Enzyme activities and carbon dioxide flux in a Sonoran desert urban ecosystem

Urban expansion into wildlands significantly changes soil processes such as nutrient cycling and organic matter processing. Knowledge of these changes is important so that the impact of urbanization on ecosystems may be assessed. We measured the activities of invertase, cellulase, and CO₂ flux in mesiscape, xeriscape, and in remnant desert patches in a rapidly urbanizing south central Arizona Sonoran desert ecosystem. In this system, mesiscapes are irrigated watered lawns, xeriscapes include low water-use vegetation, and desert remnants include undeveloped areas within the urban matrix. Invertase activity ranged from 2.4 to 15 mg glucose equivalents (GE) g^-1 24 h^-1. Invertase activities in mesiscapes during January exceeded desert remnant sites by a factor of six and xeriscape sites by a factor of two. Cellulase activity ranged from 48 to 406 μg GE g^-1 24 h^-1. Cellulase activity in mesiscapes during January significantly exceeded desert remnant and xeriscape sites by a factor of two. Mesiscape soils were up to 18.4°C cooler than xeriscape soils and had the lowest average temperatures (20.7°C). The average temperature of desert remnant soils was 27.4°C. Over the study period, CO₂ flux rates ranged from 0.212 to 1.760 g m^-2 h^-1. Maximum rates of CO₂ flux rates occurred in the spring and summer, and flux rates were lowest during the winter months. Winter peaks of enzyme activity are attributed to the onset of dormancy in C-4 grasses in the fall and establishment of winter lawns by homeowners.