AbstractCarbon (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.

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