Soil NO3− level and O2 availability are key factors in controlling N2O reduction to N2 following long-term liming of an acidic sandy soil

Abstract Liming of acidic soils has been suggested as a strategy to enhance N 2 O reduction to N 2 during heterotrophic denitrification, and mitigate N 2 O emission from N fertilised soils. However, the mechanisms involved and possible interactions of key soil parameters (NO 3 − and O 2 ) still need to be clarified. To explore to what extent soil pH controls N 2 O emissions and the associated N 2 O/(N 2 O + N 2 ) product ratio in an acidic sandy soil, we set-up three sequential incubation experiments using an unlimed control (pH 4.1) and a limed soil (pH 6.9) collected from a 50-year liming experiment. Interactions between different NO 3 − concentrations, N forms (ammonium- and nitrate) and oxygen levels (oxic and anoxic) on the liming effect of N 2 O emission and reduction were tested in these two sandy soils via direct N 2 and N 2 O measurements. Our results showed 50-year liming caused a significant increase in denitrification and soil respiration rate of the acidic sandy soil. High concentrations of NO 3 − in soil (>10 mM N in soil solution, equivalent to 44.9 mg N kg −1 soil) almost completely inhibited N 2 O reduction to N 2 (>90%) regardless of the soil pH value. With decreasing NO 3 − application rate, N 2 O reduction rate increased in both soils with the effect being more pronounced in the limed soil. Complete N 2 O reduction to N 2 in the low pH sandy soil was also observed when soil NO 3 − concentration decreased below 0.2 mM NO 3 − . Furthermore, liming evidently increased both N 2 O emissions and the N 2 O/(N 2 +N 2 O) product ratio under oxic conditions when supplied with ammonium-based fertiliser, possibly due to the coupled impact of stimulated nitrification and denitrification. Overall, our data suggest that long-term liming has the potential to both increase and decrease N 2 O emissions, depending on the soil NO 3 − level, with high soil NO 3 − levels overriding the assumed direct pH effect on N 2 O/(N 2 +N 2 O) product ratio.