Overestimation of closed chamber soil CO<sub>2</sub> effluxes at low atmospheric turbulence

Abstract. Soil respiration (Rs) is an important component of ecosystem carbon balance and accurate quantification of the diurnal and seasonal variation of Rs is crucial for correct interpretation of the response of Rs to biotic and abiotic factors, as well as for estimating annual soil CO2 efflux rates. In this study, we measured Rs hourly for one year by automated closed chambers in a temperate Danish beech forest. The data showed a clear diurnal pattern of Rs across all seasons with higher rates during night-time than during day-time. However, further analysis showed a clear negative relationship between flux rates and friction velocity (u∗) above the canopy, suggesting that Rs was overestimated at low atmospheric turbulence throughout the year due to non-steady state conditions during measurements. Filtering out data at low u∗ values removed or even inverted the observed diurnal pattern, such that the highest effluxes were now observed during day-time, and also led to a substantial decrease in the estimated annual soil CO2 efflux. By installing fans to produce continuous turbulent mixing of air around the soil chambers, we tested the hypothesis that overestimation of soil CO2 effluxes during low u∗ can be eliminated if proper mixing of air is ensured, and indeed the use of fans removed the overestimation of Rs rates during low u∗. Artificial turbulent air mixing may thus provide a method to overcome the problems of using closed chamber gas exchange measurement techniques during naturally occurring low atmospheric turbulence conditions. Other possible effects from using fans during soil CO2 efflux measurements are discussed. In conclusion, periods with low atmospheric turbulence may provide a significant source of error in Rs rates estimated by the use of closed chamber techniques and erroneous data must be filtered out to obtain unbiased diurnal patterns, accurate relationships to biotic and abiotic factors, and before estimating Rs fluxes over longer time scales.

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Overestimation of closed-chamber soil CO<sub>2</sub> effluxes at low atmospheric turbulence

Biogeosciences ◽

10.5194/bg-14-1603-2017 ◽

2017 ◽

Vol 14 (6) ◽

pp. 1603-1616 ◽

Cited By ~ 13

Author(s):

Andreas Brændholt ◽

Klaus Steenberg Larsen ◽

Andreas Ibrom ◽

Kim Pilegaard

Keyword(s):

Atmospheric Turbulence ◽

Abiotic Factors ◽

Measurement Techniques ◽

Negative Relationship ◽

Soil Co2 Efflux ◽

Co2 Efflux ◽

Diurnal Pattern ◽

Soil Co2 ◽

Closed Chamber ◽

Biotic And Abiotic Factors

Abstract. Soil respiration (Rs) is an important component of ecosystem carbon balance, and accurate quantification of the diurnal and seasonal variation of Rs is crucial for a correct interpretation of the response of Rs to biotic and abiotic factors, as well as for estimating annual soil CO2 efflux rates. In this study, we measured Rs hourly for 1 year by automated closed chambers in a temperate Danish beech forest. The data showed a clear diurnal pattern of Rs across all seasons with higher rates during night-time than during daytime. However, further analysis showed a clear negative relationship between flux rates and friction velocity (u∗) above the canopy, suggesting that Rs was overestimated at low atmospheric turbulence throughout the year due to non-steady-state conditions during measurements. Filtering out data at low u∗ values removed or even inverted the observed diurnal pattern, such that the highest effluxes were now observed during daytime, and also led to a substantial decrease in the estimated annual soil CO2 efflux. By installing fans to produce continuous turbulent mixing of air around the soil chambers, we tested the hypothesis that overestimation of soil CO2 effluxes during low u∗ can be eliminated if proper mixing of air is ensured, and indeed the use of fans removed the overestimation of Rs rates during low u∗. Artificial turbulent air mixing may thus provide a method to overcome the problems of using closed-chamber gas-exchange measurement techniques during naturally occurring low atmospheric turbulence conditions. Other possible effects from using fans during soil CO2 efflux measurements are discussed. In conclusion, periods with low atmospheric turbulence may provide a significant source of error in Rs rates estimated by the use of closed-chamber techniques and erroneous data must be filtered out to obtain unbiased diurnal patterns, accurate relationships to biotic and abiotic factors, and before estimating Rs fluxes over longer timescales.

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Corrigendum to: Seasonal patterns of soil carbon dioxide efflux from a wet¿dry tropical savanna of northern Australia

Australian Journal of Botany ◽

10.1071/bt01049_co ◽

2002 ◽

Vol 50 (3) ◽

pp. 373

Author(s):

Xiaoyong Chen ◽

Derek Eamus ◽

Lindsay B. Hutley

Keyword(s):

Root Growth ◽

Soil Temperature ◽

Dry Season ◽

Soil Co2 Efflux ◽

Co2 Efflux ◽

Tropical Savanna ◽

Northern Australia ◽

Soil Co2 ◽

Wet Season ◽

Closed Chamber

Soil CO2 efflux rates were measured in a eucalypt open forest in a tropical savanna of northern Australia, with a portable closed chamber and CO2 gas analyser. Both abiotic (soil temperature and water content) and biotic (litterfall and fine-root growth) factors that may influence soil CO2 efflux were examined. Daytime rates of soil CO2 efflux rate were consistently higher than nocturnal values. Maximal rates occurred during late afternoons when soil temperatures were also maximal and minimum values were recorded during the early morning (0400–0800 hours). Average soil CO2 efflux was 5.37 mol m–2 s–1 (range 3.5–6.7 mol m–2 s–1 during the wet season and declined to 2.20 mol m–2 s–1 (range 1.2–3.6 mol m–2 s–1) during the dry season. The amount of carbon released from soil was 14.3 t ha–1 year–1, with approximately 70&percnt; released during the wet season and 30&percnt; during the dry season. The rate of efflux was correlated with soil moisture content and soil temperature only during the wet season, when root growth and respiration were high. During the dry season there was no correlation with soil temperature. These results are discussed in relation to the carbon balance of tropical savannas.

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