Elevated carbon dioxide does not offset loss of soil carbon from a corn-soybean agroecosystem

The potential for storing additional C in U.S. Corn Belt soils - to offset rising atmospheric [CO2] - is large. Long-term cultivation has depleted substantial soil organic matter (SOM) stocks that once existed in the region's native ecosystems. In central Illinois, free-air CO2 enrichment technology was used to investigate the effects of elevated [CO2] on SOM pools in a conservation tilled corn-soybean rotation. After 5 and 6 y of CO2 enrichment, we investigated the distribution of C and N among soil fractions with varying ability to protect SOM from rapid decomposition. None of the isolated C or N pools, or bulk-soil C or N, was affected by CO2 treatment. However, the site has lost soil C and N, largely from unprotected pools, regardless of CO2 treatment since the experiment began. These findings suggest management practices have affected soil C and N stocks and dynamics more than the increased inputs from CO2-stimulated photosynthesis. - Soil carbon from microaggregate-protected and unprotected fractions decreased in a conservation tilled corn-soybean rotation despite increases in primary production from exposure to atmospheric CO2 enrichment.