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In semi-arid floodplains the average times between floods have been cited to drive metabolic and biogeochemical responses during the subsequent flooding pulse. However, the interaction effects of flood pulse duration and the length of time between floods on the carbon budget are not well understood. Using field experiments, flood pulses-dry cycles were simulated (SF plots-short flood/dry cycles: 15 flood days ? 7 dry ? 15 flood and LF plots-long flood/dry cycles: 21 flood ? 14 dry ? 21 flood) in a semi-arid floodplain in Central Spain, in order to study the effects on soil CO 2 emissions. Differences on soil water content among SF, LF and control plots were statistically significant throughout the experiment (p \ 0.01). Soil CO 2 emission rates during drying time were significantly related with the duration of previous flooding and inter-flooding intervals (R 2 = 0.52-0.64, p = 0.03). During the first stage of desiccation, the high soil water content appears to limit aerobic metabolism. Soil respiration rates similar to those of control plots measurements occurred 1-2 weeks later. Then, soil respiration increased to a maximum rate which was delayed 5-8 weeks, as high soil water content limited microbial activity. While more than 7 days of inundation promoted denitrification, organic nutrients supplied by flood water increased 1% soil respiration during drying. Differences between SF and LF plots in soil CO 2 emissions only appeared after floodplain soil had been subjected to two consecutive flood-dry cycles; 70 days after the second inundation ended, CO 2 fluxes achieved similar values in all treatments. Daily soil CO 2 emission rates during the entire study period (117 days) were comparable, independently of the flood duration and the time between floods (75.76 ± 1.59 and 77.94 ± 0.45 mmol CO 2 m -2 day -1 , in SF and LF, respectively). Flood disturbance affects site-specific microbial processes, but only during very short time periods. The mechanism by which soil microbial communities cope or adapt to new conditions needs to be reassessed in future research in order to determine the long-term effects of hydrological changes in the soil carbon balance of semi-arid floodplains.

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