Quantification of N2O fluxes from soil–plant systems may be biased by the applied gas chromatograph methodology
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With regard to measuring nitrous oxide (N2O) emissions from biological sources, there are three most widely adopted methods that use gas chromatograph with an electron capture detector (GC–ECD). They use: (a) nitrogen (N2) as the carrier gas (DN); (b) ascarite as a carbon dioxide (CO2) trap with DN (DN-Ascarite); and (c) a mixture gas of argon and methane as the carrier (AM). Additional methods that use either a mixture of argon and methane (or of CO2 and N2) as a make-up gas with the carrier nitrogen or soda lime (or ascarite) as a CO2 trap with the carrier helium have also been adopted in a few studies. To test the hypothesis that the use of DN sometimes considerably biases measurements of N2O emissions from plants, soils or soil–plant systems, experiments were conducted involving DN, AM and DN-Ascarite. When using DN, a significant relationship appeared between CO2 concentrations and the apparent N2O concentrations in air samples. The use of DN led to significantly overestimated N2O emissions from detached fresh plants in static chamber enclosures. Meanwhile, comparably lower emissions were found when using either the DN-Ascarite or AM methods. When an N2O flux (from a soil or a soil–plant system), measured by DN in combination with sampling from the enclosure of a static opaque chamber, was greater than 200 μg N m−2 h−1, no significant difference was found between DN and DN-Ascarite. When the DN-measured fluxes were within the ranges of <−30, −30–0, 0–30, 30–100 and 100–200 μg N m−2 h−1, significant differences that amounted to −72, −22, 5, 38 and 64 μg N m−2 h−1, respectively, appeared in comparison to DN-Ascarite. As a result, the DN measurements in rice–wheat and vegetable fields overestimated both annual total N2O emissions (by 7–62%, p < 0.05) and direct emission factors for applied nitrogen (by 6–65%). These results suggest the necessity of reassessing the available data determined from DN measurements before they are applied to inventory estimation. Further studies are required to explore appropriate approaches for the necessary reassessment. Our results also imply that the DN method should not be adopted for measuring N2O emissions from weak sources (e.g., with intensities less than 200 μg N m−2 h−1). In addition, we especially do not recommend the use of DN to simultaneously measure N2O and CO2 with the same ECD.
KeywordsNitrous oxide Emissions Gas chromatography Carrier gas Soil Plant Inventory
Gas chromatograph equipped with an electron capture detector.
A GC–ECD method that uses pure N2 alone as the carrier gas but does not introduce a high-concentration make-up gas with an electronegative group/atom or with an ionization potential lower than that of N2 (such as carbon dioxide or methane) into the detector while allowing carbon dioxide (CO2) in the air samples to enter the detector.
Different from the DN method only in that an ascarite (a type of sodium-hydroxide-coated silica) filter column is connected to the beginning of the separation column. As a result of this ascarite filter, CO2 and water vapor in the air samples cannot enter the detector.
A GC–ECD method that uses an argon–methane mixture instead of pure N2 as the carrier gas while having the same GC configuration as the DN method.
A GC–ECD method that uses N2 as the carrier gas and directly introduces a mixture of methane and argon (5–10% CH4 in Ar) into the detector as a make-up gas.
A GC–ECD method that uses N2 as the carrier gas and directly introduces a mixture of CO2 and N2 into the detector as a make-up gas.
A GC–ECD method that uses He as the carrier gas, along with an ascarite (or soda lime) filter column connected to the beginning of the separation column.