Comparison of background nitrous oxide emissions in agricultural and natural lands

When comparing simultaneously across all BNEA land-use types and BNEN ecosystem types, it was found that BNE from riparian, vegetable and intentional fallow areas were significantly higher than from boreal forests (P< 0.05). As mentioned above, riparian areas can receive abundant N input from surrounding crop fields and sustain anaerobic conditions, vegetable fields can have a history of previous N fertilizer additions, and intentional fallow fields register net N mineralization from soil organic matter in absence of N and water uptakes by crop plants, while boreal forests typically exhibit N limited status and they are also subjected to relatively cold temperatures.Other than boreal forests, riparian, vegetable, and intentional fallow sites, none of the other land-use or ecosystem types assessed in our meta-analysis showed significant effects on BNE. Likewise, when comparing overall BNEA vs. overall BNEN, no significant difference was found. This can suggest that although BNEN values tended lower than BNEA on median basis, we cannot generalize at this stage that BNE is different between agricultural and natural lands. Many factors can contribute to mask and confound these tendencies including interactions between land-use or ecosystem type with soil properties and climatic factors as well as previous land management history. For instance, natural lands typically exhibit higher soil organic C and N contents than agricultural lands (soil organic C medians = 45.7 vs. 23.7 g kg^-1, respectively). However, soil C:N ratios are slightly wider for natural than for agricultural lands (medians = 12.2 vs.11.2, respectively), and also natural lands exhibited much lower pH than agricultural lands (medians = 4.5 vs. 6.4, respectively). These observed variations in soil properties between natural and agricultural lands are not clearly explained. They might be caused by a differential influence of land-use or ecosystem types on these soil properties and/or conversely the result of a human-driven selection of the more fertile available lands (i.e., soils with high organic matter and neutral pH) for agricultural use.