Background nitrous oxide emissions in natural land (BNEN)

Globally, the BNEN ranged from -0.5 to 95.2 kg N₂O−N ha⁻¹ yr⁻¹, while median and mean of BNEN were 0.31 and 1.75 kg N₂O−N ha⁻¹ yr⁻¹, respectively. Of the edaphic-climatic variables assessed, only average annual air temperature show significant, weak correlation with the BNEN (ρ = 0.283, P < 0.001). This could reflect the increasing rate of N mineralization and denitrification at higher temperatures. This notion is also supported by regression analysis of BNE from tropical forest where BNE was found to be a function of annual air temperature. However, it is likely that other ecosystem factors may also interplay and determine BNE such as differences in N mobilization, and N limitation or saturation across ecosystems and biomes (Reich et al. 2005; Russell and Raich 2012). In general, previous observations indicate that certain tropical ecosystems exhibit rapid N cycling which could lead to increased N losses (Russell and Raich 2012) likely including increased BNE, while responses to atmospheric N addition in temperate and boreal regions become a function of the pre-existing N limitation or saturation status of the ecosystem (Vitousek et al. 1997). 


Amongst the ten different natural ecosystems assessed, BNE in riparian area (median and mean: 2.0 and 7.7 kg N₂O−N ha⁻¹ yr⁻¹, respectively) was significantly higher than in two other ecosystem types (i.e., boreal forest and tundra). Since NO₃⁻ concentrations decrease as a result of increased denitrification in the riparian areas (e.g., Groffman and Hanson 1997; Watts and Seitzinger 2000; Kim et al. 2009), it has been hypothesized that increased denitrification within riparian areas may increase N₂O emissions (e.g., Groffman et al. 1998; Hefting et al. 2006; Bradley et al. 2011). However, because of their landscape position, riparian zones typically receive water that has run-off or leached from neighbouring land. Therefore, riparian zone may receive significant amounts of NO₃⁻ load that originated in some other land type leading to higher N₂O emissions. The results from our meta-analysis showing significantly higher BNE in riparian areas is consistent with these existing studies.