Background/Question/Methods   Atmospheric CO2 concentrations are increasing, affecting ecosystems both directly by enhancing photosynthetic carbon assimilation, and indirectly by warming climate. Warming affects temperature-sensitive biological and biogeochemical processes, many of which feed back to the carbon cycle. Predicting ecosystem responses to these interacting processes requires multifactorial experiments under natural conditions. Corn-soy rotation is the dominant ecosystem in the midwestern United States. Because aboveground biomass does not accumulate, the carbon balance of this ecosystem depends on carbon stored as soil organic matter (SOM). SOM is controlled by inputs from plant photosynthesis, which increases with higher CO2, and by losses from microbial respiration, which increases with higher temperatures. We characterized the direction and magnitude of SOM changes in a corn-soy ecosystem under elevated CO2 and temperature. We used the DAYCENT dynamic SOM model to predict changes in soil C over three years of treatment. We simulated climate conditions predicted for 2050 using overhead infrared heaters to raise plant canopy and soil temperatures by 3.5 °C and canopy fumigation to raise CO2 levels to 550 ppm in an open field environment. We monitored heterotrophic and total respiration and calculated autotrophic respiration as Rtotal - Rheterotrophic. We measured root mass by washing roots from soil cores.

Results/Conclusions   DAYCENT simulations predicted that over three years elevated CO2 will increase soil C more than elevated heat will reduce it, resulting in a slight and variable gain of ~0.5% SOM under the combined treatments. Measurements revealed that heat increased heterotrophic respiration by 23% and decreased autotrophic respiration by 36% relative to ambient-temperature plots, resulting in no significant net change in total respiration. Heated plants showed a trend towards increased root mass in deep soil layers, suggesting that reduced Raut may have come from lowered tissue-specific root respiration. Respiration showed no significant CO2 or CO2 x temperature effects, but there was a trend toward higher total root mass in elevated CO2. Overall, belowground processes appear to be less responsive to CO2 than current models predict, suggesting that corn-soybean ecosystems may lose carbon as climate change continues.