Abstract

We studied the in situ relative contribution of soil nitrous oxide (N2O) emissions from already discovered pathways, namely the nitrification, nitrifier denitrification (ND), and heterotrophic denitrification (HD). We selected a transect from upland pastures to lowland riparian forests growing on volcanic ash soils. We used the acetylene blocking technique, and N2O was measured by gas chromatography. We also measured rates of potential organic matter mineralization and potential nitrification in laboratory. Potential mineralization increased from uplands to lowlands, while potential nitrification decreased in the same transect. In riparian forests, N2O emissions under acetylene blocking were higher than without acetylene, revealing that 22-31% of nitrogen emissions consisted of dinitrogen. Conversely, in pastures, N2O emissions under acetylene blocking were lower than without acetylene. Our results suggest that HD predominates in waterlogged, anoxic, and acidic soils with no nitrification, which are typical conditions for riparian soils. Conversely, N2O from nitrification and ND predominates in more oxic environments that are less water-saturated, have less acidic soils, and a larger nitrification rate than riparian areas (i.e., typical conditions for pasture soils). Thus, processes related to the nitrogen cycle depend on the land cover/use and soil physical-chemical properties, even in vegetation zones growing on adjacent soil types. Our results imply that different mitigation measures must be implemented to abate N2O emissions in different soil environments.