The Effect of Elevated CO2 and O3 On Soil Nitrogen Inputs and Losses in a Soybean Agroecosystem in Illinois.

It has been found that elevated atmospheric carbon dioxide (eCO₂) and tropospheric ozone (eO₃) affect belowground microbial processes, including N transformations, through plant-mediated changes. Conversely, changes in soil organic carbon sequestration and plant biomass production are constrained by N availability. Furthermore, changes in N losses to the groundwater or the atmosphere cause eutrophication of aquatic ecosystems and global climate change, respectively. Therefore, it is pertinent to assess the effect of eCO₂ and eO₃ on N inputs and losses. In a soybean agroecosystem in Illinois, we used natural abundance stable isotopes as a proxy to integrate long-term N inputs and losses, based on minimal isotopic discrimination during biological N₂-fixation, and ¹⁵N depletion associated with lost N. Our isotopic model suggested that N-inputs and losses decreased by eCO₂, whereas inputs and losses increased by eO₃. Under eCO₂, changes in inputs and losses balanced each other out, while a larger increase in inputs compared to losses under eO₃ resulted in a larger soil N content under eO₃ compared to ambient. Differences in N-losses between treatments are due to removal of soil derived N by harvest, N leaching and gaseous N emissions. Soil derived N removed by harvest was increased by eCO₂, and there was a stimulation of the cumulative N₂O flux during the growing season in 2005 but not in 2006. Consequently, the reduction in N loss under eCO₂ compared to ambient must be a result of decreased N leaching or decreased total gaseous loss. eO₃ did not affect the amount of soil N removed by harvest or the cumulative N₂O flux during the growing season. Hence, the increased N loss under eO₃ compared to ambient might be due to changes in total gaseous N loss, N₂O emissions during the non-growing season or increased leaching. In conclusion, our results warn for increased environmental risk for hazardous N loss from agroecosystems under changing atmospheric conditions.