Soil microbes burn sugar reserves instead of growing when rain returns
When dry soil gets wet again, microbes rapidly consume trehalose—a stored sugar—and release it as CO2 rather than using it for growth. The finding explains a poorly understood carbon loss mechanism in soils that has implications for agriculture, carbon sequestration efforts, and climate models.
Originaltitel: When dry soil is re-wet, trehalose is respired instead of supporting microbial growth
<p>When dry soil is re-wet there is a rapid increase in CO2 efflux and rates can remain above those of well-watered controls for one or more days. These large pulses of CO2 efflux are known as the `Birch effect. To provide experimental evidence of different pools of C fuelling the Birch effect, we incubated a drying soil with 13C6glucose, re wet the soil and quantified 13C labelling of pools (microbial biomass, trehalose, extracellular, and old C) and soil CO2 efflux. We took advantage of trehalose being the most 13C-enriched pool (& delta;13C = +518%o) to obtain direct isotopic evidence of trehalose's contribution to respiration and microbial growth. For soil incubated with 13C6-glucose, the & delta;13C of soil respiration was +35%o in dry soil, increased to 100%o in the 10 min following rewetting, and subsequently decreased. During the first 5 h after re-wetting, trehalose must have been contributing to respiration given that & delta;13C of soil respiration was more 13C enriched than trehalose-free microbial biomass (& delta;13C = +30%o), extracellular C (& delta;13C = -17.7%o), and old C (& delta;13C = -22.9%o). A four-member isotopic mixing model suggested trehalose underpinned 16% of respiration in the 1st hour after rewetting, decreasing to 7% in the fifth hour. At times beyond 5 h after rewetting, trehalose underpinned 0-4% of respiration. In the seven days following rewetting, microbial biomass increased 2292 nmol C g-1. Isotopic mass balance indicated trehalose-C could account for no more than 5% of the gross influx of C into microbial biomass, instead the increase in microbial biomass was fuelled by unlabelled or weakly labelled pools such as old C and extracellular C. Collectively these data provide direct experimental evidence C from trehalose does not significantly contribute to microbial growth in re-wet soil, but instead contributes to respiration for the first 5 h after rewetting.</p>