Elevated Carbon Dioxide and Ozone Effects On Above- and Belowground Growth and Decomposition in a No-till Soybean-Wheat System.

Elevated atmospheric carbon dioxide and ozone concentrations often have counteracting influences on many C3 crops depending on the concentration of the gases and sensitivity of the crop.  However, root growth and residue decomposition responses within this context are poorly understood. The objective of this four-year experiment was to determine the separate and combined effects of elevated carbon dioxide and ozone on above- and belowground growth and decomposition in a soybean-wheat no-till system.  Plants were treated with either ambient or elevated carbon dioxide (550 ppm) in combination with charcoal-filtered (CF) air or CF air plus ozone (1.4 x ambient ozone) using open-top field chambers.  Elevated carbon dioxide stimulated soybean and wheat biomass production and yield by 20-22% on average, but effects ranged from 10 to 29% depending on cultivar and year.  Soybean biomass and yield were suppressed with added ozone by 16% on average, but responses varied among cultivars and years as well.  Wheat yield was lowered by only 4% with elevated ozone, corresponding to lower springtime ozone concentrations compared with the summer growing season.  Soybean root length under elevated carbon dioxide was similar to the control while it was 45% lower in the added ozone treatment due to lower production and early senescence (wheat data not yet available).  In the combined elevated carbon dioxide and ozone treatment, biomass, yield and soybean root length were restored to control or higher levels.  Soybean root to shoot ratio was decreased by both elevated carbon dioxide and ozone.  After 3.5 years, litter C mass was 23 to 31% higher in the elevated carbon dioxide treatments but decomposition rates were similar to the control.  In the added ozone treatment, litter C mass was similar to the control even though 10% less residue C was added to the system.  Soybean leaf residues from the ozone treatment had higher fiber and lignin concentrations compared with the control.  At this point it is unclear how changes in inputs of above- and belowground C, nutrients and recalcitrant components with elevated carbon dioxide and ozone will interact with soil microbiology to alter soil C and N dynamics although it is expected that elevated carbon dioxide effects will predominate.