A Proposed Mechanism for the Pulse in Carbon Dioxide Production Commonly Observed Following the Rapid Rewetting of a Dry Soil

There is uncertainty about the mechanisms responsible for producing the rewetting CO2 pulse. One proThe rapid rewetting of a dry soil often yields a pulse in soil CO2 posed mechanism is that the pulse of CO2 is largely a production that persists for 2 to 6 d. This phenomenon is a common occurrence in surface soils, yet the mechanism responsible for producresult of the mineralization of nonbiomass soil organic ing the CO2 pulse has not been positively identified. We studied the matter (SOM) rendered accessible to microbial attack effects of a single drying and rewetting event on soil C pools, to by the rewetting event. According to this hypothesis, identify which specific C substrates are mineralized to produce the the drying and rewetting process disrupts aggregate observed pulse in respiration rates. We labeled two soils with structure, releasing organic matter from physical protec14C-glucose and measured the enrichment and pool sizes of the retion within aggregates and producing a pulse in microleased CO2, extractable biomass C, and extractable soil organic matter bial activity as this material is mineralized (Adu and (SOM-C) throughout a drying and rewetting cycle. After rewetting, Oades, 1978; Appel, 1998; Denef et al., 2001; Sorensen, respiration rates were 475 to 370% higher than the rates measured 1974; Utomo and Dexter, 1982). Alternatively, others before the dry down. The enrichment of the released CO2 was 1 to have proposed that microbial C, not SOM-C, is the major 2 times higher than the enrichment of the extractable biomass C pools and 10 to 20 times higher than the enrichment of the extractable substrate mineralized to produce the rewetting CO2 organic C, suggesting that the CO2 pulse was generated entirely from pulse (Bottner, 1985; Kieft et al., 1987). The rapid inthe mineralization of microbial biomass C. However, there was no crease in soil water potential associated with the rewetevidence of substantial microbial cell lysis on rewetting. We hypotheting of a dry soil causes microbes to experience osmotic size that the pulse of CO2 is generated by the rapid mineralization of shock. In general, microbial cells either lyse completely highly enriched intracellular compounds as a response by the microbial or adjust to the water potential shock by releasing intrabiomass to the rapid increase in soil water potentials. The drying and cellular osmoregulatory solutes (Halverson et al., 2000; rewetting process also releases physically protected SOM, increasing Harris, 1981). The compounds released into the soil the amount of extractable SOM-C by up to 200%. The additional environment are taken up by surviving microbes and SOM-C rendered soluble by the rewetting event did not contribute mineralized, producing the respiration pulse. Some substantially to the rewetting CO2 pulse. Overall, the rapid rewetting of a dry soil can influence soil C cycling in the short-term, by increasing studies have combined these two proposed mechanisms, the microbial mineralization of cytoplasmic solutes, and in the longersuggesting that both biomass C and nonbiomass SOM-C term, by decreasing the total amount of SOM physically protected contribute to the rewetting CO2 pulse (Scheu and Parkwithin microaggregates. inson, 1994; Van Gestel et al., 1993a; Van Gestel et al., 1991; Van Veen et al., 1985). The identification of the specific mechanisms responN studies have shown that the rapid rewetsible for producing the rewetting CO2 pulse is important ting of a dry soil can cause a large pulse in soil C if we want to understand the implications of climate mineralization rates (Birch, 1958; Clein and Schimel, change on soil C dynamics. In the future, many regions 1994; Franzluebbers et al., 2000; Jager and Bruins, 1975; of the globe may experience higher mean annual temSoulides and Allison, 1961). After a soil rewetting, CO2 peratures and greater intra-annual variability in the timproduction rates are often elevated by as much as 500% ing of precipitation events (Barrow and Hulme, 1996; compared with samples kept continuously moist, with Houghton et al., 1996; Waggoner, 1989). Under these the CO2 pulse generally persisting for a 2to 6-d period scenarios, we would expect many surface soils to experifollowing the rewetting event. Since many surface soils ence more frequent drying and rewetting events. If nonexperience large seasonal fluctuations in moisture conbiomass SOM-C is the primary source of the rewetting tent, these short-term pulses in CO2 production after CO2 pulse, an increase in the frequency of soil drying rewetting are likely to be a common occurrence in many and rewetting will increase the amount of soil C accessisoils. In arid, semi-arid, or Mediterranean environble to microbial attack, potentially decreasing the total ments, where rainfall events are infrequent and soils are amount of C sequestered in a particular soil over time. often dry, the rewetting CO2 pulse may constitute a However, if microbial biomass is the source of the rewetsignificant proportion of the total annual CO2 flux from ting CO2 pulse, an increase in the frequency of dryingsurface soils. rewetting events may increase the level of physiological stress for soil microbes, potentially reducing C mineralN. Fierer and J. P. Schimel, Dep. of Ecology, Evolution, and Marine ization and increasing C sequestration rates over time. Biology, Univ. of California, Santa Barbara, CA 93106. Received 1 We conducted an experiment with two soils from a July 2002. *Corresponding author ([email protected]). Abbreviations: SOM, soil organic matter. Published in Soil Sci. Soc. Am. J. 67:798–805 (2003).