Multiscale legacy responses of soil gas concentrations to soil moisture and temperature fluctuations

Abstract:

The sensitivity of soil carbon dynamics to climate change is a major uncertainty in carbon cycle models. Of particular interest is the response of soil biogeochemical cycles to variability in hydroclimatic states and the related quantification of soil memory. Toward this goal, the power spectra of soil hydrologic and biogeochemical states were analyzed using measurements of soil temperature, moisture, oxygen, and carbon dioxide at two sites. Power spectra indicated multiscale power law scaling across sub-hourly to annual timescales. Precipitation fluctuations were amplified by the soil biogeochemical system at monthly to annual timescales and soil moisture fluctuations were dominant across all frequencies. Soil temperature fluctuations were attenuated across all frequencies at both sites, but at one site were comparable in strength to soil moisture at sub-daily frequencies. The effect of soil hydrologic, thermal, and biogeochemical processes on gas concentration variability was evidenced by low spectral entropy relative to the white noise character of precipitation. A full mass balance model was unable to capture high-frequency soil temperature influence, indicating a gap in commonly used model assumptions. A linearized model driven by soil moisture only was shown to capture the main features of the observed and modeled gas concentration spectra. Breakpoints in the spectra corresponded to the mean diffusion rate, providing a first-order estimate of the soil biogeochemical integral timescale (∼ 1−4 hours). These methods can be used to identify biogeochemical system dynamics to develop robust, process-based soil biogeochemistry models that capture variability in addition to long-term mean values.