Soil type determines response of soil microbial activity to an atmospheric CO2 gradient

Background/Question/Methods

Rising atmospheric CO₂ will have direct effects on ecosystems in addition to consequences for climate.  We investigated belowground response of a prairie ecosystem to a preindustrial-to-future (250-500ppm) gradient of CO₂.  Three soil types are represented throughout the gradient, allowing us to ask how soil type influences microbial activity and soil carbon storage response to CO₂ treatment.  A long-term (up to 1 year) laboratory soil incubation is conducted to study both microbial activity and the allocation of soil carbon in fast versus slow-cycling pools.

Results/Conclusions

Preliminary results indicate that soil type is a strong determinant of microbial response to CO₂ treatment.  Both substrate-induced respiration (SIR) and potential carbon mineralization (Cmin) assays indicate a linear increase in microbial activity with CO₂ concentration, but this trend is present only in the clay-rich Austin and Houston soils, not the sandy Bastrop soil.  The Austin soil responded most strongly to CO₂ treatment, with SIR increasing 30% and Cmin increasing 50% along the gradient.  This increasing microbial activity may be due to greater root growth and carbon input under elevated CO₂.  Soil respiration data suggests that the soil type effects on microbial activity occur in the field; average soil respiration rates in 2009 increased linearly with CO₂ treatment in the Austin and Houston soils (both rates increased 30% along the gradient) but no significant trend occurred in the Bastrop soil.  Understanding soil type may improve predictions of how grasslands store carbon in a future high-CO₂ world.