Rainfall-induced carbon dioxide pulses result from sequential resuscitation of phylogenetically clustered microbial groups.

The pulse of carbon dioxide (CO(2)) resulting from the first rainfall after the dry summer in Mediterranean ecosystems is so large that it is well documented at the landscape scale, with the CO(2) released in a few days comparable in magnitude to the annual net carbon exchange of many terrestrial ecosystems. Although the origin of this CO(2) is debated, we show that the pulse of CO(2) is produced by a three-step resuscitation of the indigenous microbial community. Specific phylogenetic groups of microorganisms activate and contribute to the CO(2) pulse at different times after a simulation of the first rainfall following the severe summer drought. Differential resuscitation was evident within 1 h of wet-up, with three primary response strategies apparent according to patterns of relative ribosomal quantity. Most bacteria could be classified as rapid responders (within 1 h of wet-up), intermediate responders (between 3 and 24 h after wet-up), or delayed responders (24-72 h after wet-up). Relative ribosomal quantities of rapid responders were as high in the prewet dry soils as at any other time, suggesting that specific groups of organisms may be poised to respond to the wet-up event, in that they preserve their capacity to synthesize proteins rapidly. Microbial response patterns displayed phylogenetic clustering and were primarily conserved at the subphylum level, suggesting that resuscitation strategies after wet-up of dry soil may be a phylogenetically conserved ecological trait.