Emissions and spatial variability of N2O, N2 and nitrous oxide mole fraction at the field scale, revealed with 15N isotopic techniques

The accurate measurement of nitrous oxide (N2O) and dinitrogen (N2) during the denitrification process in soils is a challenge which will help to estimate the contribution of soil N2O emissions to global warming. Oxygen concentration, nitrate concentration and carbon availability are generally the main factors that control soil denitrification rate and the amount of N2O or N2 emitted. The aim of this paper is to present a database of the N2O mole fraction measured at the field scale, and to test hypotheses concerning its regulation. A 15N-nitrate tracer solution was added to 36 undisturbed soil cores on a 20 m×20 m cultivated field plot. Fluxes of CO2, N2O and N2 from the soil surface were monitored for 24 h. Soil moisture, bulk density, carbon, nitrogen and mineral nitrogen concentration were also measured to investigate possible spatial relationships between their variations and those of N2O, N2 and nitrous oxide mole fraction. Under high water content, nitrous oxide and N2 emissions were highly variable with variation coefficients of 70–140%. N2O emission rates were about twice as high as those of N2, with a total denitrification rate ranging from 269 to 3843 g N ha−1 d−1. After 24 h of incubation, the values of nitrous oxide mole fraction ranged from 0.15 to 0.94 and no significant decline during incubation time was observed. Spatial variability of N2O, N2 and nitrous oxide mole fraction was high and no spatial dependence was observed at the scale of the experimental plot. Only tenuous relationships between gaseous nitrogen emissions and soil properties (mainly nitrate concentration and moisture content) were found. Meanwhile, a positive correlation was observed between N2 and CO2 emissions. This result supports the hypothesis that an increase in soil available organic carbon leads to N2 emissions as the end product of denitrification.