Influences of Elevated Carbon Dioxide and Ozone On Soil Respiration and Carbon Accumulation in a No-till Soybean-Wheat System After Six Years.

Atmospheric carbon dioxide and ozone often have counteracting influences on many C₃ crops depending on the concentration of the gases and sensitivity of the crop and variety, but effects of these gases on plant-soil processes are poorly understood.  The objective of this six-year experiment was to determine the separate and combined effects of elevated carbon dioxide and ozone on soil respiration and carbon accumulation in a no-till soybean-wheat 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.  Plant residue C and N inputs to the soil were increased by 20 to 30% by elevated carbon dioxide and decreased by 5% by ozone.  Soybean fine root biomass was increased by 13% by carbon dioxide and inhibited by 42% by ozone.  Wheat root biomass was not significantly altered.   Soil respiration average across six growing seasons was stimulated by 33% with elevated carbon dioxide but the effect of ozone was not significant.  Soil microbial respiration was stimulated by 19 to 31% by elevated carbon dioxide.  Between 2005 and 2011, soil C and N in the 0-5 cm depth doubled in all treatments, while there was no significant change in C and N at 5-10 cm and 10-20 cm depths.  There were no significant effects of elevated carbon dioxide or ozone on soil C or N concentrations except in the final year of the study when C levels in the control and ozone treatments declined.  Apparently double-cropping and no-till contributed to increased soil C and N content while increased C and N inputs with elevated carbon dioxide stimulated decomposition.  Ozone effects were null, suggesting that inhibited decomposition compensated for reduced residue inputs.