Decadal variation in soil respiration in a Sitka spruce plantation in central Ireland and its temperature sensitivity

Soil respiration (Rs) is an important part of the terrestrial ecosystem carbon cycle and one of the important contributors to increasing atmospheric carbon dioxide concentrations. However, few studies have examined the effect of stand age on Rs and its temperature sensitivity (Q₁₀) or their associated drivers in plantation forests. This study reports on measurements of Rs, soil water content (SWC), soil temperature (ST) and other environmental variables in Sitka spruce plantation in central Ireland from 2004 to 2016. Our results showed that the annual average Rs ranged from 1.46–2.81 μmol m^-2 s^-1, while Q₁₀ varied from 3.69–6.75. Rs generally decreased as the stand aged, although it first increased and then decreased during the early stages of stand development (Y1-Y3). As stand age increases, ST rose while SWC and Q₁₀ showed the opposite trend. The cumulative emissions of carbon dioxide first increased and then decreased with stand age. Variations in Rs showed a clear seasonal pattern, with the highest values in summer and the lowest in winter and were positively correlated with ST and negatively correlated with SWC. Q₁₀ decreased with increasing ST while high or low SWC reduced Q₁₀. The magnitude of the effect of projected increases in ST and reductions in water availability due to climate change on annual carbon budgets will depend on their relative impacts during different periods of the year. A combination of the expected higher temperatures and reduced soil water contents as projected for the future will, however, reduce Q₁₀. Whilst the planting of Sitka spruce is considered to be an extremely important aspect of increasing the terrestrial carbon sink, Q₁₀ may show a decreasing response to temperature through thermal acclimation, so that the rate of soil carbon sequestration could potentially slow down or remain constant in the future.