The impacts of crop rotations and N fertilization on the activities of 12 enzymes involved in C, N, and P cycling was studied in soils of two long-term cropping systems at the Northeast Research Center (NERC) in Nashua and the Clarion-Webster Research Center (CWRC) in Kanawha, established 1979 and 1954, respectively, in Iowa. The activities of glycosidases (alpha- and beta-glycosidases, alpha- and beta-galactosidases), amidohydrolases (amidase, arylamidase, L-glutaminase, L-asparaginase, L-aspartase), and phosphatases (acid and alkaline phosphatases, and phosphodiesterase) were assayed in soils sampled in 1996 and 1997 from four replicated cropping systems taken in corn, soybean, oats, or meadow (alfalfa) that received 0 or 180 kg N ha-1 before corn.;Crop rotations significantly affected enzymes activities in soils at the NERC site in both years, and glycosidases only in soils at the CWRC site. The effect of N fertilization was inconsistent. Greatest activity values were in multicropping systems in meadow or oats, and the least in continuous corn or soybean systems. Enzyme activities were significantly correlated with C mic, and arylamidase and amidohydrolases significantly correlated with Nmic, in both years and sites. The amounts of N mineralized during 24 weeks of incubation at 30°C were significantly correlated with the activities of arylamidase and amidohydrolases at both sites. The activity of arylamidase was also significantly correlated with the amounts of N mineralized in soils from six regions of North Central U.S.;An alkaline hydrolysis method for determining the total N potentially hydrolyzable in soils was evaluated. It involves determination of the NH 4+-N produced by direct steam distillation of 1 g field-moist soil and 1M KOH, NaOH, or LiOH or with PO4-B3O 7 buffer (pH 11.8) successively every 5 min for 40 min. Calculated total hydrolyzable N (Nmax or No) values differed among soils, ranging from 401 to 1667 mg kg-1 soil and accounted for 12--56% of organic N in the soils. The Nmax or No values obtained with KOH or NaOH were significantly correlated with the values obtained by using selected N mineralization indexes, and with the activities of amidohydrolases and arylamidase in the soils.