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SOIL PHYSICS

Respiration in Boreal Forest Soil as Determined from Carbon Dioxide Concentration Profile

^a Dep. of Forest Ecology, P.O. Box 27, FI-00014 Univ. of Helsinki, Finland
^b Finnish Forest Research Institute, Jokiniemenkuja 1, 01301 Vantaa, Finland
^c Dep. of Physical Sciences, P.O. Box 64, FI-00014 Univ. of Helsinki, Finland
^d Hyytiälä Forestry Field Station, Hyytiäläntie 124, FI-35500 Korkeakoski, Finland
^e Vaisala Oyj, P.O. Box 26, FI-00421 Helsinki, Finland

^* Corresponding author (jukka.pumpanen{at}helsinki.fi).

Soil respiration forms a substantial part of the ecosystem respiration. However, the respiration measurements conducted with chambers on the soil surface do not give information on the vertical distribution of CO₂ sources and its seasonal dynamics. We used permanently installed CO₂ probes to determine the CO₂ profile in a boreal coniferous forest soil and calculated the CO₂ efflux from the concentration profile with a dynamic model. There was an increase in both soil CO₂ efflux and soil air CO₂ concentration over the 2.5-mo study period between April 15 and June 30. The CO₂ efflux determined from the concentration profile was in relatively good agreement with the CO₂ efflux measured by open dynamic chamber method. We also determined the respiration of different soil horizons and estimated the contribution of recent photosynthate to total respiration. Humus layer and A-horizon contributed 69.9%, B-horizon 19.8%, and C-horizon 10.4% of the total CO₂ efflux. The Q₁₀ values determined over 7-d periods were on average 2.54, 3.66, and 13.14 in A-, B-, and C-horizons, respectively. However, when fitted over the whole 2.5-mo time period, the Q₁₀ values were 3.56, 5.57, and 17.45 in A-, B-, and C-horizons, respectively. Based on the different temperature responses obtained over 7-d and 2.5-mo time periods, we estimated the contribution of recent photosynthate to soil respiration. In April, the respiration originating from recent photosynthate contributed about 2% of the total respiration in the A-, B-, and C-horizons and increased to 32, 35, and 24% by June 30 in the respective soil horizons.

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