Source-process partitioning of soil N2O and CO2 production: nitrogen and simulated exudate additions

Understanding the source partitioning of carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes from soil is integral for the characterization of total fluxes and the quantification of potential soil organic matter priming effects. Additionally, we utilized ¹⁵N-N₂O site preference data to analyze the process priming of microbial nitrification and denitrification on subsequent N₂O fluxes. A 32-day laboratory incubation was designed to examine the effects of artificial exudate, nitrogen fertilizer and their potential interactive effects on CO₂ and N₂O fluxes, soil organic matter source-priming and N₂O process-priming. Artificial root exudate (ARE) consisting of a mixture of 99 atom% ¹³C labelled compounds at three addition rates (0, 6.2, 12.5 mg C kg^-1 soil day^-1) was applied daily for 21 days to microcosms with or without urea fertilizer, a subset of which was labelled with 5 atom % ¹⁵N. Measurements of CO₂ and N₂O fluxes, isotopic composition and N₂O site preference were frequent throughout the duration of the experiment. Source partitioning of CO₂ fluxes showed that soil organic carbon (SOM-C) positive priming was significantly altered by additions of artificial exudate and urea (p < 0.001 and 0.001, respectively). When applied concurrently, urea addition had an antagonistic interactive effect on SOM-C sourced CO₂ fluxes (p < 0.001).  Source partitioning of N₂O flux data revealed that soil organic matter nitrogen (SOM-N) was positively primed for N₂O flux by the addition of urea fertilizer (p < 0.001), but positive SOM-N priming was reduced by an antagonistic interaction with artificial exudate application (p < 0.001). Further, examination of ¹⁵N-N₂O site preference found that the main processes by which N₂O is formed (nitrification and denitrification) were differentially process-primed by the addition or absence of ARE. Cumulative denitrification and nitrification contributions to total N₂O flux were both positively primed in the soils receiving both ARE and urea inputs relative to a control (50.0 ± 10.1 and 28.2± 8.0 μg N₂O-N kg^-1, respectively). In soils receiving only ARE application, denitrification-derived N₂O was negatively primed relative to a control and thus contributed less to overall N₂O flux (-9.5 ± 12.4 μg N₂O-N kg^-1) but nitrification-derived N₂O was positively primed (17.2 ± 9.0 μg N₂O-N kg^-1).