Linking changes in plant growth, soil structure, and N-transformers in a soybean agroecosystem under elevated CO2 and O3

Atmospheric CO₂ and O₃ concentrations are rising due to increased anthropogenic activity and can alter N cycling in terrestrial ecosystems. Elevated CO₂ and O₃ can induce changes in plant growth that can propagate to alter soil microhabitats, and subsequently the abundance and the activity of N-transforming microorganisms. We hypothesized that elevated CO₂ will promote soil aggregation by increasing plant production, and that this will induce changes in the abundance of N-cycling microorganisms. We also hypothesized that O₃ enrichment will negate the effect of CO₂ enrichment. Our hypotheses were tested at the Soybean Free-air CO₂ and O₃ Enrichment experiment in Urbana, Illinois, in which soybeans were grown under factorial CO₂ and O₃ treatments. We analyzed the soybean belowground biomass and total soil C and N, and performed microaggregate isolation for soil fractionation. Quantitative polymerase chain reaction using primers for universal eubacteria, amoA and nosZ genes were employed to determine the abundance of eubacteria, nitrifiers, and denitrifiers. Elevated CO₂ increased the root biomass, but total soil C and N concentrations were unaffected by CO₂ enrichment. Microaggregation was lowered by elevated CO₂. Ozone enrichment had no effect on any of the parameters measured. The increase in substrate availability and decrease in microaggregation under elevated CO₂ induced changes in the abundance of nitrifiers and denitrifiers. Our study indicates that higher labile substrate input to soil under elevated CO₂ may lead to lower soil aggregation, possibly due to a priming effect, and that this can change the composition of N-cycling microorganisms. Therefore, alterations in plant production and soil structure under elevated CO₂ have the potential to change terrestrial N cycling.