Interactive effects of soil salinity and nitrogen fertilizer types on nitrous oxide and ammonia fluxes

Soil salinization, impaired by climate change and poor management practices, poses a global threat, particularly in arid and semi-arid regions, leading to significant land degradation. This study aims to investigate the effects of different nitrogen (N) fertilizer sources (urea, ammonium-sulfate, and biogas waste) on CO₂, N₂O, and NH₃ emissions and soil enzyme activities in two soil types varying in salinity level (non-saline: EC = 1.15 dS m⁻¹, and saline: EC = 35.80 dS m⁻¹) in a robotized continuous-flow soil incubation system. Our results showed a sharp increase in N₂O and CO₂ emissions (up to 0.51 ± 0.02 g N₂O-N ha⁻¹ day⁻¹, 28.1 ± 3.9 kg CO₂-C ha⁻¹ day⁻¹) in non-saline soils following soil rewetting, attributed to bacterial denitrification. However, this pattern was not observed in saline soils, suggesting that salinity causes partial inhibition to the regeneration of soil organic matter mineralization and denitrification processes after rewetting. Although salinity did not alter the overall cumulative N₂O losses in any fertilizer treatment, it significantly delayed the evolution of N₂O peak during the incubation period. On the other hand, NH₃ volatilization was significantly higher in N-fertilized saline soils compared to non-saline soils (241% and 157% in ammonium-sulfate and biogas waste treatments, respectively), except for urea treatment, likely due to the decrease in nitrification rates. Furthermore, the study clearly showed lower soil enzyme activity levels for both nitrate reductase and urease activity. Interestingly, the lowest NH₃ emissions were measured in urea treatment in both soils. Overall, our findings highlight the complex interplay between soil salinity, nitrogen fertilizer sources, and microbial processes, significantly influencing gaseous nitrogen emissions and N cycling in agricultural soils. Identifying the specific fertilizer treatments that minimize or maximize gaseous nitrogen losses in varying soil salinity, may guide the selection of appropriate fertilization strategies for farmers and policymakers to mitigate environmental impacts of fertilizer use during agricultural production.