Real time monitoring of N₂O emissions from agricultural soils using FTIR spectroscopy

Emissions of N₂O from agricultural soils are an important source for this greenhouse gas. The present work examines the potential of Fourier transformed infrared (FTIR) spectroscopy coupled with a long path (LP) infrared (IR) gas cell for on-line measurement of concentration and isotopic signature of N₂O emitted from soils. Nitrous Oxide was spectrally monitored during incubations of soil samples in a closed system under different conditions. Its emission from a Grumosol (Vertisol) was measured in presence and absence of acetylene, with various additions of nitrate and glucose, under aerobic and anaerobic conditions, and for two soil thickness layers. For comparison N₂O emissions from a Terra Rossa (Cambisol) and a Hamra (Luvisol) were measured in the presence and absence of acetylene. In an additional experimental set the isotopic signature of emitted N₂O was quantified after enrichment with K¹⁵NO₃. Acetylene addition led to an increase in N₂O emissions in all three soils but at various extents. Under aerobic conditions, N₂O emission from the Grumosol became detectable only when running the experiments with a thicker soil layer (10 mm), suggesting the existence of coupled nitrification- denitrification. Nitrate addition to soils enhanced N₂O emissions especially when coupled with glucose addition. Addition of ¹⁵NO ₃^- to the Grumosol resulted in the emission of all four N₂O isotopologues: ¹⁴N₂O, ¹⁵N ₂O, ¹⁴N¹⁵NO, and ¹⁵N¹⁴NO. The observed slight delay in appearance of the species containing ¹⁵N and the relatively lower ¹⁵N enrichment of the N ₂O compared with the soil nitrate, indicate isotopic fractionation during denitrification. Yet, within the accuracy of our isotopic analysis, temporal emission patterns of ¹⁴N¹⁵NO and ¹⁵N¹⁴NO were similar indicating low possibility for "site preference" under the specific experimental conditions.