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UNDERSTANDING CARBON DYNAMICS
Globally, soils are the largest terrestrial contributor of carbon (C) flux to the atmosphere. Carbon is released from soils mainly in the form of carbon dioxide (CO2) emitted as a result of microbial and plant processes, but also as methane (CH4 ), a potent greenhouse gas with 28 times the warming potential of CO2.  The production of CO2 and CH4 in soils is closely connected to physical drivers like temperature and soil water content, which may change in response to climate change. Ecosystems like heathlands and grasslands cover large areas of the Northern hemisphere and also store large amounts of C in the soil.  At the CLIMAITE climate change research facility in Brandbjerg, Denmark, researchers from the University of Copenhagen are trying to understand C dynamics under present and manipulated climate conditions.

 

eosAC automated chambers beneath a permanent rain shelter (50% rain removal) at Brandbjerg.

THE BRANDBJERG SITE
Brandbjerg is located 60 km west of Copenhagen on a former glacial outwash plain overlain by wind deposited sand. The soil is very well-drained and nutrient poor. The site is characterized as a heath- and grassland co-dominated by heather (Calluna vulgaris) and grass (Deschampsia flexuosa). The climate at the site is temperate with a mean annual temperature of 8°C and annual precipitation close to 800 mm. From 2005-2013 a climate change experiment involving increased atmospheric CO2 concentration, temperature and drought took place at the site. Starting in 2016, a new experimental set-up was introduced with a series of permanent rainout shelters removing 40%, 50% and 66% of the annual precipitation. The Brandbjerg site is part of the AnaEE Denmark consortium (www.anaee.dk).

eosAC chambers monitoring the control plot (Oct. 30 – Dec. 12)

WHERE EOSENSE COMES IN
In the current study eosAC chambers were used to gain a better understanding of the CO2 and CH4 fluxes from the soil in late autumn and winter. As part of the study, CO2 and CH4 fluxes were observed in drought (50% precipitation removal) and control plots. The eosAC has been especially helpful with measuring in detail the temporal variability of CO2 and CH4 during a period of the year where good estimates of soil respiration are often lacking. These data can help disentangle the soil temperature and moisture control on soil-atmosphere exchange of carbon in cold periods. These data are furthermore hard to come by, and although fluxes may be low, the eosAC connected to a Los Gatos Research UGGA provides the necessary precision for high quality data.

FINDINGS THUS FAR
The CO2 fluxes  displayed a clear decrease across the measurement period. All chambers start with CO2 fluxes at or above 1 𝝻mol CO2 m-2 s-1 and subsequently plateau at values between 0.4-0.8 𝝻mol CO2 m-2 s-1 from the beginning of December onward. Comparison of the CO2 fluxes in the drought and control plots indicates a consistent decrease in CO2 efflux under drought conditions. This is in line with previous manual chamber measurements in the drought plots, where a decrease in soil respiration rates has been observed during spring measurements (March to April – data not published). The temporal variability of CO2 effluxes in the drought treatment is also smaller than the variability observed in the control plots. However, Chambers 3 and 4 also show different fluxes, which can be attributed to spatial variation at the site. Therefore, it is not entirely conclusive from this data that the drought decreases the C  turnover in the soil as the difference we observe may still be due to natural spatial variability. CH4 fluxes were consistently negative throughout the measurement period, showing that the soil continued to act as a CH4 sink in the cold period of the year. Over time CH4 fluxes in control plots decreased, indicating a similar temperature effect as was observed for CO2 efflux.

Dr. Jesper Christiansen in his rain gear checking on the eosAC chambers located in the control plot.

CONCLUSIONS
Drought conditions reduced both CO2 efflux and CH4 uptake in the soil compared to the control; this indicates that soil carbon cycling rates are slowed down under lower soil moisture content. Drought also seems to reduce the response of CO2 and CH4 fluxes to environmental variability. Overall, the eosAC chambers coupled to the Los Gatos Research UGGA were capable of capturing the high-resolution temporal variability that is required to start disentangling the relationship between the environmental drivers of CH4 and CO2 fluxes. Additionally, the high accuracy of the system allowed for the observation of small net CH4 uptakes into these soils and clearly demonstrated the difference in CH4 uptake between drought and control plots. The eosAC chambers have thus provided a rare glimpse of the cold-season fluxes for a Danish heath/grassland ecosystem and provided a robust dataset for understanding the impact of drought on soil carbon cycling.

ACKNOWLEDGMENTS
Thanks to Dr. Jesper Christiansen and Dr. Klaus Steenberg Larsen for performing the measurements and analyzing the data associated with this study.