Sources and priming of nitrous oxide production across a range of moisture contents in a soil with high organic matter

Abstract

Adding nitrogen fertilizers to agricultural soils contributes to increasing concentrations of nitrous oxide (N₂ O) in the atmosphere. However, the impacts of N addition on soil organic matter (SOM) turnover, SOM availability, and the ensuing SOM-derived N₂ O emissions remain elusive. Within this context, the net change in direction and rate of SOM-derived N₂ O production triggered by added N is termed the N₂ O priming effect. This incubation study examined the sources and priming of N₂ O production as a function of urea addition and multiple moisture contents in a soil with high SOM (55 g organic C kg^-1 ). We assessed four water-filled pore space (WFPS) conditions: 28, 40, 52, and 64%. Relative to controls receiving no N, urea addition increased N₂ O production by 2.6 times (P < .001). Cumulative N₂ O production correlated well with nitrification rates (r = .75; P = .03). We used ¹⁵ N-labeled urea to trace the added urea into N₂ O. Of the N added via urea, the recovery as N₂ O-N shifted from 0.02 to 0.17% when WFPS increased from 28 to 64% (P < .05). We also partitioned the N₂ O production into urea vs. SOM sources. More N₂ O was sourced from SOM than urea, with 59 ± 2% N₂ O originating from SOM. The magnitude of SOM-derived N₂ O under urea was larger than that of the control, revealing that positive N₂ O priming was triggered by urea addition. Upon subtracting the controls, the primed N₂ O was a consistent 19 ± 2% of the total N₂ O produced by urea-amended soils. Nevertheless, the priming magnitude rose sharply with increasing moisture by more than one order of magnitude from 4 to 48 μg N₂ O-N kg^-1 soil and in exponential mode (R² = .98). Soil moisture, SOM, and nitrification interacted to drive the sources and priming of N₂ O.