Sources and priming of soil N2O and CO2 production: Nitrogen and simulated exudate additions

Abstract

Identifying the sources of nitrous oxide (N2O) and carbon dioxide (CO2) production from soil is central to enhancing the understanding and prediction of these emissions to the atmosphere. The magnitude of N2O and CO2 production derived from soil organic matter (SOM) can asymmetrically change due to stimuli from root exudation and nitrogen additions ‒ a response termed the priming effect. We conducted an incubation to examine the effects of N and artificial root exudate (ARE) additions on the priming of SOM. We also evaluated the changes in N2O production from nitrification and denitrification by measuring 15N–N2O site preference (SP). ARE consisted of a mixture of 99 atom% 13C labelled compounds at three rates (0, 6.2, 12.5 mg C kg−1 soil day−1) applied daily to microcosms with or without urea, a subset of which was also labelled with 5 atom% 15N. Additions of ARE or urea alone caused positive priming effects; however, addition of ARE and urea concurrently resulted in an antagonistic interactive effect that diminished the N2O production derived from SOM mineralization (P < 0.05). Moreover, CO2 production from SOM decreased in urea-treated microcosms (P < 0.01) such that all soils receiving ARE and urea exhibited reduced positive priming relative to their unfertilized counterparts. Based on SP results, the contributions of denitrification and nitrification to total N2O production were both amplified due to the combined inputs of ARE and urea compared to the untreated control (49.9 ± 10.1 and 28.3 ± 8.0 μg N2O–N kg−1, respectively). In soils receiving only ARE, N2O derived from denitrification decreased relative to a control, thus reducing overall N2O production (−9.5 ± 12.3 μg N2O–N kg−1); conversely, nitrification-derived N2O was differentially augmented (+17.2 ± 9.0 μg N2O–N kg−1). Results indicate that a combination of elevated root exudation with N fertilization has the potential to asymmetrically amplify N2O emissions due to increases in both nitrification and denitrification sources.