Soil enzymes mediate biochemical transformations involving organic residue decomposition and nutrient cycling in soil. A field study was conducted to determine the activity and persistence of soil enzymes with repeated additions of different organic residues. The activities of 10 soil enzymes involved in carbon, nitrogen, phosphorus, and sulfur cycling were assayed in an Arlington coarse-loamy soil which had received 100 Mg ha-1of either poultry manure, sewage sludge, barley straw, (Hordeum vulgare) or fresh alfalfa (Medicago sativa) over a 31-month time period (25 Mg ha-1× 4 additions). The enzyme activity assayed in the amended soil was increased by an average of 2− to 4-fold by incorporation of the four organic amendments when compared with the unamended soil (tillage alone) during the 31-month study. In general, the addition of organic amendments greatly increased the enzyme activity during the first year of the experiment, but subsequent additions failed to sustain high enzyme activity in the amended soil. The straw amendment was the most effective amendment enhancing the soil enzyme activity for all the enzymes assayed except for urease. In contrast to other reported studies, soil enzyme activities were not inhibited by sewage sludge addition in this investigation. The increased level of enzyme activity in the organic-amended soil may be a reflection of the increased protective sites within the soil as a result of enhanced humus content.