The Effect of Elevated CO2 and O3 On Sources of N2O Emissions in a Soybean Agroecosystem.

Abstract

Under elevated atmospheric CO₂ (eCO₂) and tropospheric O₃ (eO₃), soil microbial processes can be altered through plant input changes. Particularly, processes underlying N₂O emissions are characterized by various controls that can be affected differently by eCO₂ and eO₃. In order to determine the effect of eCO₂ and eO₃ on sources of N₂O emissions, we set up an incubation experiment using soils from a soybean agroecosystem that was exposed to elevated CO₂ and/or O₃ in a free air concentration enrichment experiment (SoyFACE). Ammonium nitrate, isotopically enriched in either ammonium ((¹⁵NH₄)₂NO₃) or nitrate ((NH₄)₂¹⁵NO₃), was added to the soils and N₂O emissions as well as ¹⁵N content in N₂O were determined over time (12 days). We hypothesized that eCO₂ would increase total N₂O emissions and the fraction of N₂O derived from denitrification (N₂O_D), driven by an increase in root exudation and anaerobic microhabitat associated with increased soil aggregation. Under eO₃, decreased root exudation, decreased soil aggregation and decreased N₂-fixation resulting in increased uptake of soil NO₃^- and thus decreased availability of NO₃^- for denitrifiers have been observed. Therefore, we expected a decrease in total N₂O emissions and an increase in the fraction of nitrification derived N₂O (N₂O_N) under eO₃. eCO₂ tended to increase total N₂O emissions (p = 0.0703), while eO₃ had no effect. After addition of (¹⁵NH₄)₂NO₃, we observed a significant increase in ¹⁵N₂O for soils previously exposed to O₃, while significantly higher ¹⁵N₂O was found under eCO₂ on day 2 and 9 of the incubation after addition of  (NH₄)₂¹⁵NO₃. This suggests an increase in N₂O_N under eO₃ and an increase in N₂O_D under eCO₂, corroborating our hypothesis. Our results demonstrate significant effects of eCO₂ and eO₃ on sources of N₂O emissions from soybean agroecosystems, which can have important implications for predicting and mitigating future N₂O emissions.

---