With regard to measuring nitrous oxide (N 2 O) 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 (N 2 ) as the carrier gas (DN); (b) ascarite as a carbon dioxide (CO 2 ) 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 CO 2 and N 2 ) as a make-up gas with the carrier nitrogen or soda lime (or ascarite) as a CO 2 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 N 2 O 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 CO 2 concentrations and the apparent N 2 O concentrations in air samples. The use of DN led to significantly overestimated N 2 O 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 N 2 O 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 Plant Soil (vegetable fields overestimated both annual total N 2 O 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 N 2 O 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 N 2 O and CO 2 with the same ECD.

There is an ongoing discussion of the possible effects of nitrogen (N) application on methane (CH 4 ) emission from rice fields. However, the Intergovernmental Panel on Climate Change (IPCC) methodologies for estimating the national inventory of CH 4 emission from paddy rice production do not consider the effects of N addition. To assess the lack of knowledge about N addition effects on inventory estimates, we recently launched a multi-site observation campaign in major rice cultivation regions of China. The observations showed that, across various climate zones, the application of ammonium-based fertilizers at the commonly-adopted levels for fields in China (150 or 250 kg N ha −1 ) generally inhibited accumulative CH 4 emission during rice season (by 28-30% on average) as compared to no N addition. An increase in application from the moderate level of 150 kg N ha −1 to the high rate of 250 kg N ha −1 did not significantly modify CH 4 emission. Our results suggest that disregarding the effect of N fertilization by the IPCC methodologies may not significantly bias CH 4 inventory estimates of China. In regions with much lower N addition levels, however, disregarding the effect of N fertilization may result in the underestimation of Plant Soil (2010) 326:393-401 regional CH 4 emission, since these emissions were mainly derived from studies in regions with relatively high N addition rates.

No abstract