When structure means conservation: Effect of aggregate structure in controlling microbial responses to rewetting events

Rewetting events after a drought produce a pulse of soil respiration (the Birch Effect) that leads to a loss of carbon from soil, especially in Mediterranean ecosystems. Two main hypotheses have developed to explain the Birch effect: the metabolic explanation, based on the rapid consumption of intracellular osmolytes previously accumulated to survive to dry conditions, and the physical explanation, based on the consumption of carbon made accessible by physical destruction of internal structures of the soil. Here, we compared the respiration response of intact and crushed 9-4 mm aggregates from a California grassland soil under two different rewetting schemes: (1) successive short dry/wet events and (2) increased drought periods followed by a single rewetting. In intact aggregates, both microbial biomass and respiration rates were relatively stable through both experimental treatments. In crushed aggregates, through multiple short dry/wet cycles, both respiration rate and microbial biomass increased, while as drought length increased, biomass was unaffected but the magnitude of the following rewetting pulse increased. A mechanism that explains both these results is that crushing aggregates exposes occluded particular material that must be degraded into an immediately bioavailable form for microbes to take it up and metabolize it. Nitrification was generally higher in intact than crushed aggregates, suggesting the importance of physical association between nitrifiers and resources in regulating overall soil nitrification. This work suggests that physical processes are most important in driving respiration pulses through multiple rewetting cycles and that the physical association of organisms, substrates, and mineral particles are critical in controlling the functioning of the microbial landscape. (C) 2011 Elsevier Ltd. All rights reserved.