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THE CARBON CYCLE AND ARCTIC SOILS

Arctic permafrost soils hold stores of organic carbon which amount to more than twice the carbon currently in the atmosphere, or almost 1,700 Gigatons (Schuur et al., 2015). Soil respiration releases CO2 into the atmosphere and represents an important contribution to the overall carbon cycle, but it is difficult to measure in cold winter months in arctic regions. Due to the accumulation of snow and ice, most available automated chamber methods for monitoring CO2 fluxes which require freedom of movement, cannot be used. Additionally, low light conditions make it difficult to ensure sufficient power for equipment and access for maintenance or observation is difficult or impossible. For these reasons winter soil respiration is rarely measured and normally only done using chemical traps (i.e. Nobrega and Grogan, 2007) or similar non-automated methods thereby leaving large data gaps. However, given the amount of carbon stored in arctic soils and the long winter season, even small amounts of winter soil respiration could represent important contributions to the carbon cycle so a better understanding of over-winter CO2 fluxes from these soils is critical.

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Aerial photo of tundra landscape near Daring Lake Eco Tundra Research Station, NT, Canada

WHERE EOSENSE COMES IN?
The Eosense eosFD soil flux sensor was designed specifically for the challenges encountered in harsh environments like the Arctic. Using the Forced Diffusion (FD) method, the eosFD chambers continuously monitor CO2 fluxes without moving parts. This is achieved by using a membrane of a known flow rate, or diffusivity, which, in addition to measurements of CO2 concentration within the eosFD chamber, allows for calculation of CO2 emission rates (Risk et al., 2011). Aside from the membrane based approach, the eosFD offers other benefits for use in harsh, remote environments including; low power consumption (<1.6W), a weatherproof housing, ability to operate in temperatures from -20°C to +50°C, and internal data logging capabilities. While the eosFD has many design features that make it ideal for arctic CO2 flux monitoring, making accurate measurements in these harsh  environments can still prove difficult, especially if some aspect of the device unexpectedly perturbs the natural ecosystem thereby affecting the fluxes. In particular – despite the low power consumption of the instrument – there is potential for the eosFD to impact the snowpack environment through heat generation, which is where Dr. Ronald Layden of Aurora Research Institute (ARI) took over.

eosFD soil flux sensor deployed in summer near the new Inuvik to Tuktoyaktuk Highway

eosFD soil flux sensor deployed in summer near the new Inuvik to Tuktoyaktuk Highway

LAYDEN LABORATORY OBJECTIVES
Dr. Layden and the ARI set out to determine the best way to make accurate soil respiration  measurements using the Eosense eosFD, in winter, in Inuvik, NWT, Canada (68.36° N, 133.72° W). The main objective of this study was to determine if the eosFD device itself could contribute to microclimate changes that might alter the measurement results, most likely through internal heat generation. In addition the researchers hoped to obtain a set of usable flux data covering the winter months. The intervals and power consumption chosen for this study are outlined in the table below.

Interval  Max. Power  Avg. Power
5 min 3.6 W 1.4 W
10 min 1.0 W
60 min 0.7 W

Varying the measurement interval alters the power consumption and thus the heat produced by each unit.

In order to eliminate the need for solar and battery power, and to ensure data coverage throughout the winter, the eosFD soil flux sensors were set up beside the Aurora Research Institute (ARI) facility in Inuvik and connected to continuous line power. The location consisted of a disturbed gravelly area with a thin soil layer and some herbaceous vegetation. The summer vegetation is mostly colonizing species with a mixture of weeds and early successional species including foxtail and Yarrow none more than than 0.5 m in height.

Photo from above eosFD device set to record every five minutes. Note the 1-2 cm cavitation around the device.

Photo from above eosFD device set to record every five minutes. Note the 1-2 cm cavitation around the device.

Most equipment was excavated from under snow cover in late January 2017 with the assistance of Environment and Natural Resources Technology Program (ENRTP) students from the Aurora College on a clear winter day with temperatures around -35°C. The equipment had been running continuously since late August 2016. Visual observations indicated that devices sampling every 5 minutes (~1.6 W average power consumption) warmed and melted the surrounding snow into ice approximately 1 to 2 cm around the cylindrical circumference (shown bottom left). Sampling at 10-min frequency (power draw 1.0 W) produces less, but still noticeable, cavitation whereas at 60 minutes (power draw 0.7 W) no cavitation was observed but some snow around the sensor appeared to be slightly granulated. Based on the observations of the snow properties surrounding the various devices and considering the power consumption of the eosFD units as a function of sampling interval (table left) provides insight on the sampling interval limits. Intervals of greater than 10 minutes should be used to avoid cavitation and ice formation around the equipment. Intervals of more than 1 hour might further improve the microclimatic impact but need to be evaluated, because the temporal resolution loss and associated loss of statistical power from longer sampling intervals may outweigh the benefits of reduction in microenvironmental effects. Carbon dioxide fluxes from the 3 different sample intervals are shown on the time series plot on the next page. Sensors with measurement intervals that showed noticeable cavitation also tended to measure lower fluxes after snowfall, possibly due to ice formation on the ground surface which may have slowed emissions of CO2 from the soil surface below the sensor. Further support for this comes from the observation that the largest difference between the 60 minute sensor and the others occurs around the same time that snow has significantly accumulated – thereby limiting heat dissipation through convection. Based on separate temperature loggers deployed on the ground and in the air at the site and webcam pictures (data not shown), we noted that snow covered the equipment and provided insulation before the temperature dropped much below -20°C.

Soil flux measured at 5 min (blue), 10 min (green, purple) and 1 hour intervals (orange) using 4 different eosFD devices placed outside the ARI building in Inuvik. One unit (purple) was left undisturbed and ran from August, 2016 to May 2017. The other 3 were excavated for observation in late January 2017. The red line shows is the temperature as measured hourly from Inuvik airport (~12 km away). The photo and arrow show first day of total snow cover over the equipment as recorded by a webcam monitoring the site (~December 3rd at 2pm).

Soil flux measured at 5 min (blue), 10 min (green, purple) and 1 hour intervals (orange) using 4 different eosFD devices placed outside the ARI building in Inuvik. One unit (purple) was left undisturbed and ran from August, 2016 to May 2017. The other 3 were excavated for observation in late January 2017. The red line shows is the temperature as measured hourly from Inuvik airport (~12 km away). The photo and arrow show first day of total snow cover over the equipment as recorded by a webcam monitoring the site (~December 3rd at 2pm).

LET’S GO BIGGER!
Canada’s Northwest Territories represents one of the largest northern jurisdictions in the world (1,346,106 km2) with few inhabitants (44,263 inhabitants, 2017 estimate) and mostly remote and undisturbed tundra, taiga, lakes and rivers. This unspoiled natural terrain makes the NWT an ideal place to study natural processes and climate change without significant human habitation and similar confounding factors. Pilot studies like this one are helping us to understand how to measure winter respiration. In future we plan to conduct similar studies on natural terrain sites across the NWT to better understand the impact of climate warming in the Arctic and its effects on permafrost and carbon stores. The NWT is experiencing climate warming at a rate 5 times faster than other areas of the planet according to data published by the GNWT website (http://bit.ly/2xcfbMn) and experiencing temperatures as much as 4°C warmer in winter in some areas of NWT over the last 50 years.

REFERENCES
Schuur et al., 2015. Climate change and the permafrost carbon feedback, Nature 520: 171-179.
S. Nobrega, P. Grogan, 2007, Deeper Snow Enhances Winter
Respiration from Both Plant associated and Bulk Soil Carbon Pools in Birch Hummock Tundra, Ecosystems 10: 419-431.
Risk et al., 2011, Forced Diffusion soil flux: A new technique for continuous monitoring of soil gas efflux, Agricultural and Forest Meteorology 151 (12): 1622-1631.

ACKNOWLEDGMENTS
Thanks to Dr. Ronald Layden (Manager, North Slave Research Centre, Aurora Research Institute, Yellowknife) for co-authoring this case study, and his colleagues Erica Hille and Edwin Amos in Inuvik and Chris MacIntyre (St. Francis Xavier University) for assistance in the field. This work was completed with support from the NSERC College and Community Innovation Fund to Dr. Layden. This study was conducted under research license 15929.