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Biological activity in arid grasslands is strongly dependent on moisture. We examined gas exchange of biological soil crusts (biocrusts), the underlying soil biotic community, and the belowground respiratory activity of C-3 and C-4 grasses over 2 years in southeast Utah, USA. We used soil surface CO2 flux and the amount and carbon isotope composition (delta C-13) of soil CO2 as indicators of belowground and soil surface activity. Soil respiration was always below 2 mu mol m(-2) s(-1) and highly responsive to soil moisture. When moisture was available, warm spring and summer temperature was associated with higher fluxes. Moisture pulses led to enhanced soil respiration lasting for a week or more. Biological response to rain was not simply dependent on the amount of rain, but also depended on antecedent conditions (prior moisture pulses). The short-term temperature sensitivity of respiration was very dynamic, showing enhancement within 1-2 days of rain, and diminishing each day afterward. Carbon uptake occurred by cyanobacterially dominated biocrusts following moisture pulses in fall and winter, with a maximal net carbon uptake of 0.5 mu mol m(-2) s(-1), although typically the biocrusts were a net carbon source. No difference was detected in the seasonal activity of C-3 and C-4 grasses, contrasting with studies from other arid regions (where warm-versus cool-season activity is important), and highlighting the unique biophysical environment of this cold desert. Contrary to other studies, the delta C-13 of belowground respiration in the rooting zone of each photosynthetic type did not reflect the delta C-13 of C-3 and C-4 physiology.