Forest thinning and soil respiration in a Sitka spruce forest in Ireland

Forest thinning influences soil processes by altering key microclimatic conditions, root density, microbial communities, organic matter turnover and nutrient budgets. It introduces a large pulse of harvest residues (brash) to the soil surface and can alter the balance between autotrophic and heterotrophic respiration. This study determined the influence of thinning, microclimatic factors and plant productivity on carbon (C) losses through the emission of carbon dioxide (CO2) respired from thinning lines (brash lanes or BL) and the forest floor (FF: without brash) in a first rotation Sitka spruce (Picea sitchensis (Bong.) Carr.) forest in Ireland. Weekly measurements of CO2 efflux were carried out using an Infra-Red Gas Analyser connected to static chambers; while soil moisture content and soil surface temperature were measured, using theta probes and data loggers, respectively. The soil respiration measurements were also correlated with the gross primary productivity (GPP) determined by eddy covariance techniques.The highest CO2 efflux were observed at the peak of summer in July/2010 (FF = 699.20 mg CO2 m−2 h−1 and BL = 374.22 mg CO2 m−2 h−1) and were associated with maximum soil surface temperatures and higher rates of GPP. Soil temperature had a strong positive influence on the variation of CO2 from the forest (FF = 75% and BL = 59%), and the temperature sensitivity (Q10) of soil respiration from the FF (5.47) was higher than from the BL (2.72). Soil moisture was inversely correlated with soil respiration from both FF (R = −0.73, p < 0.0001) and BL (R = −0.53, p = 0.003). The combined effect of temperature and moisture gave a better description of the variability in CO2 respired from both the FF (R2 = 0.85, p < 0.0001) and BL (R2 = 0.67, p < 0.0001) than temperature and/or moisture alone. GPP was positively correlated with soil respiration with a stronger relationship observed in the FF (R2 = 0.73, p < 0.0001) than the BL (R2 = 0.45, p < 0.0001). The total C loss due to soil respiration from the FF (448.93 g C m−2 year−1) was significantly higher than BL (351.77 g C m−2 year−1). The annual soil respiratory C loss was 435.32 g C m−2 year−1 (calculated based on the contribution of the BL (14%) and FF (86%) to the total forest area).

► Thinning significantly changes the soil moisture dynamics of wet mineral gley soils.
► Soil respiration is limited by the interactive effects of temperature and moisture.
► Soil respiration is driven by the seasonal fluctuations of temperature and moisture.
► Plant productivity has a strong influence on soil respiration in Sitka spruce forest.
► Forest thinning did not have a significant effect on annual respiratory soil C losses.