Wavelet Spectra of Nitrous Oxide Emission from Hummocky Terrain during Spring Snowmelt

Abstract

Soil N2O emission data is typically highly skewed. In terrain where the spatial distribution of soil processes is controlled by topography, extreme N2O flux events can be highly localized and nonstationary. Wavelet analysis can be used to describe the spatial variation of these nonstationary processes. The objectives of this study were to use wavelet analysis to determine the spatial variation, scales of variability and their change over the snowmelt period for soil N2O flux. On a hummocky, agricultural landscape in the Dark Brown soil zone of Saskatchewan, N2O flux measurements were taken from a 128‐point linear transect five times over the spring snowmelt season of 2004. Localized variance was determined using a continuous wavelet transform (Mexican Hat) and the local spectrum was compared with the distribution of fluxes along the transect and the relative elevation. Two spatial patterns of soil N2O emission were revealed. The first was a cyclic, landscape‐element‐controlled pattern with a scale of variation that ranged between 20 and 60 m. Changes in the spatial scale were due to a shift in importance between landscape elements as sources of peak N2O flux. The second pattern was composed of non‐cyclic, localized features that were due to extreme flux events at specific landscape positions. These extreme flux events were not temporally persistent, but represent a large, non‐random contribution to mean and variance on the dates they occurred. Sample strategies to capture the full range of soil N2O emission at this site would probably require separate approaches for these two spatial patterns.