Seasonal and inter-annual variability of soil respiration at Skyttorp, a Swedish boreal forest

Abstract: The ecosystem carbon balance is the net result of CO2 uptake by photosynthesis and CO2 emission through ecosystem respiration and soil respiration is a major part of ecosystem respiration. Soil CO2 fluxes were continuously measured from August 2005 to November 2008 with an automatic chamber system at a boreal forest site in central Sweden to investigate the annual and seasonal variations of soil respiration (Rs) and its dependence on soil temperature and soil moisture. The soil temperature varied from -2°C to 21°C at 3 cm depth, and top 5-cm soil water content was in the range of 5 vol% to 40 vol%. There were no large variations of soil temperature among all the chambers on the annual time scale. The CO2 fluxes in all chambers but one varied from 0 to 12 μmol m-2 s-1 in summer and from 0 to 5 μmol m-2 s-1 during winter. The fluxes in one chamber had larger fluctuations with the range of -4 to 24 μmol m-2 s-1 in summer time because of the influences of ground vegetation in the chamber. The variation of CO2 fluxes tightly followed the variation of soil temperature on seasonal and inter-annual time scale. The temperature sensitivity Q10 calculated on the annual time scale varied from 3.07 to 3.45 among 5 chambers. A positive correlation existed between Q10 and soil water content when soil water content was in the range of 15% to 30%, and Q10 was decreasing with increasing temperature. The R10 (the soil respiration rate at 10°C) regression model provided good estimated Rs in both daytime and nighttime. The photosynthesis rate of ground vegetation in the chamber was around 5 μmol m-2 s-1 by comparing the estimated CO2 fluxes and measured CO2 fluxes in the daytime. A decline of Rs rate was found in one chamber when soil water content was higher than 33% during growing season of 2008. The seasonal and inter-annual variability of soil CO2 fluxes were mainly explained by 3-cm soil temperature variation at Skyttorp site.