Soil type mediates soil respiration response to a CO2 gradient in tallgrass prairie

Background/Question/Methods Changes in soil respiration due to rising atmospheric CO₂ have large  implications for land-atmosphere carbon balance and consequently the greenhouse effect.  We report results from tallgrass prairie exposed to a gradient of CO₂ spanning preindustrial to expected mid-century levels (250 - 500ppm).  Elongated chambers are used to expose soil monoliths planted to tallgrass prairie species to a CO₂ gradient.  Three soil series are represented along  the CO₂ gradient:  Austin (mollisol), Bastrop (alfisol), and Houston (vertisol).  Soil respiration was measured monthly with an infrared gas analyzer during the 2007 growing season (April – October).
Results/Conclusions Soil respiration responded positively to CO₂ treatment on all soils.  The response of respiration to CO₂ was linear in Bastrop and Houston soils (p < 0.05), but non-linear in Austin soil.  Interestingly, of the three soils, Houston and Austin are most similar in texture.  There is likely a seasonal dimension to the soil type x CO₂ interaction.  Early in the growing season (May), linear regressions of respiration rate vs CO₂ treatment were not significant, but in July and October were significant (p < 0.05).  Averaged across the 2007 growing season, respiration rates were highest on Houston soil and lowest on Austin soil, with Bastrop soil intermediate (8.38, 6.80, and 7.40 μmol CO₂/m₂/s, respectively).  Soil respiration responses, when combined with results on photosynthesis and plant productivity, will allow better characterization of prairie as a C sink or source under future CO₂ levels.