Emissions of nitrous oxide, dinitrogen and carbon dioxide from three soils amended with carbon substrates under varying soil matric potentials

Carbon (C) substrates are critical for regulating denitrification, a process that results in nitrous oxide (N₂O) and dinitrogen (N₂) emissions from soil. However, the impacts of C substrates on concomitant soil emissions of carbon dioxide (CO₂) and N₂O under varying soil types and soil water contents are not well studied. Three repacked Pallic grassland soils, varying in texture and phosphorus (P) status, containing NO₃⁻-¹⁵N were held at three levels of matric potential (ψ, −3, −5 and −7 kPa), while receiving daily substrate additions (acetate, glucose and water control) for 14 days. The CO₂ and N₂O emissions were measured daily. Additionally, the N₂O:(N₂ + N₂O) ratios were determined using ¹⁵N on days 3 and 14. Results showed that N₂O emissions increased exponentially as soil gas diffusivity declined, and N₂O peak emissions were higher with glucose than with acetate addition, with a range (± standard deviation) of 0.1 ± 0.0 to 42.7 ± 2.1 mg N m⁻² h⁻¹. The highest cumulative N₂O emission (2.5 ± 0.2 g N m⁻²) was measured following glucose addition with a soil ψ of −3 kPa. In comparison with added glucose, acetate resulted in a twofold increase in N₂ emissions in soils with relatively low gas diffusivities. The N₂O:(N₂O + N₂) emissions ratios varied with substrate (glucose, 0.91; acetate, 0.81) on day 3, and had declined by day 14 under substrate addition (≤0.10). Cumulative CO₂ emissions were enhanced with increasing soil gas diffusivity and were higher for soils amended with glucose (ranging from 22.5 ± 1.3 to 36.6 ± 1.8, g C m⁻²) than for those amended with acetate. Collectively, the results demonstrate that the increase of N₂O, N₂ and CO₂ emissions and changes in the N₂O:(N₂ + N₂O) ratio vary over time in response to C substrate type and soil gas diffusivity. Highlights: Co-regulation of CO₂ and N₂O emissions was assessed for varying soil types and C substrates. Soil diffusivity explained concurrently cumulative N₂O and CO₂ emissions. Acetate enhanced N₂O reduction to N₂ in three grassland soils more than glucose. C substrate effects on soil N₂O, N₂ and CO₂ emissions were soil type specific.