WaveRet analysis of wintertime and spring thaw CO2 and N2O fluxes from agricultural fields

Fluxes of N2O and CO2 are not limited to the growing season; winter and spring thaw can represent a significant emission period. The objective of this study was to apply wavelet analysis to winter and spring thaw CO2 and N2O fluxes and soil temperatures, to yield additional information about underlying processes, examining temporal patterns and relationships among them. Fluxes used in this analysis were measured over 4 years using micrometeorological methods, in a study comparing two agricultural management practices, best management (BM) and conventional (CONV) practices. Cross-wavelet transform (XWT) and wavelet coherence (WCO) were applied to daily mean time series of N2O fluxes for BM and CONV replicates and treatments, CO2 vs. N2O fluxes, CO2 flux vs. air and soil temperatures, and N2O flux vs. air and soil temperatures. N2O fluxes for replicate plots had small differences in temporal variation while N2O fluxes from BM and CONV treatments showed a large difference in their time series. XWT and WCO analysis confirmed differences in N2O fluxes between management practices due to differences in temporal trends in the time series. Field emissions of N2O and CO2 fluxes showed times of common high fluxes, such as thaw events. Nitrous oxide and CO2 flux time series showed a strong coherence with surface (air) temperatures. The relationship between N2O fluxes and temperature decreased with depth but the relationship between CO2 flux and temperature was similar for surface and at depth. The strong coherence between emissions and surface conditions does not support the suggested mechanism of trapped gas release. A release of trapped gases from below the ice formation would have been indicated by a strong coherence from CO2 and N2O with temperatures at depth as the trapping ice barrier melted. This study demonstrates the effectiveness of wavelets as a tool to investigate temporal relationships in GHG emissions, which is a relatively new application for this type of analysis. (C) 2008 Elsevier B.V. All rights reserved.