What is greenhouse gas nitrous oxide?

Atmospheric N₂O contributes to both the greenhouse effect (Wang et al. 1976) and plays a precursor role in the ozone layer depletion (Crutzen 1970). Nitrous oxide has a relatively high global warming potential (i.e., 298 times greater than carbon dioxide in a 100-year time horizon; IPCC 2006; Forster et al. 2007). Use of N fertilizers and animal manure is recognized as the main anthropogenic source of N₂O with roughly 24% of total annual emissions (Bouwman 1996; Forster et al. 2007). Nitrous oxide emissions from natural lands comprise 55% of the total global N₂O emissions (Forster et al. 2007). 

Soil N₂O can be produced and consumed via different biological pathways. Aerobic autotrophic nitrification is the stepwise oxidation of ammonia (NH₃) to hydroxylamine (NH₂OH) as first intermediate, subsequently to nitrite (NO₂⁻) as second intermediate, and finally to nitrate (NO₃⁻) (Kowalchuk and Stephen 2001). Production of N₂O can occur because of enzymatic decompositions of the substrates NH₂OH and NO₂⁻ resulting in production of N₂O and other gaseous N species (Arp and Stein 2003; Baggs 2011). Heterotrophic denitrification is the stepwise reduction of NO₃⁻ to NO₂⁻, to nitric oxide (NO), to N₂O, and ultimately to dinitrogen (N₂), where NO₃⁻ as an electron acceptor in the respiration of organic material under low oxygen conditions (Knowles 1982). Although denitrification in soils is typically associated with anaerobiosis (Bremner 1997), denitrification and the associated enzymatic activity have been detected where aerobic conditions are reestablished after an anaerobic phase (Patureau 2000). In close association with denitrification, microbial co-denitrification can source N from organic co-substrates for increased N₂O production. Additional biosyntheses of the intermediate NO₂⁻ as substrate for soil N₂O production can also happen during nitrate ammonification (Baggs 2011) and nitrifier denitrification (Wrage et al. 2001) processes. Conversely, the respiratory reduction of N₂O to N₂ represents a biological consumption pathway, which in certain cases could result in soil uptake of atmospheric N₂O (Bremner 1997). These several N transformation processes can be coupled simultaneously or sequentially in soils and a wide variety of microbes typically participates.

What is background nitrous oixde emissions?

Background N₂O emissions are defined as N₂O emission from soils that received no nitrogen (N) fertilizer and soil management (e.g., Bouwan, 1996; Lu et al., 2006; Neftel et al., 2007; Gu et al. 2009; van Beek et al., 2011). Gu et al. (2009) noted that BNE in agricultural lands originates from residual N that has remained in the soil from N addition in previous years or seasons, and other N sources, such as biological N fixation, that are present in the soil. Crop residues can provide substrate for microbial activities such as nitrification and denitrification which produce N₂O (Aulakh et al., 1991; Huang et al., 2004; Miller et al., 2008). Also soil disturbance for agricultural activities can enhance N₂O emission through changing soil microclimate (i.e., soil temperature and moisture) and soil properties (i.e., bulk density, porosity and diffusivity) (Dobbie and Smith, 2001; Saggar et al., 2011). Kim et al. (2009) observed that soil temperature and soil moisture in crop fields were significantly different from adjacent grass lands and soils in crop fields had severe soil drought and frozen which caused peak N₂O emissions following rewetting and thawing events (Kim et al., 2012).  In animal grazing grassland, treading and trampling by the animals cause soil compaction, making the soil more anaerobic and stimulating denitrification activity, thus facilitating N₂O production (van Groenigen et al., 2005; Bhandral et al., 2007; Uchida et al., 2008). In agricultural land, magnitude of BNE may be influenced by the fertilization and soil management during previous years (Bouwan, 1996).

Questions on background N2O emissions?

Despite substantial contribution (10–52%) of BNE to the overall N₂O emissions in agricultural lands (Li et al. 2001; Brown et al. 2002; Sozanska et al. 2002; Yan et al. 2003; Lu et al. 2006; Gu et al. 2009; van Beek et al., 2011), current understanding of BNE has remained at a low level and this causes conflicting interpretation and application of IPCC guideline (IPCC, 2006) for agricultural N₂O inventories (de Klein et al., 2010). To improve the methodology for N₂O inventory and accuracy of N₂O emissions estimation, the following key questions should be urgently addressed: What are the sources of BNE? Is BNEA significantly different from BNEN? Is BNE significantly different by land-use type (i.e., crop lands, pasture, paddy fields) or ecosystem type (i.e., tropical forest, temperate forest, savannah)?  What are the key processes or key controlling factors for BNE?

Background nitrous oxide emissions in agricultural land (BNEA)

Globally, the BNEA ranged from -1.8 to 56.4 kg N₂O−N ha⁻¹ yr⁻¹ and median and mean of the BNEA were 0.70 and 1.52 kg N₂O−N ha⁻¹ yr⁻¹, respectively . The median BNEA was lower than the mean BNEA implies that the mean was notably influenced by the higher valued sites. It should be noted that the mean in our meta-analysis was influenced mainly by high values of BNEA in intentional fallow fields. In earlier studies, Bouwman (1996) and Yan et al. (2003) estimated a mean BNEA of 1.0 kg N₂O−N ha⁻¹ yr⁻¹ and 1.2 kg N₂O−N ha⁻¹ yr⁻¹ (only upland) in global scale, respectively. In our meta-analysis, median BNEA was lower and mean BNEA was higher than global mean values reported by Bouwman (1996) and Yan et al. (2003). Determined median (0.43 kg N₂O−N ha⁻¹ yr⁻¹) and mean BNEA (0.65 kg N₂O−N ha⁻¹ yr⁻¹) of rice paddy in our meta-analysis were lower than mean values reported by previous studies of Yan et al. (2003) (0.81 kg N₂O−N ha⁻¹ yr⁻¹) and Akiyama et al. (2005) (1.82 kg N₂O−N ha⁻¹ yr⁻¹).
 
Analysis of BNEA data by country revealed that country medians of BNEA ranged from -0.23 to 5.0 kg N₂O−N ha⁻¹ yr⁻¹. The BNEA medians for both Finland (5.0 kg N₂O−N ha⁻¹ yr⁻¹) and Germany (2.4 kg N₂O−N ha⁻¹ yr⁻¹) were much higher than for the other countries.
 
Comparing land-use types in our meta-analysis, the BNEA medians in both intentional fallow (2.06 kg N₂O−N ha⁻¹ yr⁻¹) and vegetable (1.70) fields were significantly higher than cropland, pasture, and rice paddy. The underlying causes for these differences are not completely clear; however, there are indications that residual N from earlier N fertilizer additions during previous years in vegetable and intentional fallow fields could be sufficient to increase BNE (Parkin and Kaspar 2006; Gu et al. 2009; Hernandez-Ramirez et al. 2009a). Moreover, soil N mineralization continues to occur in fallow fields but in the absence of active N uptake by crop plants, which result in increases of substrate for greater BNE (Sauer et al. 2009; Smith et al. 2011). The absence of crop plants in fallow fields also leads to shifts in soil thermal and water regimes that can frequently be associated with altered N cycling, enhanced N mineralization (Curtin et al. 2012), and increased BNE (Sauer et al. 2009). Additional quantification of BNE from fallow fields is crucial for revealing these key effects and controlling mechanisms of vegetation on BNE in a broad variety of ecophysical conditions. Future studies are required to unravel this complexity.

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.