Abstract
Abstract. Winter soil carbon dioxide (CO2) respiration is a significant and understudied component of the global carbon (C) cycle. Winter soil CO2 fluxes can be surprisingly variable, owing to physical factors such as snowpack properties and wind. This study aimed to quantify the effects of advective transport of CO2 in soil–snow systems on the subdiurnal to diurnal (hours to days) timescale, use an enhanced diffusion model to replicate the effects of CO2 concentration depletions from persistent winds, and use a model–measure pairing to effectively explore what is happening in the field. We took continuous measurements of CO2 concentration gradients and meteorological data at a site in the Cape Breton Highlands of Nova Scotia, Canada, to determine the relationship between wind speeds and CO2 levels in snowpacks. We adapted a soil CO2 diffusion model for the soil–snow system and simulated stepwise changes in transport rate over a broad range of plausible synthetic cases. The goal was to mimic the changes we observed in CO2 snowpack concentration to help elucidate the mechanisms (diffusion, advection) responsible for observed variations. On subdiurnal to diurnal timescales with varying winds and constant snow levels, a strong negative relationship between wind speed and CO2 concentration within the snowpack was often identified. Modelling clearly demonstrated that diffusion alone was unable to replicate the high-frequency CO2 fluctuations, but simulations using above-atmospheric snowpack diffusivities (simulating advective transport within the snowpack) reproduced snow CO2 changes of the observed magnitude and speed. This confirmed that wind-induced ventilation contributed to episodic pulsed emissions from the snow surface and to suppressed snowpack concentrations. This study improves our understanding of winter CO2 dynamics to aid in continued quantification of the annual global C cycle and demonstrates a preference for continuous wintertime CO2 flux measurement systems.
Highlights
The global soil carbon (C) pool stores three times the amount of C as the atmosphere
Trace amounts of snow at NM1 and NM2 began accumulating at the beginning of data collection (11 November 2013), with appreciable (> 25 cm) snowfall at both stations occurring on 15 December 2013 and remaining through the winter
The corresponding linear regression (Fig. 3b) shows the effect that average wind speed exerted on CO2 concentration (R2 = 0.70, P < 0.001)
Summary
The global soil carbon (C) pool stores three times the amount of C as the atmosphere. Seasonal variation in soil CO2 fluxes is not always discussed in meta-analyses of global soil C studies, whether or not wintertime measurements were included in individual studies (Scharlemann et al, 2014). Despite this skewed focus, soil CO2 is still produced throughout the winter, even at −7 ◦C (Flanagan and Bunnell, 1980; Coxson and Parkinson, 1987; Brooks et al, 1996). There has been an observed decrease in northern hemispheric snow cover and an earlier onset of spring melt since the 1950s as a result of climate change (Dyer and Mote, 2006; IPCC, 2013). There has been an observed decrease in northern hemispheric snow cover and an earlier onset of spring melt since the 1950s as a result of climate change (Dyer and Mote, 2006; IPCC, 2013). Dyer and Mote (2006) indicated that these changes in snow cover are Published by Copernicus Publications on behalf of the European Geosciences Union
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