Clapp, 1980; Griffin and Laine, 1983), have contributed to the understand- ing of soil N behavior with sludge additions. However there remains a need for models that predict sludge N release especially after the termination of applica- tion (Page et al., 1983). Because N turnover in soil is highly dependent on C transformations and microbial biomass (McGill et al., 1981), an integrated approach was taken in this study. A N mineralization indicator (accumulation of NO 3 ), a C decomposition parameter (CO 2  evolution), and an estimate of microbial bio- Published January, 1989