Elevated temperature and carbon dioxide prime soil-specific increases in heterotrophic respiration

Background/Question/Methods

The former prairies of the Midwest contain large quantities of soil carbon, and frequent disturbance under intensive agricultural management increases carbon loss. Higher plant productivity from increasing atmospheric CO2 has been expected to replenish belowground carbon stocks by increasing root and litter inputs, but may instead prime microbial activity and result in further carbon losses. Previous observations at SoyFACE have shown soil carbon to be declining over time, and this trend has continued despite more than ten years of higher carbon inputs from increased plant productivity under elevated CO2, supporting the hypothesis that new carbon is priming decomposition rather than organic matter accumulation in this system.  We elevated temperature and CO2 to levels predicted for 2050 in a factorial experiment on a corn-soybean agroecosystem for three years and used mobile gas analyzers to monitor respiration by roots and soil heterotrophs. We predicted that heat would accelerate the previously observed priming of organic matter decomposition, leading to higher respiration from heated elevated-CO2 plots and accelerated carbon losses over time.

Results/Conclusions

Neither elevated CO2 nor elevated temperature, singly or in combination, affected total soil CO2 efflux. Elevated temperature stimulated heterotrophic respiration and showed a trend for an offseting decrease in respiration by roots, but the magnitude of this effect varied over time. This suggests that decomposition is primed through litter inputs at the end of the year rather than increased root exudation during the growing season. Different blocks of the experiment showed different response patterns, with heat-induced respiration changes persisting longer in blocks where differing soil type and land-use history resulted in higher initial carbon and nitrogen contents.  This dependence on initial conditions indicates that high-carbon systems will have the highest absolute losses with climate change, and supports predictions from biogeochemical process models that far from being stabilized, soil organic matter is dynamic and susceptible to remobilization whenever land use or environmental conditions change.