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

Abstract

The rapid rewetting of a dry soil often yields a pulse in soil CO2 production that persists for 2 to 6 d. This phenomenon is a common occurrence in surface soils, yet the mechanism responsible for producing the CO2 pulse has not been positively identified. We studied the effects of a single drying and rewetting event on soil C pools, to identify which specific C substrates are mineralized to produce the observed pulse in respiration rates. We labeled two soils with 14C-glucose and measured the enrichment and pool sizes of the released CO2, extractable biomass C, and extractable soil organic matter (SOM-C) throughout a drying and rewetting cycle. After rewetting, respiration rates were 475 to 370% higher than the rates measured before the dry down. The enrichment of the released CO2 was 1 to 2 times higher than the enrichment of the extractable biomass C pools and 10 to 20 times higher than the enrichment of the extractable organic C, suggesting that the CO2 pulse was generated entirely from the mineralization of microbial biomass C. However, there was no evidence of substantial microbial cell lysis on rewetting. We hypothesize that the pulse of CO2 is generated by the rapid mineralization of highly enriched intracellular compounds as a response by the microbial biomass to the rapid increase in soil water potentials. The drying and rewetting process also releases physically protected SOM, increasing the amount of extractable SOM-C by up to 200%. The additional SOM-C rendered soluble by the rewetting event did not contribute 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 the microbial mineralization of cytoplasmic solutes, and in the longer-term, by decreasing the total amount of SOM physically protected within microaggregates.