Microbial control of biogeochemical response to rewetting a dry soil: What have we learned in half a century of research?

Background/Question/Methods

In arid and semi-arid systems, the pulses of C and N mineralization following the wetting of dry soil are important in both the carbon and nitrogen economies of the terrestrial systems.  The large CO₂ pulse that occurs with the wet up of dry soil is commonly termed the “Birch Effect”.  Soil biogeochemists have been discussing the origin of and the importance of this phenomenon for many decades.  The rapid and substantial production of CO₂ following wet up has been explored in hundreds of papers since the original Birch paper was published in the 1950s.  Over this period researchers have addressed a range of questions including:

• How big is the pulse of CO₂ and how quickly does it occur?
• Is the pulse of CO₂ biotically driven or does it result from degassing of soil pores or solubilization of precipitated carbonates?
• What is the origin of the “available” carbon that fuels the CO₂ pulse?
• Who mediates the conversion of organic carbon to CO₂?
• What do the soil communities look like at the end of dry periods?
• How do nitrogen dynamics relate to the carbon dynamics during the wet up period?
• How will changing climates alter carbon and nitrogen dynamics following wet-up events?

Results/Conclusions

Much remains to be understood about the biogeochemistry of soil wet-up. In the context of carbon cycling and terrestrial system response to changing climate, what are the critical questions that require attention? The talks that follow this introductory talk should begin to provide a research map for the future.