Sensing Winter Soil Respiration Dynamics in Near-Real TimeAlexandra Contosta1, Elizabeth Burakowski1,2, Ruth Varner1, and Serita Frey3

1University of New Hampshire, Institute for the Study of Earth, Oceans, and Space, 2National Center for Atmospheric, Research, 3University of New Hampshire, Department of Natural Resources and the Environment

IntroductionWinter soil respiration plays a significant role in the global C cycle. Measurements of winter respiration and its drivers are rarely made in temperate areas despite predictions of reduced seasonal snow cover. Data collection is infrequent and is limited to the soil surface. We continuously sampled environmental drivers and CO2

fluxes from the soil profile, through the snowpack, and into the atmosphere in a temperate forest. These real-time, simultaneous measurements of snow and soil C loss and their drivers are unique in temperate areas and globally.

MethodsStudy Site. Research located at 100 year-old temperate forest, Durham, NH, USA that has an automated terrestrial sensor array controlled by a data logger (Figure 1). Data collected 2012-2014.

Carbon Dioxide Sampling System. Soil CO2 production (Psoil) and snow CO2 flux (Fsnow) determined with the flux gradient technique. Snow CO2 sequentially sampled every 12 s for 10 min from eight inlets on a snow tower, from 0-150 cm. Soil CO2 measured every 3 s at three nodes using sensors installed at -5, -15, and -30 cm at each node (Figure 2). Fluxes calculated at 80 min intervals.

Figure 1. Location of study site (A) and diagram of sensor system for sampling CO2 and environmental variables (B). Snow tower and soil CO2 sensor profiles are highlighted in red. The system also contains components for chamber measurements of CO2 flux, air, snow, and soil temperature, soil volumetric water content (VWC), precipitation, and aquatic physical and biological variables.

Artwork by: Janice Farmer

A B

Environmental Variables. Collected for estimating diffusion and modeling drivers of Fsnow and Psoil. Soil bulk density determined previously. Snow depth and density sampled daily (Figure 2). Air and soil temperature and soil VWC measured every 10 min. Soil variables measured at eight nodes at -5, -15, and -30 cm depth per node (Figure 1).

Figure 2. Snow tower (A) and snow pit for measuring snow depth and density (B). There is one snow tower at the site and three CO2 sensor profiles.

A B

Temporal Trends

Figure 3. (A) Snow depth; (B) snow water equivalent (SWE); (C) air temperature; (D) snow temperature; (E) soil temperature; and (F) soil VWC during the winters of 2012-2013 and 2013-2014. The topmost snow layer is designated as layer 1. Up to three deeper snow layers formed within each season, with layer 1 designated at the snow-soil interface.

Environmental Variables. Fluctuations were much greater than in high altitude / high latitude areas where most research on winter respiration has occurred (Figure 3).

CO2 Dynamics. Fsnow was lower but more dynamic than Psoil. Fsnow was highest in late winter when snow pack was greatest. Psoil peaked in early winter with warmer, drier soil conditions.

Figure 4. Soil CO2 production (Psoil) and snow CO2 efflux (Fsnow) during the winters of 2012-2013 and 2013-2014.

Psoil

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Environmental Drivers of CO2 Dynamics

Fsnow was positively correlated with soil temperature and was negatively related to VWC. Psoil was negatively correlated with both soil temperature and VWC (Figure 4A and B).

Figure 4. Correlations between (A) Soil temperature at -5 cm and Fsnow or Psoil; (B) Soil VWC at -15 cm and Fsnow or Psoil.

Modeling Snow and Soil CO2 DynamicsSWE, air temperature, soil temperature, and soil VWC were significant predictors of Fsnow, with VWC playing a dominant role (Figure 5). Only air temperature and soil VWC were drivers of Psoil, where higher VWC resulted in lower Psoil. Time series analysis showed that Fsnow lagged 40 days behind Psoil. This lag may be due to slow CO2 diffusion through soil to overlying snow in high VWC conditions.Figure 5. Multiple regression results for environmental drivers versus Fsnow and Psoil , and lags between Fsnow and Psoil. Standardized regression coefficients indicate the relative importance of each predictor variable.

Conclusions: Surface soil CO2 losses through the snowpack were driven by rapid changes in snow cover, surface temperature, and surface VWC while winter soil CO2 production was regulated by subsurface VWC.

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Acknowledgements: Funding was provided by the New Hampshire Agricultural Experiment Station, New Hampshire EPSCoR Ecosystems and Society, and the University of New Hampshire ADVANCE Collaborative Scholars Award.

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