Nitrogen, tillage, and residue management systems influence the ability of microorganisms to mineralize plant residues in soil. The objective of this research was to investigate the seasonal changes in autoclave extractable-N (AN) as influenced by different N rates, tillage practices and residue management systems. A field study under a continuous corn rotation was initiated in 1980, with factorial combinations of 2 or 20 g o N/m, roto-tillage or no-tillage, and residue (corn stover) returned or removed treatments. Soil water, AN, and inorganic N were measured at regular intervals over a three-year period (1984 to 1986). AN, as a soil N mineralization index, was shown to be sensitive to changes in the nitrogen, tillage, and residue management systems. Correlations between corn yields and the N mineralization index suggest that AN was sampling a biologically-active N pool. © 1990, Taylor & Francis Group, LLC. All rights reserved.