Artificial wetlands, constructed specifically to treat wastewaters, are increasingly used in addition to, or instead of, conventional water treatment methods. The treatment efficiency of these may be improved if nutrient cycling could be manipulated via soil processes. Extracellular enzyme activities in the soils of constructed wetlands were therefore examined to investigate the biogeochemistry of these treatment wetlands. Fluorescent model substrates were used to determine the relative activities of cellobiohydrolase, ~-glucosidase, N-acetylglucosaminidase, phosphatase, sulphatase and xylosidase; a colorimetric substrate was used to examine phenol oxidase. Field studies of constructed wetlands built for different purposes provided initial information about the levels and regulators of enzyme activity in these artificial soils. Enzyme activities and gas fluxes were lower than in natural wetlands; however there was an unexpected similarity in nutrient cycling between constructed wetlands and natural soil and sediment systems, which was shown by the relative activities of extracellular enzymes. Laboratory experiments clarified the relationships between enzyme activity and pH, redox potential, temperature, oxygenation, phenolic concentration and dissolved organic carbon supply, and showed that considerable potential for enzyme manipulation, and therefore improved treatment efficiency, exists. Soil temperature correlated with the activity of several enzymes, suggesting that extracellular decomposition may be faster at higher temperatures. Soil temperature was the strongest determinant of nitrous oxide and carbon dioxide fluxes; methane flux did not show any seasonal variation, and was not influenced by pH. Dissolved organic carbon concentration increased or decreased the activity of different enzymes. The microbial population appeared to be essential for denitrification, and compounds which may act as microbial inhibitors could therefore be detrimental to wetland treatment. The existing, 'natural' carbon-cycling enzyme activity of soil was enhanced by adding commercial enzymes, although the capacity of wetland soil to immobilize additional enzymes may depend on the organic material available. Extracellular enzymes are useful indicators of nutrient cycling, and represent a potential means of manipulating the treatment processes occurring in constructed wetlands.