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.