Snowpack-atmosphere gas exchanges of carbon dioxide, ozone, and nitrogen oxides at a hardwood forest site in northern Michigan

Brian Seok,Detlev Helmig,Mark W Williams,Christoph S Vogel,Daniel Liptzin,Joel D Blum

doi:10.12952/journal.elementa.000040

Abstract

Abstract Snowpack-atmosphere gas exchanges of CO2, O3, and NOx (NO + NO2) were investigated at the University of Michigan Biological Station (UMBS), a mid-latitude, low elevation hardwood forest site, during the 2007–2008 winter season. An automated trace gas sampling system was used to determine trace gas concentrations in the snowpack at multiple depths continuously throughout the snow-covered period from two adjacent plots. One natural plot and one with the soil covered by a Tedlar sheet were setup for investigating whether the primary source of measured trace gases was biogenic (i.e., from the soil) or non-biogenic (i.e., from the snowpack). The results were compared with the “White on Green” study conducted at the Niwot Ridge (NWT) Long Term Ecological Research site in Colorado. The average winter CO2 flux ± s.e. from the soil at UMBS was 0.54 ± 0.037 µmol m-2 s-1 using the gradient diffusion method and 0.71 ± 0.012 µmol m-2 s-1 using the eddy covariance method, and in a similar range as found for NWT. Observed snowpack-O3 exchange was also similar to NWT. However, nitrogen oxides (NOx) fluxes from snow at UMBS were 10 times smaller than those at NWT, and fluxes were bi-directional with the direction of the flux dependent on NOx concentrations in ambient air. The compensation point for the change in the direction of NOx flux was estimated to be 0.92 nmol mol-1. NOx in snow also showed diurnal dependency on incident radiation. These NOx dynamics in the snow at UMBS were notably different compared to NWT, and primarily determined by snow-atmosphere interactions rather than by soil NOx emissions.

Highlights

Winter historically has been perceived as a period of suppressed biogeochemical activity (Campbell et al, 2005)
nitrogen oxides (NOx) in snow showed diurnal dependency on incident radiation. These NOx dynamics in the snow at University of Michigan Biological Station (UMBS) were notably different compared to Niwot Ridge (NWT), and primarily d etermined by snow-atmosphere interactions rather than by soil NOx emissions
It was assumed that trace-gas exchange between the soil and the atmosphere halted with snow cover or when soil temperatures dropped to below 0°C (Steudler et al, 1989; Bouwman, 1990)

Summary

Introduction

Winter historically has been perceived as a period of suppressed biogeochemical activity (Campbell et al, 2005). Subsequent and recent studies showed that elevated CO2 concentration in the snowpack are not limited to high latitude or high elevation sites These studies showed how snow insulates the soil from sub-freezing ambient air temperatures (Groffman et al, 2001), that microbial activity under snow can stay active until soil temperatures reach below -5°C (Brooks et al, 2005), and that soil microbial communities during snow cover can be different from the growing season (Monson et al, 2006b). These findings challenged the traditional view of winter with respect to the annual biogeochemical cycle

Methods

Results

Discussion

Conclusion