Soil Nitrogen Transformations Under Elevated CO2 and O3 Concentrations During the Soybean Growing Season.

The increasing atmospheric concentrations of CO₂ and O₃ are likely to alter ecosystem functioning by modifying rates of plant photosynthesis and production of plant biomass. Soil microorganisms are driven by the amount and quality of plant organic material and conduct crucial ecosystem processes, such as N cycling. This study aims to investigate how soil N cycling will respond to elevated CO₂ and O₃. More specifically, it focuses on the changes in nitrifiers and denitrifiers communities, as well as N transformation processes, under elevated CO₂ and O₃ conditions within the growing season and across soil environments (i.e., rhizosphere and bulk soil) in a soybean (Glycine max (L.) Merr.) agroecosystem. Soil samples were collected at different phenological stages of the soybean plants: fourth trifoliolate leaf (V4), full pod (R4), and full maturity (R8) during the 2008 growing season at the SoyFACE experiment in Champaign-Urbana, Illinois. Elevated CO₂ increased 16S rRNA gene abundance during R8 compared to V4 and R4. Neither elevated CO₂ nor O₃ had significant effects on amoA abundance.  The nosZ gene abundance was significantly more abundant in the rhizosphere compared to the bulk soil. Although not statistically significant, elevated O₃ tended to increase the abundance of nosZ, whereas elevated CO₂ did not change nosZ abundance. Although total soil N was significantly higher under elevated than ambient O₃ conditions, elevated O₃ decreased soil mineral N through a reduction in plant material input and increased denitrification, which was indicated by the higher abundance of nosZ. Elevated CO₂ did not alter any of the parameters evaluated and elevated CO₂ and O₃ showed no interactive effects on nitrifier and denitrifier communities nor on the concentration of total and mineral N in soil. This study shows that elevated CO₂ may have limited effects on terrestrial N transformations in soybean agroecosystems, but elevated O₃ can lead to a decrease in plant N availability in both the rhizosphere and bulk soil. In conclusion, in addition to reducing photosynthetic rates, elevated O₃ can also affect ecosystem productivity by reducing the mineralization rates of plant-derived residues.