Soil nitrogen transformations under elevated carbon dioxide and ozone concentrations during the growing season of soybean

The increasing atmospheric concentrations of CO2 and O 3 are likely to alter ecosystem functioning by modifying rates of plant photosynthesis and production of plant biomass. Soil microorganisms that are driven by the amount and quality of plant organic material 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 CO 2 and O3 conditions within the growing season and across soil environments (i.e., rhizosphere and bulk soil). We determined changes in soil available N and in gene abundance of total bacteria (16S rRNA), nitrifiers (amoA) and denitrifiers (nosZ), caused by elevated CO2 and O3 in the rhizosphere and bulk soil of 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. The abundance of 16S rRNA genes ranged from 1.54 x 108 to 3.77 x 108 copies g-1 soil. Elevated CO2 increased 16S rRNA gene abundance during R8 compared to V4 and R4. The amoA gene abundance ranged from 6.5 x 106 to 1.5 x 107 copies g-1 soil, but neither elevated CO 2 nor O3 had significant effects on amoA abundance. The nosZ gene abundance ranged from 2.87 x10 5 to 4.18 x 106 copies g-1 soil and 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. In contrast to our expectations, the effects of elevated CO2 and O3 on the measured variables were not more pronounced in the rhizosphere compared to the bulk soil. 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 CO 2 did not alter any of the parameters evaluated and elevated CO 2 and O3 showed no interactive effects on nitrifier and denitrifier communities nor on the concentration of total and mineral N in soil. Sampling event, which corresponded to the different plant phenological stages did not show any interaction effects with CO2 and O 3 on N dynamics.;This study shows that elevated CO2 may have limited effects on terrestrial N transformations in soybean agroecosystems, but elevated O 3 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.